Benzene ring steric effect

The Hammett equation in the form of Eq. (4.14) or Eq. (4.15) is free of complications due to steric effects, since it is applied only to meta and para substituents. The geometry of the benzene ring ensures that groups in these positions cannot interact stoically with the site of reaction. Tables of a values for many substituents have been collected some values are given in Table 4.5, but substituent constants are available for a much wider range of... 

The rate of saponification of ethyl 2-thenoate, in contrast to ethyl 3-thenoate, was found to be considerably slower than predicted from the pKa of the acid, showing that the reactivities of thiophenes do not parallel those of benzene. The first explanation, that this was produced by a steric effect of the ring sulfur similar to the case in or /lo-substituted benzenes and in ethyl 1-naphthoate, could not be upheld when the same effect was found in ethyl 2-furoate. It was later ascribed to a stereospecific acid strengthening factor, involving the proper relation of the carboxylic hydrogen and the heteroatom, as the rate of saponification of 2-thienylacrylic acid was in agreement with that predicted from the acid constants. ... 

The low yields of 6,6 -disubstituted-2,2 -bipyridincs recorded in Table I are probably the result of steric retardation of the adsorption of 2-substituted pyridines. This view is supported by the observation that 2-methylpyridine is a much weaker poison for catalytic hydrogenations than pyridine. On the other hand, the quinolines so far examined (Table II) are more reactive but with these compounds the steric effect of the fused benzene ring could be partly compensated by the additional stabilization of the adsorbed species, since the loss of resonance energy accompanying the localization of one 71-electron would be smaller in a quinoline than in a pyridine derivative. 

The steric requirements of the enhanced interaction between arylsulfinyl or arylsulfonyl groups and the benzene ring in bis(4-hydroxyphenyl)sulfoxides or sulfones were examined by Oae and colleagues through the introduction of methyl groups in two or four of the positions ortho to SO or S02. As in the work described earlier for S02Me, effective values for the combined influence of the substituents o (obs.) were determined and compared with o (calc.) computed on the basis of strict additivity of substituent effects on the dissociation... 

These effects can occur when the active site at which a measurable phenomenon occurs is in close proximity to the substituent. Among the many systems exhibiting direct steric effects are ortho-substituted benzenes, 1, cis-substituted ethylenes, 2, and the ortho- (1,2-, 2,1- and 2,3-) and peri- (1,8-) substituted naphthalenes, 3, 4, 5 and 6, respectively. Other examples are d.v-1,2-disubstiUited cyclopropanes, c/ s-2,3-disubstituted norbornanes and ci.s-2,3-disubstituted [2.2.2]-bicyclooctanes, 7, 8 and 9, respectively. Some systems generally do not show steric effects. Vicinally substituted systems such as disubstituted methanes, 10, and 1,1-disubstituted ethenes, 11, are examples, 2,3-Disubstituted heteroarenes with five-membered rings such as thiophenes and selenophenes... 

The relative rates of acetylation in competition experiments in the [m.n]paracyclophane series 38> may be interpreted in terms of trans-annular electronic and steric effects. If the rate of acetylation of [6.6]para-cyclophane [(7), m =n =6] is is taken as one, the relative acetylation rates of the [4.4]-, [4.3]-, and [2.2]paracyclophanes are 1.6, 11, and >48, respectively. As the aromatic rings come closer together, the rate of entry of the first acetyl group into the nucleus increases, while that of the second acetyl group decreases. Both effects clearly indicate that the positive. partial charge can be distributed over both benzene rings in the monoacetylation transition state (64). 

First, we examined the kinetic parameters (Kj and k(.jt) of some orf/jo-substitu-ted compounds, as well as a control substrate. The results are shown in Table 8. The Kj and k(.jt values of a standard substrate (X, R=H) are 13.9 mM and 353 s, respectively. Introduction of a chlorine atom on the ortho-position of the benzene ring (X=Cl,R=H) accelerates the rate of reaction obviously because of its electron-withdrawing property. The steric effect of this substituent is con-... 

In the two systems so far discussed it is impossible to obtain a quantitative idea of the relative importance of the inductive and resonance effects because it is impossible to achieve the operation of one of the effects without the other. When nitrogen is the basic centre, this becomes possible by steric fixation of the nitrogen lone pair orbital in the plane of the benzene ring, which virtually eliminates its overlap with the 7r-electron orbital of the ring carbon and hence also the mesomerism. So the enhanced acidity of the anilinium ion (pAT = 4-62) as compared with methylammonium (pAfg = 10-67) has been shown (Wepster, 1952) to be half inductive and half mesomeric in origin by a consideration of the following systems ([10]-[12]) ... 

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