Steric effect of methyl group

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). 

An unsuccessful attempt has been made to determine the separate electronic and steric effects of alkyl groups on the acidities of hydrocarbons, acetophenone derivatives, and acetone derivatives CH3COCHR1R2 (at either site) by multivariational analyses of experimental and theoretical acidities for each set.15 A thermodynamic cycle has been used to estimate the aqueous phase p/C, = 22.7 1.0 for the methyl group of acetic acid and p/C, = 3.3 1.0 for the corresponding enol.16 Equilibrium acidities have been determined for several nitroaryl substituted nitroalkanes and cyanomethanes, 2,4,6-TNT, and 9-cyanofluorene17 in acetonitrile the influence of common cation BH+ on the electronic spectra of the anions obtained in the presence of strong guanidine bases (B) has been attributed to formation of two types of ion pair.18... 

Attack in the conformation 139 which leads to the Z-isomer takes place from the upper surface of the molecule, anti to the trimethylsilyl group. This face is occupied by a hydrogen group. In the conformation 138 which leads to the -isomer, this surface is occupied by a methyl group. The greater steric effect of this group leads to the small... 

The effect of methyl groups in the aromatic ring of 1-phenylethyl chloride is shown in Table 3. The small but significant rate increase of the o-CH3 group was reasonably ascribed to steric acceleration. 

A methyl or a f-butyl substituent in 99 in an ortho position dramatically increases the barrier. Thus, for a given model the modification of the barriers can be used to determine a steric scale of substituents in this particular geometrical context provided the electronic contribution of the substituent on the cross-conjugation capability of the heterocycle can be determined. This approach can be applied with success to the steric effect of alkyl groups whose electronic contribution should be very similar. 

While steric effects of alkyl groups may be present in the hydrogenation of alkylnaphthalenes they are by no means the only effect. Competitive hydrogenation of 2-methyl- and 2-t-butylnaphthalene showed that the latter was hydrogenated somewhat more slowly. In experiments with Raney nickel catalyst at 1000 psig and 200°, the ratios of tetralin isomers were the same whether the methylnaphthalene and i-butylnaphthalene were hydrogenated separately or competitively ... 

The reaction rates for quatemization of pyridazines provide a measure of the steric effect of alkyl groups. Rates for 3,6-dialkylpyridazines with methyl iodide show that the product distribution is kinetically controlled and is solvent dependent. Moreover, steric effects of substituents prevail... 

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