B. V. Tronov and E. A. Krüger.

Determination of the Relative Activity of Halides in Organic Compounds of the Cyclic Series

From the Laboratory of Organic Chemistry of the Siberian Technological Institute.

While working, still in the 1925–26 academic year, on the determination of the relative activity of halides in the benzene series by means of piperidine, and having obtained definite numerical values which already at that time made it possible to draw certain conclusions regarding the strength of the bond of a halide with the carbon of the benzene ring, we wished to supplement the data obtained by determining the activity of as large a number as possible of halogen derivatives of benzene and its derivatives.

The following compounds were taken for investigation:

\(C_6H_5Cl\); \(C_6H_5Br\); \(C_6H_5J\); \(o\text{-}C_6H_4\text{-}Cl_2\); \(p\text{-}C_6H_4\text{-}cl_2\); \(C_6Cl_6\); \(o\text{-}cl\text{-}C_6H_4\text{-}CH_3\); \(m\text{-}cl\text{-}C_6H_4\text{-}CH_3\); \(p\text{-}cl\text{-}C_6H_4\text{-}CH_3\); \(C_6H_6cl\); \(o\text{-}Br\text{-}C_6H_4\text{-}CH_3\); \(p\text{-}Br\text{-}C_6H_4\text{-}CH_3\); \(\alpha\text{-}Br\text{-}C_{10}\text{-}H_7\); \(C_7H_{13}Cl\); \(o\text{-}cl\text{-}C_6H_4\text{-}(OH)\); \(m\text{-}cl\text{-}C_6H_4(OH)\); \(p\text{-}cl\text{-}C_6H_4(OH)\); \(C_{10}H_{15}\text{-}BrO\); \(o\text{-}cl\text{-}C_6H_4\text{-}NO_2\); \(m\text{-}cl\text{-}C_6H_4NO_2\); \(p\text{-}cl\text{-}C_6H_4\text{-}NO_2\); \(C_6H_2(OH)Br_3\); \(m\text{-}Br\text{-}C_6H_4\text{-}NO_2\); \(p\text{-}Br\text{-}C_6H_4\text{-}NO_2\); \(p\text{-}Br\text{-}C_6H_4\text{-}O\text{-}CH_3\); \(C_{14}H_8Br_2\); \(o\text{-}cl\text{-}C_6H_4\text{-}NH_2\); \(m\text{-}cl\text{-}C_6H_4\text{-}NH_2\); \(p\text{-}Cl\text{-}C_6H_4\text{-}NH_2\); \(m\text{-}Br\text{-}C_6H_4\text{-}NH_2\); \(p\text{-}Br\text{-}C_6H_4\text{-}NH_2\).

To determine the activity of the halide, the substances studied were heated with an equimolecular quantity of pyridine. The reaction was carried out without a solvent, since every solvent, as is known, sometimes has a very great influence on the rate of addition of halogen compounds to amines.¹

Experimental arrangement: All the substances tested were checked for purity by boiling point and melting point and gave satisfactory results. Since halides in the benzene ring are in general exceedingly little mobile, all experiments were conducted in iron furnaces with gas heating at \(t = 250\text{--}255^\circ C\), in sealed glass tubes.

In setting up each experiment, the halogen-organic compound under test and pyridine were accurately weighed in equimolecular proportions into a test tube fitted into a refractory glass tube, which was immediately sealed.

In the case of liquid halogenated hydrocarbons, an equimolecular mixture with pyridine was prepared at once for all the experiments, and, after thorough mixing in a cylinder, portions of the prepared mixture were weighed out for each experiment.

Owing to the negligible rate of reaction at room temperature, the time during which the mixture stood at ordinary \(t\) (before and after heating in the furnaces) was not taken into account.

After heating for a sufficient time (as indicated by the first trial experiments), each tube was carefully opened,

its contents were carefully collected in a small beaker; the residues of the mixture in the tube were washed several times, first with alcohol and then with hot water, which were likewise poured into the beaker.

Most of the experiments gave considerable resinification of the products, or a deposit of carbon on the walls of the tubes, which had to be filtered off and washed; and, after heating the filtrate thus obtained, the halide was precipitated in the dark with a solution of $\mathrm{AgNO_3}$, and the mixture was left for 10–12 hours. The subsequent determination of the halide (in the form of AgGd) was always carried out gravimetrically. However, owing to the strong resinification of the products and to the admixture of organic matter with the AgGd (despite the most thorough washing), the accuracy of some determinations is very greatly reduced. In addition, it should be noted that in almost all experiments the precipitate of silver halide (AgGd) was repeatedly treated with fuming $\mathrm{HNO_3}$ and HGd, with very cautious ignition.

The results of the experiments are given in the following table:

Table No. 1.

The relative activity of the halides in the various compounds was estimated approximately from the intervals of time in which an equal percentage of halide reacted. In doing so, the calculations were everywhere carried out for one atom of halide (for example, in C₆Cl₆ the percentage found is divided by 6). This, of course, is not exact, since it is possible that in one and the same molecule several atoms may react rapidly one after another. Therefore the numbers obtained by us for polyhalide compounds do not express the actual rate of cleavage of one halide atom, but are average values. Yet they may exceed the true activity by no more than 6 times (in the case of C₆Cl₆, C₆H₆Cl₆, etc.); and in this entirely unstudied field, where, moreover, individual compounds sometimes differ in activity by tens of thousands of times, it is important to know at least the order of magnitude.

In the calculation, the activity of chlorobenzene, as the smallest, was taken as unity. But in view of the extreme position of C₆H₅Cl, comparison of it with the most active compounds would have given a large error. Therefore, in practice the comparison was first made with o—Cl—C₆H₄(OH), which occupies an intermediate position, and only afterward were the values obtained recalculated to chlorobenzene (see Table 2).

Table II.

Average relative activity of halides in organic compounds of the cyclic series.

The results obtained make it possible to draw the following most essential conclusions:

Table III.

As is evident from Table III, in the benzene series the difference in the activity of the various haloids is considerably smaller than in compounds with an open chain of carbon atoms. This is fully consistent with what might have been expected for more electronegative radicals.

Table IV.

Influence of the number of haloid atoms in the molecule on their activity.

With an increase in the number of haloid atoms in the benzene ring, their activity increases. This again is fully explained by the fact that the character of the radical, upon the entry of each haloid atom into the molecule, becomes increasingly electronegative.

Table V.

Influence of substituent groups in the benzene ring on the activity of the haloid.

This table (V) leads to the unexpected conclusion that all substituents, irrespective of their electrochemical character, loosen the bond of the haloid with the carbon of the benzene ring. Even the methyl group (\(CH_3\)) increases the activity of chlorine and bromine several-fold (6–22 times).

Of interest is the great difference in the influence of hydroxyl \((OH)\) and the \((NO_2)\) nitro group on the activity of the haloid in the isomers: hydroxyl \((OH)\) increases the activity of chlorine most strongly in the p-position, while the m-isomer occupies the middle place. The \((NO_2)\)-group increases the activity of the haloid comparatively little in the m-position (57-fold for Cl and 210-fold for Br), and very strongly (approximately another 100-fold) in the o- and p-positions.

Table VI.

Influence of the structure of the radical on the activity of the haloid in the molecule.

From the last table (VI) it is evident that in all cyclic systems of saturated character the haloid is held much more weakly in comparison with the benzene ring; for example, \((CH_3—C_6H_{10}—Cl)\) is approximately 3000 times more active than in toluenes.

The lesser strength of the benzene rings of naphthalene is also reflected in the fact that the haloid in the \(\alpha\)-position is 5–6 times more active than in benzene.

Vertical axis: %

[Graph: vertical axis, \(\%\); horizontal axis, hours.]

Explanatory Table

1. B. V. Tronov. Zh. R. F. Kh. O., chemical section, vol. LX, issue 1, 171–179.
   N. A. Menshutkin. Zh. R. F. Kh. O. 22, 395, 396 (1890).
   G. Mukhin and Krivobabko, Zh. R. F. Kh. O. 55, 211 (1923). ↩