A Demonstration FRANK I. LAMBERT1 Occidental College, Los Angeles, California
Exmm for the halogens, substituents on the benzene ring which are ortho-, para-orienting increase the rate of substitution in the ring." This statement is made in every course in organic chemistry and is usually supported by many examples on the blackboard and by kinetic data on slides or in textbook illustrations. No lecture demonstration appears to have been published t o illustrate this important area of aromatic chemistry. To be sure, we have frequently shown the mixing of bromine with benzene in the classroom as an illustration of the non-existence of simple isolated double bonds in benzene because the bromine color remains unaltered. However, the subsequent step of placing the mixture in the sunlight and observing the decolorization probably leaves too great an impression on the student's ' National Science Foundation Science Faculty Fellow, California Institute of Teohnology, 1957. Contribution No. 2279 from the Gates and Crellin Laboratories, Pasadena 4, California.
mind, if lecture experiments involving substitution in the ring are not also shown. Addition to the ring, which occurs with bromine and benzene in sunlight, could certainly be classified as atypical in aromatic .. chemistry. Several visually helpful reactions in the field of aromatic substitution were noted durine an investieation of chlorination of hydrocarbons when methylene chloride was used as the solvent. The most interesting effect, activation by several methyl groups on the ring, is not rapid enough to be visually useful in bromination in petroleum ether, methylene chloride, carbon tetrachloride, or acetic acid, or chlorination in carbon tetrachloride. Chlorine in acetic acid is somewhat too reactive for good differentiation. Lecture demonstrations should include the obvious and the more subtle. Such variations are easily possible with a chlorine test reagent prepared by passing chlorine from a lecture cylin-
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der into methylene chloride until a yellow solution is obtained. Test tubes of a size large enough to be visible in the lecture room may then be partially filled with the test solution placed in plastic or other non-obscuring rack, and dropping bottles of the aromatic compounds lined up behind them. Admittedly, the discharge of color of a chlorine solution is not dramatically visible for a very large class, but the effects described below should be quite apparent to a group of fifty-especially if a large white card is placed behind the test tube rack. Differencei n Activation between -R, -X, and-C02H Groups and -OH, -OR, and -NR2 Groups. An amount (dependent upon the size of the test tubes and the concentratiou of the chlorine test reagent) of benzene, toluene, chlorobenzene, phenol (in CHGlz solution), anisole, N,N-dimethylaniline, and benzoic acid (in CH2C12) is added to the respective test tube in front of the reagent bottle. Immediate decolorization is observed in the tubes containing the phenol, anisole, and dimethylaniline, because of the very great activation of the ring by electron donating groups in these compounds. (Aniline will form brown oxidized products.) Effect of Catalyst. The preceding experiment indicated the groups which are powerfully activating. By shaking an equal volume of concentrated sulfuric acid with chlorine-methylene chloride solutions containing a small amount of benzene, toluene, and o- or p-xylene the instructor can show that the methyl group is a weakly activating substituent: the top layer of the mixture slowly decolorizes in the case of toluene, whereas no reaction occurs with benzene. Glass-stoppered bottles or cylindrical separatory funnels should be used. Caution should be exercised on releasing pressure to avoid sulfuric acid spatters because of the heat of the reaction and the low boiling point of methylene chloride. The more rapid decolorization of the top layer in the o-xylene or p-xylene bottle is indicative of the inductive and resonance effects of the additional methyl group. Reinforcement of Activatia by Weakly Activating Groups. Ortho-, meta- and para-xylene, t-butylbenzene, mesitylene, and m-dichlorobenzene are added to test tubes ccntaining the chlorine test solution. Immediate decolorization occurs with mesitylene and rapid loss of color takes place with m-xylene. (m-Xylene requires from two to six minutes in the concentrations we have used.) No change is observed with the other compounds. Thus, only when two or more methyl groups are activating the same position does quick reaction take place. The general inductive effect of one more methyl group than toluene, as is the case in oand p-xylene, is much less effective than the resonance combined with the inductive effect shown in m-xylene. m-Dichlorobenzene is included in this series to prove to the thorough doubter that the mere location of
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1958
groups meta t o each other on the ring is not the cause of easy substitution. Tertiary butylbenzene is used to show that multiple methyl groups do not exert their effect through a saturated carbon (cf. mesitylene with the same number of methyl groups in the molecule). 1,2,4-Trimethylbenzene, 1,2,3-trimethylbenzene and more highly methylated benzenes react readily in this test, of course. Deactivating Effect of the Halogens and the Nitro Group. 4-Chloro-1,3dimethylbenzene (prepared from 2,4dimethylaniline by the diazotization process) is added to the chlorine solution. No decolorization occurs as compared with the quick reaction of chlorine with unsubstituted m-xylene. Here the deactivating influence of the halogens is evident. However, in bromomesitylene decolorization can be observed over a five to fifteen minute period; three methyl grlups are able to overcome the deactivation by the bromine. The more powerful deactivating influence of the nitro group can be shown by testing nitromesitylene (colorless in CH,C12 solution) with the chlorine reagent. No decolorization occurs in the class hour. The same effects of halogen and nitro groups can be shown in more readily available compounds: anisole (immediate decolorization), p-chloroanisole (five to ten minutes for decolorization), . . and p-nitroanisole (no decolorization). Thus, in a series of simple experiments it can be visually demonstrated that -OH,-OR,and -NR2 powerfully activate the benzene ring. Methyl groups activate to a much lower degree; a catalyst was necessary to chlorinate toluene. Such activation effectscan be seen to be cumulative especially when two or more methyl groups are meta to each other. (The application to qualitative analysis in distinguishing between m-xylene and o- and p-xylene is obvious, albeit of minor interest here.) I n addition, by this simple technique the halogens can be shown to be deactivating and the nitro group even more powerfully deactivating toward substitution of chlorine in the benzene ring. NOTES: The experiments are visually more effective if a concentrated solution of chlorine in methylene chloride is used, but there are some difficulties when this is done. Several milliliters of each reagent must be used; rather large amounts of hydrogen chloride are boiled out of the anisole and phenol test solutions. As with all lecture demonstrations, the procedure described should be checked prior to the lecture. The chlorine-methylene chloride sdution is stable for a t least a week in the dark but reacts to yield polychloromethanes in sunlight. The experiments should not be run in sunshine because of the possibility of non-uniform exposure of the tubes to radiation and consequent confusing results.
