Alkyl groups electron-donating effect

AH ahphatic aldehydes and most ketones react to form cyanohydrins. The lower reactivity of ketones, relative to aldehydes, is attributed to a combination of electron-donating effects and increased steric hindrance of the second alkyl group in the ketones. The magnitude of the equiUbrium constants for the addition of hydrogen cyanide to a carbonyl group is a measure of the stabiUty of the cyanohydrin relative to the carbonyl compound plus hydrogen cyanide ... 

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. 

An alternative product might be the ort/zo-substituted analogue. Alkyl groups are ortho and para directors for further electrophilic substitution. This follows from stabilization of one of the resonance forms by the electron-donating effect of an alkyl group. This is seen in the para substitution case, and extrapolation to ortho substitution shows a similar stabilization. 

Amine synthesis needs a separate chapter because the C-X disconnection la used for ethers, sulfides and the like in chapter 4 is not suitable for amines. The problem is that the product of the first alkylation 2 is at least as nucleophilic as the starting material 1 (if not more so because of the electron-donating effect of each alkyl group) and further alkylation occurs giving the tertiary amine 3 or even the quaternary ammonium salt 4. It is no use adding just one equivalent of Mel as the first formed product 1 will compete with the starting material 2 for Mel. 

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. 

In the case of A -alkyl amino acids, the oxazohum ion 18 is formed even in the absence of base catalysis because of the electron-donating effect of the A -alkyl group,thus increasing the risk of racemization (Scheme 8). Although proline can be considered as an N-alkylated amino acid, formation of the oxazolium salt is not observed, probably as a consequence of the steric constraints of the five-membered ring which disfavors cyclization.P l As a result, in... 

For organic bases such as amines, due to the electron donation effect and steric effect of alkyl groups, the alkalinity is in the order primary amine > secondary amine > tertiary amine. 

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