Prof. B. V. Tronov

On the Question of the Mechanism of Nitration and Other Substitution Reactions in the Benzene Nucleus

Tomsk

1930

Prof. B. V. Tronov.

On the Question of the Mechanism of Nitration and Other Substitution Reactions in the Benzene Nucleus

The enormous technical importance of the reaction of nitration of organic compounds, especially compounds of the benzene series, has led to the appearance of a large number of works concerning various aspects of this process. Many authors, by means of experimental investigations and theoretical considerations—recently often based on the electron theory—attempt to give one explanation or another of the mechanism of nitration; but it must be said that none of the proposed explanations has yet become generally accepted.

The simple scheme of exchange decomposition:

\[ \mathrm{RH} + \mathrm{HO}\cdot\mathrm{NO_2} \longrightarrow \mathrm{R}\cdot\mathrm{NO_2} + \mathrm{H_2O} \]

has long since been replaced by more complex schemes, according to which the first stage of the substitution process is the formation of an addition product. This idea, in general form, was expressed already by Kekulé. At the present time some investigators think that the nitric acid molecule, having dissociated into \(\mathrm{NO_2}\) and \(\mathrm{HO}\), adds to the substance being nitrated; others consider addition in another order to be more probable: in the molecule of the hydrocarbon or other organic substance the bond between \(\mathrm{C}\) and \(\mathrm{H}\) is broken, and then the radical and hydrogen add to nitric acid.

The first assumption has in recent years been especially advanced by H. Wieland\(^1\). Under the action of \(\mathrm{HNO_3}\) on ethylene, the nitric ester of \(\beta\)-nitroethyl alcohol was isolated; its formation is explained as follows:

\[ \begin{aligned} \mathrm{CH_2} \\ \| \\ \mathrm{CH_2} \end{aligned} \ +\ \mathrm{NO_2\cdot OH} \longrightarrow \begin{aligned} &\mathrm{CH_2\cdot NO_2}\\ &|\\ &\mathrm{CH_2OH} \end{aligned} \ +\ \mathrm{HNO_3} \longrightarrow \begin{aligned} &\mathrm{CH_2\cdot NO_2}\\ &|\\ &\mathrm{CH_2\cdot ONO_2} \end{aligned} \ +\ \mathrm{H_2O}. \]

Investigation of the products of the interaction of \(\alpha,\alpha\)-diphenylethylene with \(\mathrm{HNO_3}\)\(^2\) points to the same reaction mechanism:

\[ (\mathrm{C_6H_5})_2\mathrm{C}=\mathrm{CH_2} \longrightarrow (\mathrm{C_6H_5})_2\mathrm{C(OH)\cdot CH_2NO_2} \longrightarrow (\mathrm{C_6H_5})_2\mathrm{C}=\mathrm{CH\cdot NO_2}. \]

By analogy with this, for benzene and other aromatic compounds it is also assumed that nitric acid first adds to the double bond of the benzene ring, and then a molecule of water is split off:

\[ \mathrm{C_6H_6} + \mathrm{NO_2\cdot OH} \longrightarrow \mathrm{C_6H_6(NO_2)(OH)} \longrightarrow \mathrm{C_6H_5NO_2} + \mathrm{H_2O}. \]

\(^1\) H. Wieland, E. Sakellarios. Ber. 53, 20 [1920].
H. Wieland, F. Rahn. Ber. 54, 1770 [1921].

\(^2\) R. Anschütz, A. Gilbert. Ber. 54, 1857 [1921]; 57, 1697 [1924].

Similarly, other processes analogous to nitration are also written, for example, the sulfonation of benzene compounds. Thus, for the action of aminosulfonic acid on simple phenol ethers¹ we find the equation:

\[ \ce{ C6H5OR + HO.SO2NH2 -> [C6H5(OH)(OR)(SO2NH2)] -> C6H4(OR)(SO3NH4) } \]

At times even the formation of three-membered rings is admitted. For example, the benzylation of the nucleus of aromatic hydrocarbons on heating with the benzyl ether of benzenesulfonic acid is explained by Földi² as follows: first the ester decomposes into the free acid \(\ce{C6H5SO2OH}\) and the divalent radical \(\ce{=CH.C6H5}\), which then opens one of the double bonds of benzene:

\[ \ce{ C6H6 + :CH.C6H5 -> [C6H6(CH.C6H5)] -> C6H5.CH2.C6H5 } \]

However, such an interpretation of all the reactions listed cannot be considered free of shortcomings. First of all, the intermediate products formed here contain a cyclohexadiene system of bonds, and the subsequent, often almost quantitative conversion in one direction (under the conditions of benzene nitration—into nitrobenzene) is rather improbable. This expectation is fully confirmed by the results of the action of nitrogen dioxide on benzene. Ethylenic hydrocarbons and other unsaturated compounds very readily add \(\ce{2NO2}\) at the site of a double or triple bond, for example ethylene:⁴

\[ \ce{ CH2=CH2 + 2NO2\ (or\ N2O4) -> CH2NO2-CH2NO2 } \]

Likewise, isolated⁵ halogen derivatives of ethylene. \(\ce{C2Cl4}\)⁶ gives \(\ce{CCl3NO2}\), \(\ce{CCl2NO2}\), etc. Diallil gives tetranitro derivatives.⁷ In conjugated systems of double bonds (1,4-diphenylbutadiene-1,3), addition to the terminal units has been established⁸ (1,4).

The addition products can split off \(\ce{HNO2}\), forming nitro derivatives of unsaturated hydrocarbons:

\[ \ce{ (C6H5)2C(NO2).CH2(NO2) (+KOH) -> (C6H5)2C=CH.NO2 + HNO2 } \]

Benzene does not react with nitrogen dioxide in the cold, but when heated in a sealed tube to \(80^\circ\) it gives only a little nitrobenzene, and chiefly passes at once into 1,3,5-trinitrobenzene.\(^1\)

\[ \ce{C6H6 + 6NO2 -> C6H6(NO2)6 -> C6H3(NO2)3 + 3HNO2} \]

Thus, as might have been supposed, after the rupture of one double bond the molecule becomes unstable and rapidly adds another \(4\,\ce{NO2}\). It is very probable that the same would also occur with nitric acid, if it really added to the double bond of the ring. Meanwhile, with \(\ce{HNO3}\) at a low temperature, mainly mononitrobenzene is obtained.

This compels one to seek another explanation of the reaction.

It was suggested long ago that in nitration, first of all, an anhydride is formed from two molecules of \(\ce{HNO3}\), and that it is this which produces the nitrating action. This conclusion was reached by Giersbach and Kessler\(^2\) on the basis of measurements of the rate of nitration of benzene by nitric acid in a nitrobenzene solution. The rate proved to be proportional to the square of the acid concentration, i.e. the reaction is bimolecular with respect to the acid.

It is difficult to admit spontaneous dehydration of \(\ce{HNO3}\) with formation of \(\ce{N2O5}\), which avidly absorbs water; but in recent years this scheme has again been advanced by Schaarschmidt\(^3\) for the reaction with a mixture of \(\ce{HNO3}\) and \(\ce{H2SO4}\). He considers that sulfuric acid removes water from nitric acid, while the anhydride adds to the benzene nucleus:

\[ \ce{C6H6 + N2O5 -> C6H6(NO2)(ONO2) -> C6H5NO2 + HNO3} \]

Separate experiments with nitric anhydride\(^4\) showed that pure \(\ce{N2O5}\) is a very strong nitrating agent, which already at \(0^\circ\) almost instantaneously converts \(\ce{C6H6}\) into \(\ce{C6H5NO2}\), and converts nitrobenzene into dinitrobenzene. But further nitration to trinitrobenzene does not occur even at \(76^\circ\). In this theory, therefore, the principal defect of the original conception of the addition of \(\ce{NO2}\) and \(\ce{OH}\) is retained.

In 1923–24 I\(^5\), and also S. S. Nametkin and A. S. Zabrodina\(^6\) in 1925, proposed a scheme according to which the first

\(^1\) H. Wieland, Ber. 54, 1776 (1921).
\(^2\) J. Gierschbach, A. Kessler, Zeit. Phys. Chem. II, 676 [1898].
A. Kessler, Zeit. Phys. Chem. II, 691 [1888].
\(^3\) Schaarschmidt, Zeit. angew. Chem. 39, 1457 [1926].
\(^4\) L. B. Haines, H. Adkins, J. Am. Chem. Soc. 47, 1419–26 [1925].
\(^5\) B. V. Trorov, “Izv. Tomsk. Tekhn. In-ta,” 45, No. 3, pp. 1–8 [1924]; “Proceedings of the 4th Mendeleev Congress,” p. 157. See also: Ya. I. Mikhailenko, “Carbon Compounds,” part I, 2nd ed., p. 109 [1923].
\(^6\) Zh. R. Kh. O. 57, 87–96 [1925].

stage of the nitration process is the addition of the hydrocarbon to nitric acid:

\[ \begin{gathered} \text{benzene} + \mathrm{N{-}O\cdots OH} \;\longrightarrow\; \text{addition product} \;\longrightarrow\; \text{nitrobenzene}+\mathrm{H_2O}. \end{gathered} \]

This interpretation of the reaction is applicable both to aromatic and to fatty hydrocarbons and other compounds. It is in no way contradicted by the results of experiments with ethylene and other unsaturated hydrocarbons. The possibility is not excluded that, for example, ethylene with nitric acid first gives not a nitroethyl alcohol, but simply is nitrated, if only as follows:

\[ \begin{gathered} \mathrm{CH_2{=}CH_2} +\mathrm{N{-}O\cdots OH} \;\longrightarrow\; \mathrm{CH_2{=}CH{-}N(OH){-}OH} \;\longrightarrow\; \mathrm{CH_2{=}CH{-}N(=O)O} +\mathrm{H_2O}. \end{gathered} \]

and only then can a molecule of \(\mathrm{HNO_3}\) add on, with formation of \(\mathrm{CH_2NO_2\cdot CH_2ONO_2}\). Here, it is true, one has to allow the addition of hydrogen to the less hydrogenated unit, and of the acid residue to the \(\mathrm{CH_2}\) group, but this is readily explained by the theory of alternating polarity:

\[ \begin{gathered} \begin{array}{ccc} + & & + \quad -\\ \mathrm{CH_2} & & \mathrm{CH_2(ONO_2)}\\ \big| & +\, - \;+\mathrm{H(ONO_2)}\;\longrightarrow & \big|\\ - \quad + \quad - & & + \quad -\\ \mathrm{CH\cdot NO_2} & & \mathrm{CH_2\cdot NO_2} \end{array} \end{gathered} \]

In general, the latter scheme (with addition of the substance being nitrated to nitric acid, and not the reverse), as far as can be seen, accords better with the facts. But it, like the first, cannot explain why nitric acid in nitration cannot be replaced by esters. Methyl nitrate is even stored and often sold in benzene solution. Ethyl nitrate and isoamyl nitrate stood with me for about three years in sealed tubes with xylene, pseudocumene, and anisole, but even these readily nitrated compounds did not enter into reaction. And yet both cleavage at the point of the bond of OH with nitrogen, and addition of the nitrated molecule to \(\mathrm{HNO_3}\) with rupture of the double bond of nitrogen with oxygen, ought to proceed with approximately equal ease for the free acid and its esters.

Since this is not so, one must think that in the process of nitration some participation is taken by the hydrogen atom of nitric acid. With benzene it can attach only to one of the carbons of the ring. Such an addition—complex and, of course, unstable—appears quite possible, if one accepts...

the Lowry theory of the polarity of double bonds. When applied to Kekulé’s old formula, this theory gives the following distribution of electrons in benzene.

                         H
                         ..
                         C
              ..       .     .       ..
          H : C   .           .   C : H
              ..                   ..
                 C                 C : H
          H  .     .           .. 
                         C
                         .   .
                             H

The nuclei of three carbon atoms are each surrounded by only six valence (outer) electrons; the other three have 8 electrons each, two of them forming a free electron pair that has no positive charge on the other side. To one of such electron pairs, most probably, the nucleus of the hydrogen of nitric acid is attracted:

                         H
                         ..
                       ..C.
                    ..        H : O : N : O :
              H : C      C : H       : O :
                  ..      ..
                 C        C : H
              H .          .
                    ..   .
                       C   : H
                       ..

The formula as written still seems to give nothing for solving the question of the mechanism of nitration. But if one takes into account the results of the observations of Hirschbach and Kessler, which speak in favor of the participation in the reaction not of one but of two molecules of nitric acid, then the following course of the process may be considered probable:

Attracted to a carbon atom of the benzene ring, an HNO₃ molecule makes the hydrogen standing at this carbon more mobile. The latter is detached and attaches itself to the oxygen of a second acid molecule, with rupture of the double bond between N and O:

      CH                 HNO3                         CH        HNO3                         CH
     /  \               . .                          /  \       . .                          /  \
  HC    C—H          O                              HC    C—N=O                              HC    C—NO2
   ||   ||          //                                ||   |                                   |    ||
  HC    CH     + N=O  ———→                         HC    CH                                  HC    CH + H2O + HNO3
     \  /          \                                  \  /       \   \                         \  /
      CH            OH                                 CH        OH  OH                         CH

Important confirmation of this interpretation of the reaction mechanism may be found in the fact that ethyl nitrate vigorously nitrates benzene and toluene in the presence of aluminum chloride¹). In AlCl₃ the Al atom is capable of complex formation and naturally can play the role of the hydrogen of the first

¹) E. Boedtker, Bl. [4] 3, 726 [1908]; Chem. Zbl. 1908. II, 403.

molecules of $\mathrm{HNO_3}$. The second molecule reacts by means of the double bond that is present and retains its activity also in esters. The scheme for the nitration of benzene will here be as follows:

\[ \mathrm{ C_6H_6 + AlCl_3 \longrightarrow C_6H_6\!\cdot\!AlCl_3 + N(=O)(O)(OC_2H_5) \longrightarrow C_6H_6{-}N(OH)(OC_2H_5){=}O\!\cdot\!AlCl_3 \longrightarrow C_6H_5NO_2 + HOC_2H_5 + AlCl_3 . } \]

It is likewise possible that nitration proceeds with a mixture of $\mathrm{HNO_3}$ and $\mathrm{H_2SO_4}$. Sulfuric acid can replace the first molecule of $\mathrm{HNO_3}$ or $\mathrm{AlCl_3}$.

Without entering into a discussion of the numerous theories concerning the question of the regularities of substitution in the benzene ring, let us consider what consequence, in this question, follows from the scheme of the nitration reaction proposed here. For this purpose we shall again use Lowry’s theory and write the electronic formulas, for example, of chlorobenzene, toluene, phenol, nitrobenzene, benzonitrile, and benzoic acid.

If at one of the carbon atoms of the benzene ring there is $\mathrm{Cl}$, then the 8 electrons surrounding its nucleus can prevent the carbon from having an extra pair of electrons of the double bond. This carbon, and also (by virtue of the transmitted induced polarity) both carbon atoms in the meta positions, will have only 6 valence electrons each, while around the nuclei of the ortho and para carbons there must be located three free electron pairs, i.e. in all there will be 8 electrons each. In toluene the nucleus of the C atom of the side chain is again surrounded by eight electrons, and the same arrangement of electrons in the ring is to be expected. The same is true for phenol.

\[ \begin{array}{ccc} \begin{array}{c} \ddot{\ }\\[-0.4em] :\mathrm{Cl}:\\[-0.2em] \dot{\ }\\[-0.2em] \mathrm{C}\\ \cdots\quad\cdots\\ \mathrm{H:C}\quad\quad\mathrm{C:H}\\ \mathrm{H:C}\quad\quad\mathrm{C:H}\\ \cdots\quad\cdots\\ \mathrm{C}\\ \mathrm{H} \end{array} & \begin{array}{c} \mathrm{H}\\[-0.2em] \ddot{\ }\\[-0.2em] \mathrm{H:C:H}\\[-0.2em] \ddot{\ }\\[-0.2em] \mathrm{C}\\ \cdots\quad\cdots\\ \mathrm{H:C}\quad\quad\mathrm{C:H}\\ \mathrm{H:C}\quad\quad\mathrm{C:H}\\ \cdots\quad\cdots\\ \mathrm{C}\\ \mathrm{H} \end{array} & \begin{array}{c} \mathrm{H}\\[-0.2em] :\mathrm{O}:\\[-0.2em] \ddot{\ }\\[-0.2em] \mathrm{C}\\ \cdots\quad\cdots\\ \mathrm{H:C}\quad\quad\mathrm{C:H}\\ \mathrm{H:C}\quad\quad\mathrm{C:H}\\ \cdots\quad\cdots\\ \mathrm{C}\\ \mathrm{H} \end{array} \end{array} \]

It is clear that in all these compounds the complex bond with the hydrogen of nitric acid is possible only in the ortho and para positions, and consequently the ortho and para positions must be nitrated.

In \(C_6H_5 \cdot NO_2\), \(C_6H_5 \cdot CN\), \(C_6H_5 \cdot COOH\), directly adjacent to the carbon of the benzene nucleus there is an atom \(N\) or \(C\) joined by a double or triple bond to the atom (or atoms) of a more electronegative element. The free pairs of electrons of these bonds are here taken up by \(O\) (in \(NO_2\) and \(COOH\)) or \(N\) (in \(CN\)), while \(N\) in \(NO_2\), or \(C\) in \(CN\) and \(COOH\), remains with an incomplete number of electrons—6 (or 4). The nearest carbon of the benzene ring must, as far as possible, make up this deficiency and draws to itself a pair of electrons in the ring. In general we obtain the following electronic structures:

        :O:                         :O:
         N                           C
:O:      :   C        :N:C:C:    H:O:   :C:
      H:C     C:H      H:C   C:H      H:C     C:H
      H:C     C:H      H:C   C:H      H:C     C:H
         C              C              C
         H              H              H

Complex addition of hydrogen, nitric acid, aluminum, etc. must take place at the meta-carbon atoms; and the nitro group is directed there as well.

If one takes more complete series of substituents directing into the ortho-para or into the meta-position¹:

\(o - p:\) \(Cl,\ Br,\ J,\ OH,\ OR,\ SR,\ NH_2,\ N(CH_3)_2,\ NH \cdot CO \cdot CH_3,\ NH \cdot CO \cdot C_6P_5,\ NH \cdot CO \cdot NH_2,\ NH \cdot NO_2,\ -N{=}N-,\ --N--N--,\ CH_3,\ CH_2R,\ C_6H_5,\)

\[ \begin{gathered} \quad\quad\quad\quad\quad\quad\quad\quad O \end{gathered} \]

\(CH_2Cl,\ CH_2 \cdot CN,\ CH_2 \cdot OR,\ CH_2 \cdot COOH,\ CH_2 \cdot CH_2COOH,\ CH_2 \cdot CHOH \cdot COOH,\)

\(CH_2 \cdot CH(NH_2) \cdot COOH,\ CH_2 \cdot NH \cdot CO \cdot CH_3,\ O \cdot PO(OH)_2;\)

\(m:\) \(SO_2,\ SO_3H,\ NO_2,\ NH_3 \cdot O \cdot SO_3H,\ NH_3 \cdot O \cdot NO_2,\ CN,\ CCl_3,\)

\(CF_3,\ CHO,\ COOH,\ COR,\ CO \cdot CH_2Br,\ CH_2 \cdot NO_2,\ CH(NH_2) \cdot COOH,\)

\(CO \cdot NH \cdot CH_2 \cdot COOH,\)

then, almost without exception, in the meta-directing groups, at the atom directly adjacent to the benzene ring, the nucleus is surrounded by an incomplete number of electrons. The following deviate: \(SO_2,\ SO_3H,\ CCl_3,\ CF_3,\ NH_2NO_2,\ CH(CH_2) \cdot COOH,\ NH_3 \cdot O \cdot NO_2\), etc. In the first four substituents it may be significant that sulfur and carbon are connected for the most part with three (only in \(SO_2\), with two) atoms of very electronegative elements, whose nuclei apparently strongly draw electrons to themselves and make the neighboring atomic nuclei little saturated. \(CH_2 \cdot NO_2\) may function in the tautomeric form \(CH{=}NOOH\). For salts of amines it has been experimentally proved that into the meta-position

is oriented by the ionized group $\mathrm{NH_3}$. The excess positive charge of this group, of course, must attract to the neighboring carbon a free pair of electrons.

\[ \left[ \begin{array}{c} \mathrm{H}\,\cdot\,\,\cdot\,\mathrm{H}\\ \mathrm{N}\\ \mathrm{H}\,\cdot\,\,\cdot\,\,\cdot\\ \mathrm{C}:\!\\ \cdot\,\,\cdot\\ \mathrm{H:C}\qquad\qquad \mathrm{C:H}\\ \mathrm{H:C}\qquad\qquad \mathrm{C:H}\\ \cdot\,\,\cdot\mathrm{C}\cdot\,\,\cdot\\ :\!\\ \mathrm{H} \end{array} \right]^+ \]

The addition of other salts suppresses dissociation and decreases the yield of the meta nitro product. This was observed, for example, in the nitration of dimethylbenzylamine in a sulfuric-acid medium with the addition of ammonium sulfate¹. The yield of $m$-nitro-dimethylbenzylamine in concentrated solution was 44%; upon dilution it rose almost to 52%, while in the presence of $(\mathrm{NH_4})_2\mathrm{SO_4}$ it fell to 34%.

Nitration of 1-benzylpiperidine² gave 72% of the meta compound, with $\mathrm{NH_4NO_3}$—56%, and with the more readily soluble $\mathrm{NH(CH_3)_3NO_3}$—even 16%.

Very characteristic figures are found in the work of Ingold and co-workers. Nitration, under identical conditions, of a series of salts of aromatic amines differing from one another in the degree of dissociation and in the distance of the nitrogen atom from the benzene ring gave the following quantities of meta nitro derivatives³:

In the nitration of compounds of N, P, As, Sb the following results were obtained⁴:

Everywhere, factors that increase the degree of electrolytic dissociation of the salt simultaneously also increase the yield of the meta-nitro compound. The approach of the charge of the ammonium (etc.) ion to the benzene ring has the same effect. Of course, cases are possible in which different influences counteract one another [compare \(C_6H_5 \cdot N(CH_3)_3X\) and \(C_6H_5 \cdot CH_2 \cdot N(CH_3)_3X\)].

The diazonium group, owing to the strong dissociation of diazonium salts, having a positive charge and, in addition, a triple bond between the nitrogen atoms, must direct \(NO_2\) (and others) especially energetically into the meta position. And indeed, there is an indication¹ that \(-N(X)\equiv N\) surpasses all other substituents in this respect.

Ortho-para-directing atoms and groups in all compounds for which a quite definite formula can be written are adjacent to the benzene nucleus with a full octet of electrons. Sometimes there are double and even triple bonds here, as in the molecules \(C_6H_5 \cdot N=N \cdot C_6H_5\), \(C_6H_5 \cdot CH=CH \cdot COOH\)², \(C_6H_5-C-C \cdot COOH\)² (on nitration gives only \(6\%\) meta-substitution, whereas \(C_6H_5-C\equiv N\)—\(81\%\)), but always between identical atoms, so that an arrangement is also possible in which the excess electrons will be at the atom nearest to the benzene ring.

It is quite natural that ortho-para substitution is greatly favored by a free negative charge, which must repel electrons from the atoms nearest to it. Experiments on the nitration of compounds containing the group \(CH_2 \cdot SO_2R\)³ showed that, indeed, the greatest yield of para and ortho products (and the smallest of meta) is obtained when \(R=OH\). Even replacement of the hydroxyl hydrogen by methyl raises the amount of the metanitro derivative formed from \(13.7\%\) to \(32.4\%\).

The series of substituents given above have only approximate significance. Almost always, nitration and other reactions lead to a mixture of all three isomers: ortho, meta, and para, and only one of the substitution types predominates. In some cases the principal direction of the process also depends on the reagent with which we act upon a given benzene compound. Thus \(C_6H_5 \cdot CHCl_2\) and \(C_6H_5 \cdot CCl_3\) on nitration give chiefly the meta product, but on chlorination the para product.

All this can quite well be explained by the fact that the particular distribution of electrons in the molecule, determined by the substituent, is likewise only the predominant one in each individual case. There are indications even of the existence of electronic isomers. Thus, for metanitrotoluene two forms have been established, to which the following structures are assigned⁴ (shown schematically):

\[ \begin{array}{ccc} \begin{array}{c} \stackrel{+}{(CH_3)}\\[-2pt] -\!\!|\\[-2pt] C\\[-2pt] {}^{+}HC\!\!\diagup\!\!\!\diagup \quad \diagdown\!\!\!\diagdown CH^{+}\\ | \qquad\qquad |\\ {}^{-}HC \qquad\quad C(NO_2)\\ \quad \diagdown \qquad \diagup\\ {}^{+}CH \end{array} & \text{and} & \begin{array}{c} \stackrel{-}{(CH_3)}\\[-2pt] +\!\!|\\[-2pt] C\\[-2pt] {}^{-}HC\!\!\diagup\!\!\!\diagup \quad \diagdown\!\!\!\diagdown CH^{-}\\ | \qquad\qquad |\\ {}^{+}HC \qquad\quad C(NO_2)^{+-}\\ \quad \diagdown \qquad \diagup\\ {}^{-}CH \end{array} \end{array} \]

In the first, the charges are arranged according to the nitro group (i.e., consideration is given to how the electrons must be situated in it, and then the construction is carried through to \(\mathrm{CH_3}\) according to the principle of alternation \(+\) and \(-\)); in the second, according to the methyl group. Since \(\mathrm{NO_2}\) is the more strongly orienting substituent, it is natural that the first isomer should be more stable.

In other compounds the same kind of isomerism is possible, but the isomers, apparently, almost always very readily pass into one another. The final result of one process or another is determined by the relative content of these isomers and by the rate of interaction of each of them with the reagent taken.

I shall say nothing here about the rate of nitration of various compounds. This rate, for any substituent present in the benzene nucleus, must also depend on the electronic structure of the molecule and, consequently, may be connected in a definite way with the directing action of the substituent. But, evidently, the dependence is too complex. Even the rather general rule, at first sight, that ortho–para directing groups accelerate nitration and meta-directing groups retard it is by no means always justified\(^1\). It is well applicable only to certain strongly orienting groups: \(\mathrm{OH}\), \(\mathrm{NH_2}\), \(\mathrm{NO_2}\), \(\mathrm{CN}\), and some others.

The considerations developed in this work may also be applied to the explanation of other substitution processes in the benzene series. Sulfonation proceeds, in all probability, quite analogously to nitration. The acceleration of the action of halogens in the presence of \(\mathrm{AlCl_3}\) is readily explained:

\[ \begin{array}{c} \chemfig{*6(=CH-CH=CH-CH=CH-HC)} \;+\;\mathrm{AlCl_3} \;\longrightarrow\; \chemfig{*6(=CH-C(-[::45]\mathrm{AlCl_3})-H-CH=CH-CH-HC)} \;+\;\mathrm{Br{-}Br} \;\longrightarrow\; \chemfig{*6(=CH-CBr=CH-CH=CH-HC)} \;+\;\mathrm{HBr}+\mathrm{AlCl_3} \end{array} \]

and so forth.

\(^1\) As is shown, for example, by the results of measurements of the rate of nitration of benzene and its homologues and derivatives in a nitrobenzene medium (B. V. Tronov and G. Ya. Ber).

1. H. A. Schoutissen, R. T. ch. P. B. 40, 763—74; Chem. Zbl. 1928. I, 804. ↩↩↩↩

2. J. W. Baker, K. E. Cooper, Ch. K. Ingold. Chem. Zbl. 1928. I, 2603. ↩↩↩↩

3. Ch. K. Ingold, E. H. Ingold, F. R. Shaw. J. ch. S. Lond. 1927, 813—32. ↩↩

4. R. H. Clarke, R. N. Crozier. Chem. Zbl. 1926. II, 20. ↩↩↩

5. F. Guthrie. A. 116, 247 [1860]; 119, 83 [1861]. ↩

6. H. Biltz. Ber. 35, 1528 [1902]. ↩

7. L. Henry. Ber. 2, 279 [1869]. ↩

8. H. Wieland, H. Stenzl. Ber. 40, 4828 [1907]. ↩

9. H. Wieland, F. Rahn. Ber. 54, 1770 [1921]. ↩