Electronic effects of alkyl groups

Consider first the electronic effect of alkyl groups versus hydrogen atoms attached to C=0 Recall from Section 17 2 that alkyl substituents stabilize C=0 making a ketone carbonyl more stable than an aldehyde carbonyl As with all equilibria factors... 

Sebastian summarized some other experimental evidence that supports this view of the electronic effect of alkyl groups. Sebastian, J. F. /. Chem. Educ. 1971, 48, 97. 

The destabilising influence of the electron-donating inductive effect of alkyl groups is seen in the observed carbanion stability sequence ... 

Experimental evidence concerning the relative rates for SnI reactions of halides is listed in Table 6.7. The differences in reactivity reflect stmctural features that stabilize the intermediate carbocation. Carbocations are stabilized by the electron-donating effect of alkyl groups, which help to disperse the positive charge. We have noted that alkyl groups have a modest electron-donating effect (see Section 4.3.3). In carbocations, this is not a simple inductive effect, but results from overlap of the a C-H (or C-C) bond into the vacant p orbital of the carbocation. This leads to a favourable delocalization of the positive charge. 

Insertion of electron-donor ethyl groups into both a-positions in thiophene raises the /3-atom exchange rate by almost four orders of magnitude (1.6 x 10 and 2 x 10 sec 0- The strong activating effect of alkyl groups also occurs in the isomeric thienothiophenes 1 and 2 where the / exchange rate approaches that of the a in unsubstituted compounds. There is, however, some difference between 3,4-dideutero-... 

In a reactivity study conducted by competitive methods, the influence of 3-alkyl groups (Me and Et) has been found to be rather peculiar.135 3-Alkylpyridines are attacked at the adjacent 2-position more easily than pyridine itself in spite of the weak adverse electronic effect of the substituent. However, at the 6-position the attack occurs more slowly, as expected. Of the several possible reasons for this behavior, probably the most likely is the establishment of a weak attractive interaction between the a carbon of the 3-alkyl group and phenyllithium, which would favor orientation for attack at the 2- rather than the 6-position and overcome the small electronic effect of the group. 

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. 

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... 

A quantitative scale for the structural effect of various silyl groups is established, as shown in entry 57 of Table 1, by the rates of solvolysis of 40 triorganosilyl chlorides in aqueous dioxane under neutral conditions69. The structural effect involves the steric effect and, in some examples, the electronic effect. Because little difference exists in the electronic effect among alkyl groups, their steric effect at silicon follows the order primary < secondary < tertiary substituents. 

Thiazoles readily undergo AT-alkylation by alkyl halides or tosylates (Menshutkin reaction). The sensitivity of this SN2 quaternization reaction to the molecular environment of the nitrogen atom has been used to evaluate, in a quantitative way, steric and electronic effects of ring substituents. The electronic effect of alkyl substituents (unperturbed by any steric effect) may be evaluated from the rate constants for the reaction of 5-alkylthiazoles with methyl iodide (in nitrobenzene at 30 °C) Table 19 shows that introducing a methyl group at the 5-position corresponds to an acceleration of the rate by a factor of 2 but that each addition of a methyl ramification to the 5-alkyl group enhances the rate only by a factor of 1.1. The data in Table 19 fit well with a Hammett-Taft equation (3) ... 

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. 

Both radicals and carbocations are electron deficient because they lack an octet around the carbon atom. Like carbocations, radicals are stabilized by the electron-donating effect of alkyl groups, making more highly substituted radicals more stable. This effect is confirmed by the bond-dissociation enthalpies shown in Figure 4-7 Less energy is required to break a C—H bond to form a more highly substituted radical. 

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