My PhD dissertation - Institut Fresnel

3.3.6 Relevance of the Data Assessed and Influence of the Steric
Strains on Alkane Cleavage
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Determination of the relative ease of cleavage for several alkyl substituents was
initially meant to assess their corresponding carbanion stabilities and thus, their related
basicities in solution. Although no accurate reference data are available concerning the
basicities of the species studied herein, the order of cleavage promptness for all the alkyl
substituents perfectly agreed with the experimental reactivity of each corresponding alkyl
carbanions considered. Gas phase acidities of few alkanes have been determined by C.H.
DePuy, R. Damrauer et al. [108] and show a different order for the relative stability of
carbanions:
-C(CH3)3 > -CH3 > -CH(CH3)2, -C2H5 (1)
It is well known that gas phase acidities are dramatically different from those
observed in solution
[ ] 168 . The four effects currently believed to account for these tremendous
variations are the polarization of the carbanions species, the possible resonance and inductive
effects and the hybridization of the carbon bearing the negative charge. In the case of simple
alkanes, the stabilization of the negative charge at the carbanionic center is mainly due to the
[ ]
polarization of the alkyl residue 169 . It is then of evidence that large alkyl anions like tertbutyl
are more efficiently stabilized than smaller ones like methyl group, and thus methane is
less acidic than 2-methylpropane in the gas phase. The stabilization of the same alkyl
carbanions in solution are rather due to solvation, or charge delocalization by mesomeric or
inductive effects. Therefore, the order of relative acidity of given substrates in solution are
usually inversed compared to the gas phase values since steric hindrance usually lowers the
carbanion solvation. For this study, the relative stabilities of alkyl carbanions determined in
solution have been compared to the corresponding gas phase data described in the literature.
According to C.H. DePuy, R. Damrauer et al. [108] , the experimental gas phase acidity order
(1) was unexpected. Furthermore, these authors admitted that the effects of successive αmethyl
substitutions on carbanion stability showed less consistency than that of β-methyl
substitutions, and could hardly be rationalized.
Although well studied in its general features, the exact mechanism still has to be
settled. In this context, the conclusions made from the results collected have to me moderated
due to the fact that steric strains could help favoring one expulsion rather than the other, in the
rate determining step. As a matter of fact, if the concerted mechanism turned out to be the
effective one, two cyclic transition states could be envisaged, as proposed in chapter 3.3.1,