1I 84
GEORGES. HAMMOND AND KATIIARINE J. DOUGLAS ICONTRIBUTION FROM THE CHEMICAL
Aromatic Nitration.
LABORATORIES OF IOWA
Vol. s1
S T A T E COLLEGE]
111. The ortho to para Ratio in Nitration of Benzonitrile
BY GEORGES. H A M M O NAND D ~KATHARINE J. D O U G L A S ~ RECEIVED SEPTEMBER 19, 1958 The nitration of benzonitrile by nitric acid in 7270 perchloric acid gives approximately 80% m-nitrobenzonitrile. The oand p-isomers are also produced in the ratio 3 3 t o 1. Since the linear cyano group can provide no steric assistance to onitration, i t is concluded that the high value of the ortholpara ratio reflects the tendency of the unsaturated substituent t o continue a weak electron withdrawal from the ring even in the transition states involved in electrophilic substitution reac~ ~ ' casts ~ ~ ~doubt ~ on thp significance ( I f special tions This is in accord with the earlier suggestions of other a u t t ~ o r s and steric effects
The study of the ortho to para ratio obtained on nitration of various substituted benzenes has led to the conclusion by Dewar3 that the high values of the orfho/para ratio found among the by-products in the nitration of monosubstituted benzenes bearing the so-called m-directing groups could be explained by the "naive" LChO molecular orbital approximation if the selection of the parameters were sufficiently judicious. His results indicated that only in cases where the substituent is very electronegative would the p-position be electronically less deactivated than the o-position. His results are in qualitative agreement with expcriment, but the appropriate electronegativity pararreters for the various substituents are very uncertain. Brown4 has shown that when the substituent is a conjugated hydrocarbon system, the uncertainty about the electronegativity parameter disappears and the molecular orbital approximation provides an unequivocal prediction of the relative reactivities a t the various nuclear positions. He, as well as Dewar, attributes the observed superior reactivity of the p-position in biphenyl to steric effects rather than to failure in the h 1 0 theory. Hammond, ;\Iodic and Hedges? froiI1 their study of the ortho to para ratios obtained in the nitration of 2,5-dibron:o- and 2,klichloronitrobenzene have suggested a mechanism by which ortho direction by non-linear, unsaturated substituents could be facilitated. According to this nJechanism, the negative end of the dipole in a group such as XOZ, COOH, CHO, etc.. attracts the o-hydrogen and facilitates the approach of the electrophilic attacking group so that the proton is displaced in one concerted process arid appears first on the neighboring group. The picture was similar to that suggested earlier by Eapworth and Robinson6 to account for ortho activation. I11 order to determine if this steric influence is real, we have undertaken the study of the isomer ratio obtained on nitration of benzonitrile. Previous work' showed that the m-isomer was the main product and that the chief by-product was onitrobenzonitrile. However, the analytical method ( I ) Division of Chemistry, California Institute of Technology, Paqadena, Calif. (2) Department of Chemistry, Kewcomb College, S e w Orleans, La. ( 3 ) >I, J. S. Dewar, J . Chem. SOC.,403 (1919). (4) R. D. Brown, THISJ O U R N A L , 7 5 , 4077 (1953). ( 3 ) G. S. Hammond, IT. J. Modic and R. M. Hedges, ibid., 75, 1388 ( 1 95:+) , (f;) A. Lapworth and R. Robinson, .Ifem. Proc. MancAesfer Lir. arid phii. sot.. 78,4 3 (1928). (7) J. W. Baker, E(. E. Cooper and C. I
