Electron-donating effects, of alkyl groups

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. 

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. 

One reason for this difference in acidity has to do with the effect of solvation. With an unhindered alcohol, water molecules can easily surround, solvate, and hence stabilize the alkoxide anion that would form by loss of the alcohol proton to a base. As a consequence of this stabilization, formation of the alcohol s conjugate base is easier, and therefore its acidity is increased. If the R group of the alcohol is bulky, solvation of the alkoxide anion is hindered. Stabilization of the conjugate base is not as effective, and consequently the hindered alcohol is a weaker acid. Another reason that hindered alcohols are less acidic has to do with the inductive electron-donating effect of alkyl groups. The alkyl groups of a hindered alcohol donate electron density, making formation of an alkoxide anion more difficult than with a less hindered alcohol. 

DIRECTING ELECTRON-DONATING EFFECTS OF ALKYL GROUPS... 

Directing Electron-Donating Effects of Alkyl Groups CHAPTER 16... 

Instead of a hyperconjugative effect, some have described the stabilization of cations by alkyl substitution as due to the generic electron donating ability of alkyl groups. In this light, discuss the HIA value for2-methylallyl shown. 

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

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. 

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... 

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