Nitro group, steric effects

From the standpoint of geometrical considerations, the major difference is in the far greater steric requirements of the nitro group. This could result in either primary or secondary steric effects. Nevertheless, primary steric effects do not seem to be necessarily distinguishable by direct kinetic comparison. A classic example is the puzzling similarity of the activation parameters of 2-chloropyrimidine and 2,6-dinitrochlorobenzene (reaction with piperidine in ethanol), which has been described by Chapman and Rees as fortuitous. However, that nitro groups do cause (retarding) primary steric effects has been neatly shown at peri positions in the reaction with alkoxides (see Section IV,C, l,c). 

Secondary steric effects of nitro groups are more easily detected by comparing the reactivities with those of aza derivatives. For example, in structure 20 the rate depression on passing from methyl to -butyl is only 2.5-fold and can be attributed to an inductive effect, whereas in structure 21 a similar change involves the factor 16, which can be attributed in part to steric inhibition of resonance (S.I.R.) of thep-N02 group (reaction with piperidine). 

Steric hindrance to activation by carboaromatic nitro groups is well-known, but there seems to be no analogy in the chemistry of azines. The lone-pair of azine-nitrogen has a steric effect comparable " to, somewhat greater than, or somewhat less " than a hydrogen atom. It is not certain whether bulky groups such as i-butyl produce a steric distortion of the lone-pair orbital and whether activation or deactivation results. 

The process tolerates alkyl, methoxy, tertiary amino, and nitro groups at the salicylaldehyde. However, the yields of the nitro- and the amino-substituted salicy-laldehydes are rather low. Best results were obtained with aldehydes containing electron-donating groups (entries 4, 6, and 7). Steric effects clearly also play an im-... 

The decarboxylation of mercuric 3- and 4-nitro-l,8-naphthalene-dicarboxylates has been studied to determine substituent electronic effects [Eq. (83)] (95,96), since significant steric effects are unlikely. There is disagreement over the products. In an early study, 3- and 4-nitro-l,8-naphthalenedicarboxylates were found to undergo decarboxylation predominantly in the ring without the nitro group (95), consistent... 

Stewart33, who proposed a parallel between the rate of esterification of 2-substituted benzoic acids and the molecular weights of the substituents (the nitro group strongly deviating from this relationship) was the first to attempt to relate the steric effect of a... 

In the second edition of his book33 Stewart proposed a parallel between the rate of esterification of 2-substituted benzoic acids and the molecular weights of the substituents, the nitro group strongly deviating from this relationship. The first actual attempt to define a set of steric parameters is due to Kindler39. It was unsuccessful these parameters were later shown to be a function of electrical effects. The first successful parametrization of the steric effect is due to Taft40, who defined the steric parameter Es for aliphatic systems by the expression ... 

However, the introduction of sterically hindered substituents at the p-car bon atom of nitroalkene (42) completely changes the ring-chain tautomerism of conjugated nitroalkenes. Apparently, steric hindrance caused by two bulky Bu groups in product (42a) (Scheme 3.47) prevents effective conjugation of the jt systems of the C,C double bond and the nitro group, thus causing its deviation from the plane of the C=C bond as a result of which isomer (47a) becomes thermodynamically more favorable. 

Steric effects are also apparent in the activities of several mutagens in series 28. In Fig. 29 the 4-tert-butyl group in 281, the 4-nitro substituent in 28h and perhaps the CHO in 28f result in lower activity than would be expected on electronic grounds alone. 

The effect on 15N and 13C chemical shifts in anilines75-77 and nitrobenzenes74 of a reduced resonance interaction caused by ortho substituents which sterically twist the amino and nitro groups from coplanarity with the aryl ring has been extensively investigated and is fairly well documented. 

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