Alkyl group electron donation

Are alkyl groups electron-donating groups electron-withdrawing groups (circle one) ... 

The filled bond orbitals of adjacent alkyl groups may donate electrons by 7r-type overlap (see Figures 3.19 and 3.20 in Chapter 3) to an adjacent carbocationic center. Thus, an alkyl group may be considered to be an X-type substituent. The highest combination of the C—H bonding orbitals of a methyl group has a n donor capability intermediate to that of the nonbonded orbitals of and F. The donor abilities of a bonds were discussed in Chapter 4. 

L = H, alkyl, aryl, electron-donating group (OAc), electron-withdrawing group (CN)... 

We will not dwell again upon the factors that stabilize carbenium ions. Suffice it to say that increasing the number of alkyl and electron donating groups stabilizes these structures. 

Protonation (or addition of any Lewis acid) to an unsymmetrical double bond leads preferentially to the more stable carbocation. The energy of carbocations is strongly influenced by stabilizing resonance effects. Substituents bearing a lone pair of electrons are especially effective at stabilization, but even alkyl groups can donate electrons to the empty 2/> orbital of a carbocation. In cases in which inductive and resonance effects oppose each other, resonance usually wins. 

Experimental measurements of dipole moments give size but not direction We normally deduce the overall direction by examining the directions of individual bond dipoles With alkenes the basic question concerns the alkyl groups attached to C=C Does an alkyl group donate electrons to or withdraw electrons from a double bond d This question can be approached by comparing the effect of an alkyl group methyl for exam pie with other substituents... 

Instead of stabilizing the carbonyl group by electron donation as alkyl substituents do trifluoromethyl groups destabilize it by withdrawing electrons A less stabilized carbonyl group IS associated with a greater equilibrium constant for addition... 

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

Monomer Reactivity. The nature of the side chain R group exerts considerable influence on the reactivity of vinyl ethers toward cationic polymerization. The rate is fastest when the alkyl substituent is branched and electron-donating. Aromatic vinyl ethers are inherently less reactive and susceptible to side reactions. These observations are shown in Table 2. 

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 ... 

---