Inductive effect of alkyl groups

The relative magnitude of the inductive effect of alkyl groups is normally found to follow the order,... 

Xrx is a parameter characterizing the homologous series RX. The values of /j,r are direct measures of the polar inductive effects of alkyl groups relative to that of methyl and correlate well with Taft s a values. Substituent-induced IP shifts can thus be handled by linear free energy relationships (LFER) of the Hammett pcr-type. 

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. 

We know how to estimate the stability of a carbonium ion, on the basis of inductive effects and resonance. Because of the electron-releasing inductive effect of alkyl groups, stability and hence rate of formation of the simple alkyl cations follows the sequence 3 > T > l ... 

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 inductive effect of alkyl groups in alkyl isocyanates, such as -butyl isocyanate, stabilizes the silylurethane bonds. This improves thermal and hydrolytic resistance over aryl isocyanates. In this context, phenyl isocyanate reacts with sepiolite, giving large amounts of diphenyl urea, wha-eas butyl isocyanate gives stable grafted compounds, producing low amounts of secondary products (dibutyl urea) (101). Because diisocyanates (e.g., hexamethylene diisocyanate and 2,4-... 

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... 

Noteworthy points in the chapter on Electrophilic Substitution are the surprising findings, based on n.m.r. parameters of a model system, that the polar inductive effect of alkyl groups attached to p -carbon is zero (ref. 12). The correlation of the ortfto-directing effect in the metallation of aromatics (refs. 76—79) and a detailed study of the acylation of the pyrrole anion rationalizing the results on the HSAB principle (refs. 265, 266) are worthy of mention. 

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