Polar effects of alkyl groups

Polar effects of alkyl groups 243 Steric effects of alkyl groups 246 Kinetic effects of aryl substituents 252 Selectivity relationships and transition-state shifts in arylolefin bromination 256... 

In the nineteen sixties, there was some confusion about the value of p, the reaction constant for polar effects of alkyl groups on bromination rates, as calculated according to Taft s equation, log (k/k0) = p a. For 22 alkenes substituted by one, two or three linear alkyl groups, there is a satisfactory relation (16) between their reactivities, log k, in methanol and the sum of their... 

While p for the polar effects of alkyl groups does not vary with the solvent, pn for the polar effects of aromatic substituents is solvent-dependent. For example, there is a fairly linear log/log relationship between the bromination rates of styrenes in acetic acid and methanol (36) with a slope higher than unity, to be compared with 0.99 in (24). The p+-value for styrenes... 

Polarization effect of alkyl group, which may include some inductive effect. 

P. de la Mare, L. Fowden, E, D. Hughes, C. K. Ingold, andj. Mackie,/. Ghent. Soc., 3200 (1955). XLIX. Analysis of Steric and Polar Effects of Alkyl Groups in Bimolecular Nucleophilic Substitution, with Special Reference to Halogen Exchange. 

Fig. 9 Comparison of polar and steric effects of alkyl groups on bromination rates of linear ( ), branched (O) and adamantyl (A) alkenes in acetic acid and in methanol (Ruasse and Zhang, 1984 Ruasse et al., 1990). Polar effects are identical in both solvents [full line, eq. (24)], but steric effects differ. Deviations of branched alkenes are attributed to steric inhibition of nucleophilic solvation by methanol.

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. 

Special Reference to the Polar Effects of Substituents. Part XI. The Aldehydecyanohydrin Reaction Evidence for the Mesomeric Effects of Alkyl Groups. J. chem. Soc. (London) 1942, 191. 

Carbon-13 chemical shifts of representative aldehydes [284] and ketones [285-288] are collected in Tables 4.27 and 4.28. Inspection of the data shows that a, / , and y effects are up to 7, 2, and — 1 ppm, respectively. These increments are significantly smaller compared with those reported for alkyl carbons. Obviously, the electron releasing effect of alkyl groups (( +(-/-effect) slightly attenuates positive polarization of the carbonyl carbons. 

This reversal illustrates the conflict between the -/ effect of alkyl groups and polarization stabilization of charge. For carbanions, the two effects oppose each other in the gas phase for cations, they are in the same direction. In solution, the polarizability effects on relative stability are much less important because the medium is highly polarizable, and access of the solvent to the charged center is more important than charge polarization within the molecule. 

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... 

Brauman and Blair have argued that the polarization stabilization effect of alkyl groups should be essentially the same for negative as for corresponding positive ions [27]. Qualitative results for acidities of amines and alcohols were compared with corresponding results for basicities of amines in support of their argument. 

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. 

Gallo, 1983), and how the polar effects disappear totally this result is probably fortuitous and again due to the relationship between a and Es. It must therefore be concluded that parameter scales are inadequate to describe the kinetic influence of alkyl groups in bromination and in electrophilic additions in general. 

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