B. V. Tronov and L. V. Ladygina

Types of Bond Cleavage in Molecules of Organic Compounds.

I

In modern organic chemistry, alongside the study of natural organic substances and the synthesis of new compounds, a significant place is occupied by the study of processes. More and more works are appearing that concern the mechanism of reactions and the influence, on the direction and rate of a given reaction, of various atoms or atomic groups that are constituents of the reacting molecules.

Chemists are very interested in the question of the strength of the bonds of hydrocarbon and other organic radicals with oxygen, nitrogen, halogens, etc. The material on this question has been collected most fully in the article by M. Tiffeneau and A. Orekhov, “On the reactivity of molecules and the strength of bonds in molecules”¹.

The authors of the review mentioned distinguish the following concepts:

a) Bond strength (solidité des liaisons, force d’attache) of a given radical with the atom from which it is directly detached in the reaction under study.

b) Affinity capacity (capacité affinitaire).

Basing themselves chiefly on Werner’s views², they assume that every atom possesses a definite “sum of affinity,” and the greater the share of affinity that it expends on bonding with some other atom, the less remains for the other bonds of the first atom. Thus, in the molecule of hexaphenylethane the bond between the two central carbons is weakened and is readily broken owing to the great affinity capacity of the phenyl groups. Each phenyl saturates more than one third of the affinity of the ethane carbon, and almost nothing remains for the middle bond of the molecule. Thus the concept of “affinity capacity” is an expression of the influence that a given atom or radical exerts on other parts of the molecule. To determine the magnitude of this “capacity,” reactions involving the cleavage of more distant bonds are studied.

c) Migratory aptitude (aptiltude migratrice), i.e., the ability of an atom or radical to move within the same molecule.

Among the methods for determining bond strength proper, Tiffeneau and Orekhov cite:

1) Cleavage of unsymmetrically constructed aromatic ketones by sodium amide³.

\[ \begin{aligned} &\mathrm{R{-}C{-}R'} + \mathrm{NaNH_2} \;\longrightarrow\; \mathrm{R{-}C{-}R'} \;\longrightarrow\; \mathrm{RH} + \mathrm{R'C} \;+\;\mathrm{H_2O} \;\longrightarrow \\[-2mm] &\qquad\ \ \Big\|\,\mathrm{O} \qquad\qquad \begin{matrix} \mathrm{ONa}\\[-1mm] \wedge\\[-1mm] \mathrm{ONa}\ \mathrm{NH_2} \end{matrix} \qquad\qquad \begin{matrix} \mathrm{NH}\\[-1mm] \mathrm{O} \end{matrix} \\[2mm] &\longrightarrow\ \mathrm{R'{-}C} \qquad\longrightarrow\ \mathrm{R'C} \qquad\left\{\ \text{or } \mathrm{R'H\cdots R{-}C}\ \right\} \\[-1mm] &\qquad\quad \mathrm{NH} \qquad\qquad\ \ \mathrm{NH_2} \qquad\qquad\qquad\quad \mathrm{NH_2} \end{aligned} \]

For the relative strength of the bond, the following numbers were obtained in this way:

2) Decomposition of simple ethers by metallic sodium (P. P. Shorygin¹):

\[ \mathrm{R—OR'} + 2\,\mathrm{Na} \to \mathrm{RONa} + \mathrm{NaR'} \quad \text{or} \quad \mathrm{RNa} + \mathrm{NaOR'}. \]

In the order of increasing strength of the bond to oxygen, the radicals are arranged as follows:

\[ \mathrm{C_6H_5.CH_2 < C_2H_5 < C_5H_{11} < \alpha{-}C_{10}H_7,\ \beta{-}C_{10}H_7.} \]

3) The action of cyanogen bromide on amines, arsines, and sulfides, more precisely—the heating of the addition products obtained with $\mathrm{BrCN}$²:

\[ \begin{array}{c} \mathrm{RR'R''N—CN \to RBr + R'R''NCN}\ \text{etc.} \\ \ \ \ \ \ | \\ \ \ \ \ \ \mathrm{Br} \end{array} \]

For the strength of the bond of hydrocarbon radicals to nitrogen, the following series were obtained:

\[ \begin{gathered} \mathrm{CH_3CH{=}CH—CH_2 < CH_2{=}CH—CH_2—} \\ \mathrm{— < C_6H_5.CH_2 < CH_3 < C_2H_5 < C_3H_7 < C_4H_9 < Ar} \end{gathered} \]

The aromatic radicals were arranged as follows:

\[ \mathrm{\beta{-}C_{10}H_7 < \alpha{-}C_{10}H_7 < C_6H_5 < CH_3.C_6H_4 < C_2H_5.C_6H_4 < C_6H_5.C_6H_4.} \]

Radicals of the benzyl type:

\[ \begin{gathered} \mathrm{p{-}C_6H_5.C_6H_4.CH_2 < p{-}C_2H_5.C_6H_4.CH_2 <} \\ \mathrm{p{-}CH_3.C_6H_4.CH_2 < C_6H_5.CH_2 < \alpha C_{10}H_7.CH_2 < \beta{-}C_{10}H_7.CH_2.} \end{gathered} \]

The study of sulfur and arsenic compounds gives almost identical series:

\[ \begin{gathered} \mathrm{S:\ C_6H_5.CH_2 < CH_2{=}CH—CH_2 < CH_3 < C_2H_5 < C_4H_9} \\ \mathrm{As:\ CH_3 < C_2H_5 < C_3H_7 < C_4H_9 \ldots < C_6H_5.} \end{gathered} \]

Much more data are given on the question of the capacity of affinity of various radicals. To determine the capacity of affinity, the following have been indicated:

1) Reduction of substituted phenylhydrazines:³

\[ \mathrm{Ar—NH—NH_2 \to ArNH_2 + NH_3.} \]

The change in the reaction rate (the reducing agent is $\mathrm{SnCl_2}$) characterizes the influence of the nature of the radical $\mathrm{Ar}$ on the strength of the bond between the nitrogen atoms.

2) Hydrolysis of substituted benzoxazoles¹:

$$ \begin{array}{cc} \begin{array}{c} \mathrm{N}\\[-2mm] \diagdown\quad |\\[-1mm] |\quad | \qquad \mathrm{C}-\mathrm{R}\\[-1mm] \diagup\quad \diagdown\\[-1mm] \qquad \mathrm{O} \end{array} & \begin{array}{c} \mathrm{NH}-\mathrm{COR}\\[-2mm] \diagdown\quad |\\[-1mm] |\quad |\\[-1mm] \diagup\quad \diagdown\\[-1mm] \qquad \mathrm{OH} \end{array} \end{array} $$

3) Hydrolysis of substituted benzyl chlorides².
The rate of action of sodium ethylate was measured:

$$ \mathrm{Ar}-\mathrm{CH_2Cl}+\mathrm{C_2H_5ONa} \longrightarrow \mathrm{Ar}-\mathrm{CH_2OC_2H_5}+\mathrm{NaCl}. $$

4) Dissociation of hexaarylethanes³:

$$ \mathrm{Ar_3C}-\mathrm{CAr_3}\longrightarrow 2\,\mathrm{Ar_3C}. $$

The change in the dissociation constants under the influence of substituents in the benzene rings is studied.

5) Halochromism of triarylcarbinols⁴. The colored solutions of these compounds in concentrated sulfuric acid are decolorized upon dilution with alcohol. From the amount of alcohol that must be added for complete decolorization of the solution, the basicity of the carbinol is determined, and from it one judges the capacity of affinity of the radicals entering into the composition of the molecule.

6) Dehydration of pinacones⁵:

$$ \begin{array}{cccccc} & \mathrm{R} & & \mathrm{R} & & \mathrm{R}\qquad\qquad \mathrm{R}\\[1mm] & | & & | & & |\\[-1mm] \mathrm{C}-\mathrm{C} & \longrightarrow \mathrm{H_2O}+& \mathrm{C}-\mathrm{C} & \longrightarrow & \mathrm{R}-\mathrm{C}-\mathrm{CR_3}\\[-1mm] & & & | & & \|\\[-1mm] \mathrm{R}\ \mathrm{OH}\ \mathrm{OH}\ \mathrm{R} & & \mathrm{R}\ \mathrm{O}\ \mathrm{R} & & & \mathrm{O} \end{array} $$

The hydroxyl leaves from that carbon atom which is bound to radicals possessing the greater capacity of affinity.

7) Isomerization of substituted oxides⁶:

$$ \begin{array}{c} \mathrm{R}-\mathrm{CH}-\mathrm{CH}-\mathrm{R'} \longrightarrow \mathrm{R}-\mathrm{CH_2CO}-\mathrm{R'} .\\[-1mm] \quad \diagdown\ \diagup\\[-1mm] \quad\ \mathrm{O} \end{array} $$

Here also, from the site of the predominant bond cleavage, the radical with the greater capacity of affinity is recognized.

Tiffeneau and Orekhov give the following table of relative values of the capacity of affinity, determined by various methods:

TABLE 1.

From the figures cited it is evident that the capacity for affinity is indeed a fairly definite quantity, characteristic of the given radical. The relations only vary within comparatively small limits depending on the method employed.

To determine migratory aptitude, the article under discussion cites the dehydration reaction of pinacones,¹) which here is carried out for the most part in the following form:

\[ \begin{array}{ccccc} R & R & R & R & R\\[4pt] & C{-}C & \longrightarrow & C{-}C & \longrightarrow\ R{-}C{-}C{-}R'\\ & | \quad | & & | \quad | & \qquad\quad \|\\ R' & OH\quad OH & R' & \quad O{-}R' & R'\quad O \end{array} \]

Both halves of the molecule are identical and give up hydroxyl equally readily, but after the elimination of water it is predominantly either \(R\) or \(R'\) that migrates to the other position.

The semipinacolinic rearrangement is reduced to the same thing, e.g.²)

\[ \begin{array}{cc} & CH_3 \qquad C_6H_5\\ C_6H_5{-}CHOH{-}COH & \longrightarrow \quad CH{-}CO{-}CH_3\\ & C_2H_5 \qquad C_2H_5 \end{array} \]

the hydrolysis of certain iodohydrins³):

\[ C_6H_5{-}CHI{-}C(OH)\begin{matrix}/C_2H_5\\ \backslash CH_3\end{matrix} \ \longrightarrow\ \begin{matrix}C_6H_5\\ C_2H_5\end{matrix} > CH{-}CO{-}CH_3; \]

deamination of alcohols, as

\[ \begin{array}{c} C_6H_5\\ \diagdown \end{array} C{-}CH_2{-}CH(NH_2){-}C_6H_5 \ \longrightarrow\ x{-}C_{10}H_7.CO.CH(C_6H_5)_2\ ^3) \]
\[ x{-}C_{10}H_7 \begin{array}{c} \\[-10pt]\diagup\\[-2pt] \end{array} \qquad \begin{array}{c} |\\[-2pt]OH \end{array} \]

or

\[ \begin{array}{c} C_6H_5\\ \diagdown \end{array} >C{-}CH_2{-}NH_2 \ \longrightarrow\ C_6H_5{-}CO{-}CH_2{-}C_{10}H_7\ ^4) \]
\[ C_{10}H_7 \begin{array}{c} \diagup\\[-2pt] \end{array} \qquad \begin{array}{c} |\\[-2pt]OH \end{array} \]

The study of all these rearrangements led to the following series of radicals (arranged in order of decreasing migratory aptitude):

\[ p{-}C_6H_4F\ (\text{relative value }1.86) > C_6H_5\ \text{and}\ p{-}C_6H_4J\ (1) > \]
\[ > p{-}C_6H_4Br\ (0.75) > p{-}C_6H_4Cl\ (0.66); \]

\[ \begin{array}{rcl} p{-}CH_3.C_6H_4 & & \\ p{-}CH_3O.C_6H_4 & > C_6H_5 > p{-}Br.C_6H_4 > p{-}Cl.C_6H_4 > \end{array} \begin{array}{l} o\text{- and }m{-}ClC_6H_4\\ o\text{- and }m{-}BrC_6H_4; \end{array} \]

\[ C_6H_5.CH_2 > C_2H_5;\ C_4H_9 > C_3H_7 > CH_3;\ C_2H_5 > CH_3. \]

When \(\alpha\)-naphthyl was compared with phenyl, contradictory results were obtained.

Summing up all the material collected in the article, Tiffeneau and Orekhov do not draw any general theoretical conclusion. For

¹) Montagne, Koopal. Rec. tr. ch. 26, 253, 273; 29, 150 (1910); Koopal. Rec. tr. ch. 34, 115; Thörner; Zincke. Ber. 11, 1988; Tiffeneau. Ann. ch. phys. 8th series, 10, 360.
²) Tiffeneau, Lévy. Bl. soc. ch. 4th series, 33, 759; Lévy, Bl. soc. ch. 33, 1655.
³) Mc. Kenzie, R. Roger. Journ. Chem. Soc. 125, 845.
⁴) Luc. C. r. 180, 145.

for each individual property (bond strength, affinity capacity, and migratory ability), the series and relative numbers obtained by different methods of determination agree rather well. But the values of bond strength, affinity capacity, and migratory ability for each given radical seem to be entirely independent of one another and often do not vary with the nature of the radical in the way one might have expected.

Thus, for the first four members of the series of limiting radicals, determination of the strength of their bond with gold according to Braun (action of cyanogen bromide on tertiary amines) gives:

\[ \mathrm{C_6H_5,\ C_2H_5,\ C_3H_7,\ CH_3,} \]

whereas, for affinity capacity, the study of the products of acylation of pinacones leads to a series with a sharply expressed periodicity:

\[ \mathrm{CH_3 : C_2H_5 : C_6H_5 : C_{10}H_7 = 20 : 1 : 16 : 10.} \]

The ability of radicals to migrate would seem to have to vary inversely with the strength of their bonds to the atom from which they are detached. In fact, however, with the greatest bond strength—for example, with N, S, As—aromatic radicals migrate considerably more readily than aliphatic ones, and the weakest of all are radicals of the benzyl type; but in pinacol rearrangement the phenyl group migrates easily, the limiting radicals with more difficulty, and benzyl occupies an intermediate position.

The authors of the article draw no definite conclusions about the dependence of the quantities under consideration on other properties of radicals. They point out, for example, that atoms or groups of atoms of identical electrochemical character may exert opposite effects on the affinity capacity and bond strength of the benzene ring; even one and the same atom or radical may have opposite effects if it occupies different positions. Thus, the nitro group in the para position greatly increases affinity capacity, while in the meta position it noticeably decreases it; chlorine and iodine in the ortho and para positions also increase it, in the meta position they decrease it, and bromine always lowers affinity capacity. The same is true for the magnitude of molecular weight. The heavy radical \(\mathrm{C_6H_5}\) has a greater migratory ability than the considerably lighter \(\mathrm{CH_3}\) and \(\mathrm{H}\); fluorine further intensifies the tendency toward migration, whereas iodine, making the phenyl group almost three times heavier, does not affect migratory ability.

The review compiled by Tiffeneau and Orekhov by no means embraces all the material available in the literature on the question at issue. Thus, among the methods for the direct determination of bond strength, there are not mentioned, for example, various reactions of cleavage and exchange of halides, and the cleavage of simple and complex esters by hydrohalic acids. For evaluating the affinity capacity of radicals, the determination of the constants of electrolytic dissociation of acids and amines, the hydrolysis of esters, etc., must be considered extremely important. In practice, however, it was impossible to include all this here, since the field is too large: the course of all reactions depends on the nature of the radicals entering into the composition of the molecule.

Of the more recent works that are of a general character or that give much experimental data, we shall mention first of all the papers of K. Kindler¹ — “The strength of the bond of organic radicals and reactivity”: I) “Addition of hydrogen sulfide to nitriles and saponification of esters”; II) “Saponification of esters, addition of hydrogen sulfide to nitriles, and conversion of alkyl chlorides into simple ethers.”

In the first paper the author gives a series of numbers characterizing the bond strength of various aromatic radicals with the carbons of the nitrile and carboxyl groups, calculated from the rate constants of the reactions of addition of $\mathrm{H_2S}$ to $\mathrm{RCN}$ and hydrolysis of $\mathrm{R.COOC_2H_5}$:

\[ \mathrm{R.C \equiv N + H.SH \rightarrow R.C(=NH)(SH)} \]

\[ \mathrm{R.C(=O)-OC_2H_5 + NaOH \rightarrow R.C(ONa)(OH)(OC_2H_5) \rightarrow R.C(=O)(ONa) + C_2H_5OH.} \]

In both cases the triple bond $\mathrm{C \equiv N}$ or the double bond $\mathrm{C=O}$ is directly broken, and thus the strength of the $\mathrm{R-C}$ bond is determined only indirectly. Therefore these methods are rather related to the question of the capacity of affinity.

In the table compiled by Kindler, for comparison, there are also given numbers taken from the literature and obtained by other methods (dissociation of hexaarylethanes, hydrolysis of oxazoles, halochromy of carbinols).

TABLE 2.

¹ K. Kindler. Lieb. Ann. 450, 1–20 [1926]; 452, 90–12 [1927].

The numbers in the different columns indicate, in every case, agreement as to the order of magnitude of the capacity of the medium.

A similar dependence on the nature of the radical \(R\) was found by Kindler in reduction reactions of carboxylic acids, their esters, and amides.

As a result the author arrives at a conclusion that somewhat contradicts what we see in Tiffeneau and Orekhov. In his opinion, the strength of the bond of aryls with \(C\), \(O\), or \(C\), \(N\) is decisively influenced by the electrochemical character of the substituents in the benzene ring; positive atoms and groups \((NH_2, CH_3)\) increase it, while negative ones \((NO_2, Cl, Br, J)\) decrease it. Kindler explains this by using Stark’s theory.

In the second article several more series of numbers are added, expressing the relative strength of the bond \(R—COOC_2H_5\) (more precisely, again, the quantity reciprocal to the capacity of the medium). Besides new series of aromatic radicals, aliphatic radicals now also appear, as well as mixed aliphatic-aromatic radicals. The following table gives the bond-strength values for the simplest radicals of these types.

TABLE 3.

The strength of the bond of \(R\) with \(COOC_2H_5\) is also compared with the constants of electrolytic dissociation of \(RCOOH\) and \(ROH\). A complete parallelism is observed.

Several works by K. Auwers and co-workers are devoted to the question of the strength of the bond of hydrocarbon and other radicals with nitrogen. The thermal decomposition of indazolium salts was studied.¹) Compounds of the following types were taken:

\[ \mathrm{I}\quad \left[ \begin{array}{c} CH\\ C_6H_4 \quad N\!\cdot\!R\\ N\!\cdot\!C_6H_5 \end{array} \right]J \qquad \mathrm{II}\quad \left[ \begin{array}{c} CH\\ C_6H_4 \quad N\!\cdot\!CH_3\\ N\!\cdot\!R \end{array} \right]J \]

¹) As the strength of the bond of \(R\) with the halogen, the value reciprocal to the rate constant of the reaction \(RX + NaOC_2H_5\) was taken.

²) The rate of the reaction \(CH_3CN : H_2S\) is too great. Therefore \(CH_3CH_2CN\) and \(CH_3CH_2CH_2CN\) were compared only with one another.

³) K. Auwers, H. Dusterdieck, H. Kleiner. Ber. 61, 100 (1928).

On heating, the halogen leaves together with the radical \(R\) or \(CH_3\). The ease of cleavage of either depends both on the nature of \(R\) and on its position in the molecule. Experiment shows that the radical is held more firmly at the nitrogen atom standing next to the benzene ring.

In Table 4, on the left are given figures indicating what percentage of indazole salts of scheme I was converted on heating into 2-alkylindazole; on the right—what percentage of the 1-alkyl-2-methylindazole salt of scheme II was converted into 1-alkylindazole.

TABLE 4.

Of the results, special note should be made of the establishment of the fact of the extraordinary weakness of the vinyl bond. It is also interesting that secondary \((CH_3)_2CH\) differs only very little from normal propyl. Otherwise the data obtained correspond to the usual conception of the radicals investigated by the authors.

In the following works, benzimidazoles \(^{1}\), tetrahydroindazoles, pyrazoles, and imidazoles \(^{2}\) were considered. Here the authors give only qualitative information on the direction of the reactions, since quantitative determinations were very difficult.

The arrangement of radicals according to bond strength in most cases proved to be the usual one \((C_6H_5.CH_2 < CH_3 < C_2H_5 < C_3H_7 < C_4H_9)\), but there are also exceptions. In tetrahydroindazoles a more facile cleavage of \(C_3H_7\) than \(C_2H_5\) was observed. With some radicals there occurred,

\(^{1}\) K. v. Auwers, W. Mauss. Ber. 61, 2411 [1928].
\(^{2}\) K. Auwers, J. Conrad, A. Ernecke. Lieb. Ann. 469, 82. [1929].

besides the splitting off of alkyl iodide, also the transfer of radicals from one position to another. In studying such processes, one has to deal simultaneously with bond strength, with migratory capacity and, of course, also with the relative valence demand.

Such is the action of sulfuric acid on dialkylphenyloxytriazolines,¹ for example:

\[ \begin{array}{ccc} \begin{array}{c} \mathrm{CH_3}\\ |\\ \mathrm{CH_3{-}C \qquad C{-}CH_3}\\[0.8em] \mathrm{H{-}C \qquad N}\\[-0.2em] \mathrm{OH}\qquad \mathrm{N.C_6H_5} \end{array} & \longrightarrow & \begin{array}{c} \mathrm{CH_3}\\ |\\ \mathrm{CH_3{-}C \qquad C{-}CH_3}\\[0.8em] \mathrm{C}\\[-0.2em] \mathrm{N.C_6H_5} \end{array} \\[2em] && \longrightarrow \begin{array}{c} \mathrm{CH_3{-}C \qquad C{-}CH_3}\\[0.8em] \mathrm{CH_3{-}C \qquad N}\\[-0.2em] \mathrm{N.C_6H_5} \end{array} \end{array} \]

The rate was measured approximately and the reaction products were studied. It turned out that the \((\mathrm{CH_3})_2\)-derivative is transformed in 50 min.; \(\mathrm{CH_3C_2H_5}\) and \((\mathrm{C_2H_5})\), in 2 min.; the \(\mathrm{CH_3C_3H_7}\) and \((\mathrm{C_3H_7})_2\)-derivatives react to completion instantaneously; \(\mathrm{CH_3C_6H_5CH_3}\)—only in 70 min.; \(\mathrm{CH_3C_3H_7}\) and \((\mathrm{C_3H_7})_2\)—very rapidly; \(\mathrm{C_2H_5}\)—only in \(1\frac12\) hours.

As regards bond strength, it was found that \(\mathrm{CH_3}\) is held very firmly; \(n\text{-}\mathrm{C_4H_9}\), \(\mathrm{CH_3{=}CH.CH_2}\) and \(\mathrm{C_6H_5.CH_2}\) are split off readily; \(\mathrm{C_3H_5}\), \(\mathrm{C_4H_7}\), \(i\text{-}\mathrm{C_3H_7}\) occupy an intermediate position.

In certain cases it is not water but alcohol that is split off. For example, with two propyls:

\[ \begin{array}{ccc} \begin{array}{c} \mathrm{C_3H_7}\\ |\\ \mathrm{C_3H_7{-}C \qquad C{-}CH_3}\\[1.0em] \mathrm{OH{-}C \qquad N}\\[-0.2em] \mathrm{H}\qquad \mathrm{N.C_6H_5} \end{array} & \longrightarrow \mathrm{C_3H_7OH}\;-\; & \begin{array}{c} \mathrm{CH_3{-}C \qquad C{-}CH_3}\\[1.0em] \mathrm{HC \qquad N}\\[-0.2em] \mathrm{N.C_6H_5} \end{array} \end{array} \]

The reaction proceeds in the same way if one of the radicals is \(\mathrm{C_3H_7}\), \(\mathrm{C_4H_9}\), \(\mathrm{C_3H_5}\), \(\mathrm{C_6H_5.CH_2}\), and the other is \(\mathrm{CH_3}\). In the ethyl-propyl derivative, \(\mathrm{C_2H_5}\) migrates, while \(\mathrm{C_3H_7}\) remains in place.

Auwers and Düsterdick² determined the valence demand of radicals from the cleavage of 2-alkylcoumaranone semicarbazones. The following series was obtained:

\[ \mathrm{CH_3 > C_2H_5 > C_4H_9 > C_3H_7}, \]

i.e. the alkyls arranged themselves in a new order.

From the facts listed, Auwers concludes that no definite dependence can be indicated between bond strength (Haftfestigkeit), migratory capacity (Wanderungsfähigkeit) and valence demand (Valenzbeanspruchung). Nor can absolute significance be assigned to an existing series of bond strengths of radicals.

The latter proposition is confirmed by the results of measuring the strength of the bond of hydrocarbon radicals with metal in organomercury compounds.³ The products of incomplete decomposition of these compounds under the action of HCl were determined:

\[ \mathrm{R{-}Hg{-}R' + HCl \rightarrow RHgCl + HR'} \quad \text{or} \quad \mathrm{R'HgCl + HR}. \]

¹ K. v. Auwers, A. Fersen. Lieb. Ann. 462, 104 (1928).
² K. v. Auwers, H. Düsterdick. J. pr. Ch. [2], 118, 53—66.
³ M. S. Karesch, R. Marker. Chem. Zbl. 1927, I, 1153.

The experiments led to such an arrangement of radicals, which is in complete contradiction with the usual notion of their properties.

The series obtained was approximately the following (in order of decreasing strength of the bond with Hg):

\[ \begin{array}{l} \mathrm{C_2H_5}\\ \mathrm{C_3H_7}\\ \mathrm{C_4H_9}\\ i\text{-}\mathrm{C_5H_{11}}\\ \mathrm{C_6H_5\cdot CH_2} \end{array} \quad\left|\quad > \mathrm{C}-\mathrm{C_5H_{11}} > \mathrm{CH_3} > \mathrm{C_7H_5} > \quad\right|\quad \begin{array}{l} \mathrm{C_{10}H_7}\\ o\text{-}\mathrm{CH_3O\cdot C_6H_4}\\ \mathrm{C_6H_4(CH_3)}\\ p\text{-}\mathrm{CH_3\cdot C_6H_4} \end{array} \]

Let us mention still another modification of the halo-chromic method, which apparently makes it possible to judge the capacity for affinity not only of aromatic radicals but also of the most varied other radicals.

Many unsaturated compounds give, with polynitro compounds of the benzene series (good nitro derivatives with three \(\mathrm{NO_2}\)) and with certain salts (iodide salts of the alkali metals proved best), colored complexes¹. For example, from acetone with trinitrobenzene and NaJ there is formed a substance of the proposed structure:

\[ \begin{array}{ccccc} & \mathrm{O} & \mathrm{CH_3} & & \mathrm{OC(CH_3)_2} \\ & & | & & : \\ (\mathrm{O_2N})_2\mathrm{C_6H_3N} = \mathrm{O} \ldots \mathrm{C} = \mathrm{O} \ldots \mathrm{NaJ} \\ & & | & & : \\ & & \mathrm{CH_3} & & \mathrm{OC(CH_3)_2} \end{array} \]

These unstable ternary compounds are decolorized upon addition of alcohol and, by determining the concentration (practically, determining the amount of alcohol added until decolorization), one can compare the capacity for complex formation both of different unsaturated compounds with one another and of the remaining components of the system.

We give tables of experiments with \(\mathrm{CH_3\cdot C_6H_2(NO_2)_3}\) and NaJ, and with \(\mathrm{C_6H_3(NO_2)_3}\) and KJ.

TABLE 5.

TABLE 6.

In the experiment the complex is always taken in a solution containing an excess of the unsaturated compound, which, consequently, must be liquid. Therefore the method is not suitable for excessively complex radicals. But, on the other hand, we can take the most diverse classes of organic substances: ketones, aldehydes, unsaturated alcohols, esters, nitriles, amines of the pyridine type, and introduce into them various fatty and other sufficiently simple radicals, which will give several separate series. This method, incidentally, will serve precisely as a supplement to the Baeyer method, since there one has to deal exclusively with radicals of great complexity.

II.

Let us consider the question of the strength of the bond, the valence capacity, and the migratory ability from the electron-theoretical standpoint. In doing so we shall use only the most schematic notions of the structure of the atom, without entering into the details of electron motion.

The concept of “valence capacity” expresses the influence of some atom or group of atoms on parts of the molecule not directly connected with the given atom or radical. It had long been noticed that this influence depends not only on the nature of the radical, but also on its position in the molecule. As the table given in the article by Tiffeneau and Orekhov (Table I) shows, sometimes one and the same atom or group of atoms exerts even an opposite influence when attached, for example, to different links of some benzene derivative. We find a very striking example in comparing the dissociation constants of benzoic acid and the three oxybenzoic acids¹:

\[ \begin{aligned} \mathrm{C_6H_5.COOH} &= 6.6 \cdot 10^{-5} \\ \mathrm{o{-}HO.C_6H_5.COOH} &= 106 \cdot 10^{-5} \\ \mathrm{m{-}HO.C_6H_5.COOH} &= 8.3 \cdot 10^{-5} \\ \mathrm{p{-}HO.C_6H_5.COOH} &= 2.9 \cdot 10^{-5} \end{aligned} \]

A hydroxyl group in the ortho position to the carboxyl group very strongly increases the acidic properties of the molecule; in the meta position it also increases them, but considerably less, and finally, in the para position it reduces the constant by half.

From the point of view of the electronic theory, the influence of a substituent standing at one of the links of any chain must be of a twofold kind. If, for example, instead of hydrogen we introduce into some link of a carbon chain an atom with a larger effective nuclear charge, then it more strongly attracts the surrounding electrons, neutralizes them to a greater extent; by this a part of the positive charge of the nucleus associated with this carbon atom is liberated, and in turn this nucleus begins more fully to neutralize the neighboring electrons, etc. In general the positive charges of all the atomic nuclei of the molecule appear, as it were, to increase. As the distance from the substituent increases, its action weakens.

The same substituent also acts otherwise. The electrons, or electronic orbits, around each nucleus tend to arrange themselves as symmetrically as possible. If a pair of electrons is attracted toward the greater positive charge and, consequently, is displaced away from the carbon nucleus to which the substituent has become attached, then this also compels the remaining electrons of the given carbon atom to become farther from its nucleus and closer to the nuclei of the other atoms bound to it. At the next link of the chain the electrons are displaced closer to the nucleus. As a result, an alternating expansion and contraction of the electron shells is obtained:

$$ \begin{array}{cccc} \mathrm{H} & \mathrm{H} & \mathrm{H} & \mathrm{H}\\ \ddots & \ddots & \ddots & \ddots\\ :\mathrm{Cl}: & \mathrm{C} & :\mathrm{C}: & \mathrm{C} & :\mathrm{C}:\mathrm{H}\\ \ddots & \ddots & \ddots & \ddots\\ \mathrm{H} & \mathrm{H} & \mathrm{H} & \mathrm{H} \end{array} $$

Some carbon atoms (the nuclei of which have attracted the electrons more closely) have become, as it were, more electronegative; others have become electropositive. Schematically one may express this influence (alternating polarity)\(^1\) as follows:

$$ \mathrm{Cl}.\mathrm{CH}_2.\mathrm{CH}_2.\mathrm{CH}_2.\mathrm{CH}_3. $$

It is clear that the periodic action of a substituent on the electron shells also gradually diminishes with distance.

The considerations presented give the concept of the “affinity capacity” a rational foundation and make it possible to explain many facts that at first sight seem strange. However, in certain questions concerning the influence of atoms and groups remote from the bond that is broken in the reaction under study, it is still rather difficult to reach an understanding. Such facts as, for example, the weakening of the acidic properties of benzoic acid under the influence of hydroxyl in the para position still require additional hypotheses.

Therefore, in the present article we shall examine only the concepts of bond strength and migratory capacity, and moreover only as applied to simpler radicals.

Of these two concepts the second cannot be regarded as independent, since for a radical to pass from one place in a molecule to another there must first be a rupture of the bond with that atom with which the given—

\(^1\) On this see:

A. Lapworth, Journ. Chem. Soc. 121, 416—427 (1923).
W. O. Kermack, R. Robinson, Journ. Ch. Soc. 121, 427—440 (1923).
K. Hojendahl, Journ. Ch. Soc. 125, 1381 (1924).

radical was directly attached. Tiffeneau and Orekhov note that here not only the strength of the bond being broken is important, but also the attraction from that part of the molecule to which the radical goes. However, this difference is essentially of the same order as the difference in the action of different reagents. In almost all cases the ease of breaking the bond between two atoms depends both on the strength of this bond and on the affinity of one or both atoms for the reagent acting on the given substance; whether this reagent is outside or inside the molecule is, in principle, immaterial. One can speak of a pure rupture of a bond only in certain dissociation processes, and even then without a solvent.

In organic chemistry one most often has to deal with reactions in which bonds of carbon with carbon, or with atoms of other, usually metalloid, elements are broken. Concerning the mechanism of this rupture from the electronic point of view, one of us¹) expressed—applied to the bond of carbon with a halogen—the following opinion: the bond between C and some halogen, for example Cl, is formed by two electrons situated between the nuclei of both atoms. Since the halogen has a considerably greater effective nuclear charge (a charge partly neutralized by the electrons of the inner shells of the atom), the electrons must be located closer to it, and upon rupture of the C—Cl bond one of two things may occur: either both electrons will go with the chlorine nucleus, or they will separate, one to each nucleus:

\[ \mathrm{R:Cl} \to \mathrm{R^+ + :Cl^-} \quad \text{and} \quad \mathrm{R\cdot \ \cdot Cl} \to \mathrm{R\cdot + Cl\cdot}. \]

In the first case ions are obtained, i.e. the substance decomposes according to the type of hydrohalic acids; in the second—neutral atoms (decomposition according to the type of molecules of free halogens).

How the bond is broken in each given reaction depends both on the nature of the organic radical and on the reagent acting on the halogen compound. Experience shows that organic halogen compounds are indeed divided into two groups, and in exactly the same way the substances that can remove a halogen from these compounds are divided into two groups²). Primary halogen compounds with one halogen atom react more rapidly with salts (most often NaI and KI are used) of alkali metals and with amines. Secondary and tertiary compounds, as well as polyhalogen compounds in general, enter more rapidly into interaction with alkalis and alcoholates. Some reagents, such as AgNO₃, occupy an intermediate position. The indicated regularity is clearly visible, for example, from the following table³), expressing the relative activity of certain halogen derivatives of ethane and propane.

¹) B. V. Tronov. “Activity of halogens in organic compounds as dependent on the electrochemical character of the element with which the halogen is bonded.” Zh. R. Kh. O. 58, 1278—1301 (1926).

²) The same is noted by P. I. Petrenko-Kritchenko and I. I. Molodykh. Proceedings of the 4th Mendeleev Congress (abstracts), p. 142 (1925).

³) Compiled from data in the works:
B. V. Tronov. Zh. R. Kh. O. 58 (1926), 1278—1301.
B. V. Tronov and A. I. Gershevich. Zh. R. Kh. O. 59 (1927), 727—739.

TABLE 7.

Since reagents acting on organic halogen compounds can react both with free halogens and with hydrohalic acids, it is very difficult to predict in advance when neutral cleavage occurs and when ionic cleavage occurs. On the basis of the old conception of the electrochemical character of radicals, one of us\(^1\) put forward the supposition that primary carbon halides should be more capable of ionic cleavage; such electropositive radicals as methyl should more readily split off in the form of positive ions. Hence it followed that in tertiary halogen compounds, on the contrary, neutral rupture of the bond should occur more readily.

However, the application of modern electronic theories leads to a different conclusion. In CH\(_3\)X and —CH\(_2\)X, the carbon bonded to the halogen bears three or two hydrogen atoms, whose nuclei attract electrons comparatively weakly. The electrons of the carbon–hydrogen bonds are therefore situated closer to the nucleus of the carbon atom; but since symmetrically constructed systems of electrons (or electron orbits) are the most stable, the electrons of the other bonds, including the bond with the halogen, must also shift somewhat toward the carbon. As a result, the latter becomes, as it were, more electronegative, and upon elimination of the halogen the neutral cleavage of the molecule is facilitated. Therefore primary hydrocarbon residues, and especially methyl, while electropositive in their action on the more remote parts of the molecule (entering into a more complex group of atoms, they diminish the ability of this group to give a negative ion and strengthen the tendency toward formation of a positive ion, i.e., they weaken the dissociation of acids and strengthen the dissociation of bases), are at the same time comparatively electronegative when it is a question of the elimination of that radical itself.

We have the opposite picture in tertiary carbon halides, especially with the tertiary butyl radical. The hydrogens of the methyl

\(^1\) B. V. Tronov, loc. cit.

groups make the carbon atoms of these groups more electronegative. Owing to the alternating polarity, the neighboring carbon, i.e. the carbon of the tertiary link, should become electropositive, and in general the radical should more readily split off in the form of a positive ion.

The conclusion obtained is precisely the reverse of the first supposition.

Experimental verification is hampered by the circumstance that there are no sharp boundaries between groups of reacting compounds. Most probably, every halogen compound and every substance acting upon it are capable of reacting according to both (and perhaps all three possible) types, and in each given case only one of them predominates. Moreover, it is by no means possible to say in advance that ionic and neutral activity must necessarily change, depending on the structure of the substance, in directly opposite ways. Thus methyl halogen compounds react quite readily with all compounds capable of removing a halogen, whereas halobenzenes enter into interaction only with difficulty with alkalis, amines, and salts. It must be thought that halogens in general are held very firmly by the carbons of the benzene ring and are split off with difficulty both in the form of ions and in the form of neutral atoms.

The same considerations may also be applied to the rupture of bonds in other reactions.

For a more detailed elucidation of the questions arising here, a study was undertaken of the processes of elimination of halogens from the molecules of organic compounds in as numerous and varied examples as possible, and a fairly large number of cases of some reactions of elimination of hydrocarbon and other organic radicals from oxygen in simple and complex ethers were also traced.

In the case of halogen compounds, the rate of interaction with pyridine and piperidine without solvent and with sodium methylate in methyl alcohol was determined. Tables 8 and 9 contain a summary of data¹ relating to monohalogen derivatives of some simplest aliphatic, alicyclic, and aromatic hydrocarbons. To the literature data have also been added the results of measurements of the rate of reaction with potassium iodide in acetone² and some data from the work of Conrad and Brückner³ on the rate of reaction with alcoholates. The activity of compounds with the normal propyl radical has everywhere been taken as unity.

Experiments with halogen derivatives of benzene were carried out at high temperatures (up to 250°) and were reduced to comparison with the remaining data by means of the temperature coefficient found for other compounds. The numbers obtained therefore have only an approximate character. For iodobenzene and pyridine there is also given the result (the number below) of an experiment at ordinary temperature (which lasted more than 4 years).

TABLE 8.

Note to Table No. 8:
1) For \(\mathrm{C_3H_7F}\) there were no measurements. Only \(i\text{-}\mathrm{C_5H_{11}F}\) and \(\mathrm{C_6H_5F}\) were compared, and, in order to reduce to the same unit, the activity of \(i\text{-}\mathrm{C_5H_{11}F}\) was taken as 0,44 times the ratio of the activities—\(\mathrm{C_5H_{11}Cl}\) and \(\mathrm{C_3H_7Cl}\).

TABLE 9.

On the question of the strength of the bond of hydrocarbon radicals with oxygen, the literature contains, in addition to the already mentioned papers of P. I. Shorygin on the action of sodium on simple ethers, a number of other works concerning the decomposition of simple and complex ethers by hydrohalic acids.

For simple ethers, Silva¹) and Lippert²) collected extensive material. They derived certain regularities in the cleavage of radicals, but without establishing quantitative relations. There are a number of works on the hydrolysis of simple ethers and especially acetals under the influence of acids. According to the rate of cleavage of derivatives of the simplest aldehydes and ketones, alcohol radicals are arranged as follows:³)

\[ \mathrm{S{-}C_4H_9 < i{-}C_3H_7 < i{-}C_4H_9 < C_3H_7 < C_4H_9 < C_2H_5 < CH_3.} \]

Liden⁴) used the reaction with acetyl bromide:

\[ \mathrm{CH_3COOBr + ROR' \rightarrow CH_3COOR + R'Br \ and\ CH_3COOR' + RBr} \]

¹) R. D. Silva, Ann. d. ch. et d. phys. [5], 7, 429; Ref. Ber. 8, 1352; 9, 952 [1878]
²) W. Lippert, Lieb. Ann. 276, 148—199 [1893].
³) A. Skrabal, J. Sawink, Zeitsch. phys. Ch. 122, 357 [1926].
⁴) R. Lidén, Chem. Zbl. 1928 II, 2133.

The reaction products were studied. Experiments were also carried out with mixtures of esters, and the author determined which ester reacts more readily. For ethyl isoamyl ether the relative mobility of the radicals was found to be:

\[ \mathrm{C}_5\mathrm{H}_{11} : \mathrm{C}_2\mathrm{H}_5 = 1 : 2.183. \]

For mixtures of ethers, the relative molar reactivity was determined, the value of which depends on all four radicals. Of the general conclusions one may note that, with an increase in the carbon content in the alkyls of aliphatic-aromatic ethers, the activity of these ethers decreases.

Zapper¹ investigated the decomposition of esters of certain organic acids by hydrogen halides. On the question of the mobility of alcohol radicals he came to the same conclusion as the authors of other analogous works: benzyl is split off more readily than the other radicals studied; next come the fatty radicals, and phenyl is not split off at all.

The numerous works on the esterification and hydrolysis of esters give almost no idea of the strength of the bond of hydrocarbon residues with oxygen, since the mechanism of the process here is more complex². With carbonic acids the reaction proceeds almost exclusively as follows:

\[ \begin{array}{ccc} \mathrm{O} & \mathrm{OR'} & \mathrm{OR'} \\[0.4em] \mathrm{R.C{-}OH} + \mathrm{R'O\cdot H} \;\rightleftarrows\; \mathrm{R.C{-}OH} \;\rightleftarrows\; \mathrm{R.C{=}O} + \mathrm{H_2O}, \\[0.4em] & \mathrm{OH} & \end{array} \]

and the dependence of the rate on the nature of the alcohol is often just the reverse of what is obtained with hydrogen halides. Only the hydrogen halide acids give results having a direct bearing on the question of the strength of the carbon–oxygen bond. Using the indicated method, P. I. Petrenko-Kritchenko, V. Bogatskii and N. Lubman³ found that, in the rate of reaction with HCl, tertiary alcohols occupy first place and primary alcohols last.

Ratio of the constants:

\[ \mathrm{CH_3CH_2OH} : \mathrm{CH_3CHOH.CH_3} : (\mathrm{CH_3})_3\mathrm{COH} = 0.0348 : 0.0477 : 0.267 \]

\[ \mathrm{C_6H_5CH_2OH} : (\mathrm{C_6H_5})_2\mathrm{CHOH} : (\mathrm{C_6H_5})_3\mathrm{COH} = 0.0372 : 0.6060 : 2.848. \]

Experiments with aqueous hydrogen halide acids⁴ likewise showed that secondary alcohols react faster than primary ones.

In our laboratory the rate of decomposition of a series of simple and compound ethers by hydrogen bromide in an acetic-acid medium was determined⁵. In order that the reaction could proceed in only one direction,

We took simple ethers with a very large difference in the mobility of the radicals. Such are phenyl ethers. Only for tert-butyl was it necessary to take the ethyl tert-butyl ether, since the phenyl ether cannot be obtained. This, however, is of no great significance, because here too practically only one radical (the tertiary one) is split off.

Of the complex esters it was best to work with the acetates. The experiments were carried out in acetic-acid solution, and with esters of other acids an exchange of acid residues might occur.

The results of the determinations are given in Table 10. The rate constants for the cleavage of phenylpropyl and acetoxypropyl esters were taken as unity. The table also includes some data on the rate of addition of certain esters of mineral acids to pyridine. The decomposition of ethers by hydrogen halides is nevertheless a rather complicated process. According to Hantzsch¹, first a complex addition of the acid hydrogen to the oxygen of the ether takes place, then a rearrangement into the true compound of tetravalent oxygen, and only then is the hydrogen halide split off:

\[ \mathrm{ROR'} + \mathrm{HX} \;\to\; \mathrm{R{-}O{-}R'} \;\to\; \mathrm{R{-}O{-}R'} \;\to\; \mathrm{R{-}O} + \mathrm{R'X} \]
\[ \phantom{\mathrm{ROR'} + \mathrm{HX}\;\to\;}\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;|\qquad\qquad\quad /\!\!\backslash\qquad\qquad\quad \]
\[ \phantom{\mathrm{ROR'} + \mathrm{HX}\;\to\;}\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\mathrm{HX}\qquad\qquad \mathrm{H}\;\;\mathrm{X}\qquad\qquad \mathrm{H} \]

The nature of the radicals \(R\) and \(R'\) naturally affects each stage of the reaction and may affect them differently. The interaction of ethers with tertiary amines proceeds according to a simpler scheme, for example, for nitric-acid esters:

\[ \mathrm{R_3N} \;+\; \mathrm{R{-}ONO_2} \;\to\; \mathrm{R_3N{-}ONO_2}. \]

Therefore, in principle, this method should be recognized as more suitable for determining the strength of the \(R\)–\(O\) bond. In practice, however, we encounter a whole series of experimental difficulties. Only esters of strong acids react at a noticeable rate (such as \(\mathrm{HNO_3}\), \(\mathrm{H_2SO_4}\)), and such esters can by no means be obtained for all alcohols (for example, secondary and tertiary ones cannot be prepared).

The measurement of the reaction rate itself is also rather difficult. Owing to these obstacles, only information on a few radicals can be included in the table.²

TABLE 10.

| Radicals | \multicolumn{4}{c}{Relative reaction rate} |
|---|---:|---:|---:|---:|
| | \(\mathrm{C_6H_5OR + HBr}\) | \(\mathrm{CH_3COOR + HBr}\) | \(\mathrm{R.ONO_2 +}\)
\(+\) pyridine | \(\mathrm{R_2SO_4 +}\)
\(+\) pyridine |
| \(\mathrm{CH_3{-}}\) | 3.23 | 7.22 | 80 | 74 |
| \(\mathrm{CH_3.CH_2{-}}\) | 1.32 | 1.14 | 3 | 1 |
| \(\mathrm{CH_3.CH_2.CH_2{-}}\) | 1 | 1 | — | — |

| RADICALS | \multicolumn{4}{c}{Relative reaction rate} |
|---|---:|---:|---:|---:|
| | $\mathrm{C_2H_5OR + HBr}$ | $\mathrm{CH_3COOR + HBr}$ | $\mathrm{R \cdot ONO_2 +}$ pyridine | $\mathrm{R_2SO_4 +}$ pyridine |
| $\mathrm{(CH_3)_2CH{-}}$ | 11.3 | 1.22 | — | — |
| $\mathrm{CH_3 \cdot CH_2 \cdot CH_2 \cdot CH_2{-}}$ | 0.99 | 0.67 | -- | — |
| $\mathrm{CH_3 \cdot CH_2 \cdot CH \cdot CH_3}$ | 7.39 | 1.11 | — | — |
| $\mathrm{(CH_3)_2 \cdot CH \cdot CH_2{-}}$ | 0.63 | 0.11 | — | — |
| $\mathrm{(CH_3)_3C{-}}$ | $\approx 7 \cdot 10^{1}$ | $\approx 2 \cdot 10^{?}$ | — | — |
| $\mathrm{(CH_3)_2CH \cdot CH_2 \cdot CH_3}$ | 0.78 | 0.56 | 1 | — |
| $\mathrm{(CH_3)_2C \cdot CH_2CH_3}$ | -- | $\approx 3 \cdot 10^{?}$ | -- | -- |
| $\mathrm{CH_3 \cdot CH_2 \cdot CH_2 \cdot CH_2 \cdot CH_2 \cdot CH_2{-}}$ | 0.34 | 0.83 | — | |
| $\mathrm{CH_3 \cdot CH_2 \cdot CH{-}}$ | | | | |
| $\mathrm{CH_3 \cdot CH_2 \cdot CH_2{-}}$ | 3.86 | 0.58 | — | — |
| $\mathrm{CH_2{=}CH \ CH_2{-}}$ | 53.3 | 15.5 | — | — |
| $\mathrm{C_6H_5{-}}$ | 0.01? | 0 | — | |
| $\mathrm{C_6H_5 \cdot CH_2{-}}$ | $2.8 \cdot 10^{?}$ | 230 | -- | — |
| $\mathrm{C_6H_5 \cdot CH_2 \cdot CH_2{-}}$ | — | 0.23 | — | -- |

When comparing Tables 8, 9, and 10, the difference between the two groups of halides, and now also of oxygen-containing compounds, on the one hand, and the two groups of reagents, on the other, becomes still more clearly apparent. The considerations stated above make it more probable that the reactions of primary halogenated hydrocarbons with iodide salts and with amines have a neutral character, whereas under the action of alkalis and alcoholates on tertiary halide compounds ionic decomposition of the latter takes place, the radical being split off in the form of a positive ion.

A particularly good confirmation of this was furnished by the results of one of the recent works—the measurement of the activity of alicyclic halogenated hydrocarbons. In their structure, halocyclohexanes belong to secondary compounds and, as such, in the type of bond cleavage should occupy an intermediate position between primary and tertiary compounds. However, closure of the ring can somewhat affect the distribution of electrons in the molecule. From the standpoint of the theory of alternating polarity, we obtain the following arrangement of positive and negative carbon atoms, for example, in bromocyclohexyl:

\[ \begin{array}{c} \quad \overset{+}{\mathrm{CH_2}}\quad \overset{-}{\mathrm{CH_2}}\\[2mm] \mathrm{H_2C}^{-}\ \ \diagup\qquad\qquad\diagdown\ \ \overset{+}{\mathrm{CHBr}}^{-}\\[2mm] \quad \diagdown\qquad\qquad\diagup\\[-1mm] \quad \overset{+}{\mathrm{CH_2}}\quad \overset{-}{\mathrm{CH_2}} \end{array} \]

the carbon bonded to bromine is not only under the direct influence of the halide, but the influence is transmitted also through the chain (even in two directions) and again reaches this atom with the same sign. The electropositive character of the radical upon cleavage of the halide must therefore be especially strongly expressed. As is seen from the table, the halide derivatives of cyclohexane, in comparison with other secondary halide compounds, react more readily with alkalis and alcoholates, and with greater difficulty with amines and salts, and in their chemical properties they resemble tertiary alkyl halides. Halogen derivatives of alicyclic hydrocarbons with an odd number of ring members, where such a regular alternation \(+\) and \(-\) cannot occur, should be more similar to secondary compounds with an open chain. Experiment shows that this conclusion likewise fully corresponds to reality.¹)

In the cleavage of simple and complex esters by hydrohalic acids, the tertiary radicals are distinguished still more by their mobility. It must be thought that here an ionic reaction especially predominates.

In general, in determining the strength of the bond between organic radicals and halides or oxygen, we obtain very different series depending on the reagent with which we act on the compound being studied. One series corresponds to ionic decomposition, another to neutral decomposition. There is no complete opposition between these series. However, a real opposition in the series of radicals should be expected only in the case when the rupture of the bond is precisely the reverse, i.e., if the radical departs in the form of a negative ion. This can most easily occur when carbon is joined to some metal. Such a case has so far been studied only on the example of mercury compounds, where indeed phenyl proved to be more mobile than aliphatic hydrocarbon residues.

The same rupture of the bond is also possible where a hydrocarbon radical is torn away from a carbon atom. This evidently includes the pinacolone rearrangement. From the electronic point of view the reaction is written, for example, as follows:²)

\[ \begin{aligned} &\begin{array}{c} \mathrm{C_6H_5}\qquad \mathrm{CH_3}\\[-2pt] \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \, |\\[-2pt] \mathrm{C}-\mathrm{C}\\[-2pt] | \qquad |\\[-2pt] \mathrm{C_6H_5}\ \mathrm{OH}\quad \mathrm{OH}\ \mathrm{CH_3}\\[-2pt] \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\[-2pt] \mathrm{H^+}\ \mathrm{N}^{-} \end{array} \;\longrightarrow\; \begin{array}{c} \mathrm{C_6H_5}\quad \mathrm{CH_3}\\[-2pt] \mathrm{C_6H_5}\!:\!\mathrm{C}\quad :\!\mathrm{C}\!:\!\mathrm{CH_3}\\[-2pt] \quad\quad :\!\mathrm{O}\!:\\[-2pt] \quad\quad \mathrm{X} \end{array} \;\cdot\; \begin{array}{c} \mathrm{C_6H_5}\quad \mathrm{CH_3}\\[-2pt] \mathrm{C_6H_5}\!:\!\mathrm{C}\quad :\!\mathrm{C}\!:\!\mathrm{CH_3}\ \longrightarrow\\[-2pt] \quad\quad :\!\mathrm{O}\!:\\[-2pt] \quad\quad +\\[-2pt] \quad\quad \mathrm{H} \end{array} \\[10pt] &\longrightarrow\; \begin{array}{c} \mathrm{C_6H_5}\ \mathrm{CH_3}\\[-2pt] \mathrm{C_6H_5}\!:\!\mathrm{C}\quad :\!\mathrm{C}\!:\!\mathrm{CH_3}\\[-2pt] \quad\quad :\!\mathrm{O}\!: \end{array} \;\cdot\; \begin{array}{c} \mathrm{C_6H_5}\ \mathrm{CH_3}\\[-2pt] \mathrm{C_6H_5}\!:\!\mathrm{C}\quad :\!\mathrm{C}\\[-2pt] \quad\quad \mathrm{CH_3}\quad :\!\mathrm{O}\!: \end{array} \end{aligned} \]

After the elimination of water and of the loosely attached acid—the catalyst—a compound is obtained in which one carbon has around its nucleus only 6 valence electrons, while the other has 8, and

¹) J. Loevenich, H. Utsch, P. Moldrickx, E. Schaefer. Ber. 62, 3084 [1929].

²) Massao Migita. Chem. Zbl. 1929, I, 1218.

Moreover, on the other side of the oxygen there are no positive nuclei. This very unstable system passes into a more stable carbonium one, which can occur only by the migration of one of the radicals together with a pair of electrons, i.e., in the form of a negative ion. It is not surprising that the relative ease of migration of hydrocarbon residues here is, in general, the same as in the decomposition of organometallic compounds.

There are indications in the literature that even rupture of the bond of carbon with oxygen and with halogens can, albeit rarely, proceed in the same direction. Study¹ of the reaction:
$\mathrm{C_6H_5C(OC_2H_5)=NC_5H_4 \rightarrow\!\rightarrow C_5H_6CON(C_6H_5)_2}$ (on heating), with various substituents in the benzene rings, led to such relationships as have to be explained by cleavage of the radical from oxygen in the form of a negative ion. In a series of works² on iodine derivatives of resorcinol, metaphenylenediamine, and mesitylene, the liberation of free iodine by these compounds with hydrochloric acid was established; this indicates that the halogen here is detached from carbon in the form of a positive ion.

Thus, to the two types of bond rupture of organic radicals with atoms of halogens, oxygen, carbon, and other elements indicated in the article “The activity of halides in organic compounds as a function of the electrochemical character of the radical with which the halogen is bound”³—ionic, with detachment of the given radical in the form of a positive ion, and neutral—there must be added a third, also ionic, but in the direction opposite to the first: the radical leaves in the form of a negative ion.

If we take the reactions most characteristic for each of these types: 1) $\mathrm{ROR'}$ or $\mathrm{RCOOR' + Hqal}$, and also $\mathrm{Rqal + ROMe}$; 2) $\mathrm{Rqal + MeJ}$ or an amine; and 3) the pinacol rearrangement or the decomposition of organometallic compounds by acids, then the principal groups of hydrocarbon radicals are arranged, according to ease of detachment in order of decreasing reaction rate, as follows:

1) Tertiary—unsaturated radicals of the allyl and benzyl type—secondary aliphatic, alicyclic, and methyl-primary normal—primary branched—aromatic.

2) Unsaturated of the allyl and benzyl type—methyl—other saturated normal primary—primary with a branched chain—secondary, tertiary, cyclohexyl—aromatic, bound directly to the carbon of the benzene ring.

3) Aromatic—alicyclic and mixed—aliphatic.

As for the relationship between these series, theoretically one should expect an opposite sequence of radicals in the two kinds of ionic decomposition. As far as can be seen from the available, rather scanty material, such a sequence is generally observed for large groups of radicals, but it is difficult to say anything definite about subdivisions within these groups, since negative detachment of the radical has been too little studied. In the neutral type of bond rupture, the groups of aliphatic and aromatic hydrocarbon residues are arranged in the same way as in the positive-ionic type.

The compilation of more detailed series and, in particular, the study of the activity of organometallic compounds and other compounds in which cleavage of the organic radical can occur in the form of a negative ion will not only make it possible to predict better the direction of a given reaction, but will also help to resolve a question of fundamental importance: whether the three types of radical cleavage described above—two ionic and one neutral—cannot be reduced to only two, positive and negative ionic ones.

Electrons (or electronic orbitals) tend to arrange themselves in pairs, and the possibility is not excluded that a pair of moving electrons constitutes such a stable system that in reactions it does not separate, but passes in its entirety with one of the atoms. This would be a return to the iono-electronic theory of A. M. Berkenheim¹ and Ya. I. Mikhailenko,² but with the difference that now it would always be necessary to admit bond rupture in both directions in almost all compounds. It would have to be accepted that, although, for example, the molecule $\mathrm{CH_3Cl}$ decomposes predominantly into the ions $[\mathrm{CH_3}]^{\cdot}$ and $\mathrm{Cl}^{\cdot}$, to a small extent decomposition also proceeds into $[\mathrm{CH_3}]'$ and $\mathrm{Cl}\cdot$.

All the data presented in the present work point fairly clearly to the existence of two types of bond rupture. The third—neutral—has not been proved, since it may turn out that where such decomposition is assumed, some of the molecules in fact rupture according to one ionic type, and some according to the other. The electronic theory of the structure of matter is still insufficiently developed to clarify this question by purely theoretical reasoning and calculations. New experimental material is needed that would make it possible to give at least some quantitative relations between the courses of reactions of opposite types. At present, however, the negatively ionic cleavage of radicals has scarcely been elucidated even qualitatively.

It is very important, for example, to determine the bond strength of tertiary radicals with metals.

1. “Electronic Chemistry of Organic Compounds,” Zh. R. Kh. O., 1917. ↩↩↩↩↩↩↩↩↩↩

2. “Carbon Compounds,” Part I (Tomsk, 1919 and 1923), Part II (Tomsk, 1921). ↩↩↩↩↩↩↩↩

3. B. V. Tronov. Zh. R. Kh. O. 58, 1278–1301 (1926). ↩↩↩↩↩↩

4. J. F. Norris, M. Watt, R. Thomas, Chem. Zbl. 1926, II, 556. ↩↩

5. B. V. Tronov, L. N. Dyakonova-Shul’ts, O. I. Gulyaeva, and N. S. Nikiforova, Zh. R. Kh. O. 59, 545—562.
   B. W. Tronow, L. W. Ladigina: “Über die Haftfestigkeit von organischen Radikalen an Sauerstoff in Äthern.” Ber. 62, 2844—2850 [1929].
   B. W. Tronow, N. Ch. Ssibgatullin: “Über die Haftfestigkeit von Kohlenwasserstoffresten an Sauerstoff in Essigsäure Estern.” Ber. 62, 2850—2856 [1929]. ↩↩

6. Tiffeneau, Orekhoff, loc. cit. p. 297. ↩