Electron donation, from alkyl groups

The half-wave reduction potentials for a series of annelated 1,4 naphthoquinones (102-106) increase upon alkylation, and decrease as ring size decreases (Table 13). The more cathodic reduction potentials of 2,3-dimethylnaphtho-l,4-qui-none (106, 0.846 V) and l,2,3,4-tetrahydro-9,10-anthroquinone (105,0.854 V) as compared to 1,4-naphthoquinone (0.685 V) are expected from inductive electron donation of alkyl groups. A decrease in reduction potential from 105 to 2,3-cyclobutanaphtho-l,4-quinone (103) (0.695 V) as ring size decreases is observed such that the reduction potential of 103 is only slightly higher than the parent 1,4-naphthoquinone. 

For organic reactions in which positive charge develops on carbon, structural effects on reactivity are enhanced in weakly nucleophilic solvents (14). The effect of alkyl substitution on SN reactions can be quantified by the substituent parameter p, which is much more negative in weakly nucleophilic media [e.g., TFA (14, 20), HFIP (4), or TFE (4)] than in normal alcohol solvents. Apparently in weakly cation solvating media, the electron demand for stabilization of positive charge is met from within the molecule [e.g., by enhanced electron donation from adjacent alkyl groups (4)]. 

PP (65,67,70,71,344). The observed enhancement has been explained by a combination of steric and electronic influence on the chain-termination reaction. Enhanced rigidity of the ligand framework combined with a direct steric interaction of the alkyl group with the growing polymer chain decreases the rate of P-elimination. Electron donation from the alkyl substituent may further decrease the local Lewis acidity of the central metal atom, thus reducing its tendency for /3-H abstraction. 

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... 

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