Inductive effects of other groups

The degree of shielding depends on the density of the circulating electrons, and, as a first, very rough approximation, the degree of shielding of a proton on a carbon atom will depend on the inductive effect of other groups attached to the carbon atom. The difference in the absorption position of a particular proton from the absorption position of a reference proton is called the chemical shift of the particular proton. 

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

This is precisely opposite to what would be expected from the inductive effects of alkyl groups, and the observations are likely to be the result primarily of solvation (hydrogen bonding) effects. Note, the cations shown all have negative pATa values. In other words, they are very strong acids and will lose a proton readily. Conversely, the non-protonated compounds are weak bases. 

Baker-Nathan effect. Effect originally observed in the reaction of p-substituted benzyl bromides with pyridine and other processes in which the observed rates are opposite to those predicted by the electron-releasing inductive effect of alkyl groups, i.e., CH3 > CH3CH2 > (CH3)2CH > C(CH3)3. To explain it, a type of electron delocalization involving 2 electrons was proposed, termed hyperconjugation, which manifests itself in systems in which a saturated carbon atom attached to an unsaturated carbon or one with an empty orbital bears at least one hydrogen atom. 

The charge capacity effect is responsible for Ihe well known inductive effect of alkyl groups (see page 196). It is also important in hard and soft acid-base theory (see Chapter 9), and causes several other unexpected elfects.32 It is basically a polarization effect in which larger atoms and groups can acquire or donate large amounts of electron density without unfavorable energy chances.n... 

The effect of alkyl groups in the 5-position on the reactivity of the thiazole nitrogen is analogous to that found for 3-alkylpyridines, in other words, a simple inductive effect. In passing from the unsubstituted heterocycle to the methyl derivative, the rate constant doubles a further increase in substitution produces a much less pronounced variation. 

However, there is evidence against d-orbital overlap and the stabilizing effects have been attributed to other causes. In the case of a PhS substituent, carbanion stabilization is thought to be due to a combination of the inductive and polarizability effects of the group, and d-pn resonance and... 

Peroxybenzoic acid readily oxidizes aryl and alkyl sulphoxides in acetone, methylene chloride or chloroform solutions, to the sulphone in high yield . The reaction is second order and acid catalysed as is the reaction with peracetic acid . The rate of oxidation is about five times faster than when peracetic acid is used. Other work considering the oxidation of sulphoxides with peracids gathered kinetic evidence and showed that the reaction was indeed second order and that the reaction involved nucleophilic attack by the sulphoxide sulphur atom on the peracid moiety. A further study by the same authors showed that with benzyl and phenyl alkyl sulphoxides the rate of reaction was very sensitive to the inductive effect of the alkyl group. Support for the nucleophilic attack by the sulphur atom on the peracid in acidic solution was forthcoming from other sources . ... 

One approach refers to local charge densities [188] the other one uses localization energies as a reactivity index [189]. In any event, the inductive effect of the methylene group, formed in the first protonation step, has to be taken into account if two sites provide comparable reactivity indices. 

It is perhaps worth mentioning that substitution of other groups for the C-methyl group in amidinium ions has an effect on their pA a-values, which is best correlated with Hammett s -constants (p = —1T98), and the same is true of C-substituted N-phenylami-dinium ions (p = —12-08) (Charton, 1965). This is in line with the suggestion made on page 286, that -constants are a measure of the inductive and mesomeric interactions of groups with a carbon in the sp state of hybridization. 

These p/Tg-values show that the inductive effect of the C=C bond accounts for about one half of the reduction in the basicity of the enamine nitrogen atom, the other half being due to the mesomeric effect (Stamhuis et al., 1965). The localization energy of the lone pair on the nitrogen, calculated from the second Ap/iTa, is only about T6-1-7 kcal mole (6 7-7 l kj mol ). There are indications from the substituent effects on the stretching frequency of the C=CH2 bond that this group is an electron sink of the same kind... 

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