Alkyl group, electronic effects

In the absence of special electronic effects, alkyl groups show a clear dependence on the size of the migrating group. In general, smaller groups migrate before larger ones ... 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

The effect of introducing electron-donating alkyl groups into the benzene nucleus is found to be small ... 

This developing positive charge is most effectively delocalized in the transition state leading to a 3° carbocation because of the presence of three electron-releasing alkyl groups. 

Ethene does not polymerize by the cationic mechanism because it does not have sufficiently effective electron-donating groups to permit easy formation of the intermediate growing-chain cation. 2-Methylpropene has electron-donating alkyl groups and polymerizes much more easily than ethene by this type of mechanism. The usual catalysts for cationic polymerization of 2-methylpropene are sulfuric acid, hydrogen fluoride, or a complex of boron... 

An important example of this effect is the decrease in equilibrium constants for ketones as compared to aldehydes. The replacement of the aldehyde hydrogen of acetaldehyde (K = 1.3) with a methyl group, to produce acetone (K = 2 X 10-3), results in a decrease in the equilibrium constant for hydration by a factor of approximately 1000. The inductive effect of the electron-donating alkyl group also helps shift the equilibrium for ketones toward the reactant. 

Natta et al. (167,188,287,298,312) have built a strong case in favor of a coordinated anionic mechanism in which an electropositive metal complexes and polarizes the monomer and a polymer anion adds to the positively polarized carbon of the monomer. One of the points which was used to support the anionic mechanism was that the order of reactivity for ethylene, propylene and butene is opposite to that of cationic catalysts. The lower reactivity of propylene and butene versus ethylene was attributed to the electron releasing alkyl groups (287), but steric hindrance is believed to be a better explanation. Support for the steric effect is indicated by the influence of bulk placed at some distance from the double bond (116). For example, reactivity decreases sharply in the order pentene-1 > 4-methylpentene-l > 4,4-dimethylpentene-l, although basicity of the double bonds must change only very slightly. 

The use of a Cp or other alkylated Cp ligand instead of unsubstituted Cp in CpCo chemistry is preferred for two reasons. First, there is less of a tendency to form oUgomers and polymers with type I and III compounds, which ensures higher solubility secondly, there seems to be a favorable electronic effect. Electron-donating alkyl groups on the Cp... 

To predict whether a substituted benzene is more or less electron rich than benzene itself, we must consider the net balance of both the inductive and the resonance effects. Alkyl groups, for instance, donate electrons by an inductive effect, but they have no resonance effect because they lack nonbonded electron pairs or n bonds. Thus, any alkyl-substituted benzene is more electron rich than benzene itself. 

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