Page 330 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

There is also a good deal of information available on carbanion stability in 311
solution. 84 These data are derived from equilibrium measurements analogous to acid
dissociation constants, in which the extent of deprotonation of hydrocarbons by strong SECTION 3.4
base is determined: Electronic Substituent
Effects on Reaction
X – CH – H + B – X – CH 2 – + BH Intermediates
2
Only a few hydrocarbon derivatives can be directly measured in aqueous solution, but
extensive studies have been done in other solvents, in particular DMSO. We consider
this solution data in Chapter 6.
3.4.3. Radical Intermediates

Carbon radicals have an unpaired electron in a nonbonding orbital. The possible
hybridization schemes are shown below.

• • R
R R • •
R C C R C C C C R R C C •
R R R
R R R
3
2
sp / sp 3 p / sp 2 sp / sp 2 p / sp sp / sp
The methyl radical is close to planarity. 85 Experimental 86 and computational 87 results
indicate that simple alkyl radicals are shallow pyramids with low barriers for inversion,
2
which is consistent with p/sp hybridization. The ethenyl radical is bent with a
88
C−C−H bond angle of 137 . High-level MO calculations arrive at a similar
2
2
structure. 89 This geometry indicates sp /sp hybridization. The alkenyl radicals can
90
readily invert through the linear p/sp radical as a transition structure. A major struc-
tural effect in alkyl (beyond methyl) and alkenyl radicals is a marked weakening of
the -C−H bonds, which occurs by interaction of the -C−H and the SOMO (singly
occupied molecular orbital). According to both computational (CBS-4) and thermody-
91
namic cycles, the strength of the -C−H bond is only 30–35 kcal/mol. This leads to
one of the characteristic bimolecular reactions of alkyl radicals—disproportionation to
an alkane and an alkene. Similar values pertain to -C−H bonds in vinyl radicals.
• HC CH • RCH CH • RCH CH 2
CH 2 CH 2 2
H H 35.6 kcal/mol H
35.2 kcal / mol •CH CH 3 CH R
2
2 CH R 2
disproportionation
84
E. Buncel and J. M. Dust, Carbanion Chemistry, Oxford University Press, Oxford, 2003.
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L. A. Curtiss and J. A. Pople, J. Chem. Phys., 88, 7405 (1988); K. A. Peterson and T. H. Dunning, J.
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X. -M. Zhang, J. Org. Chem., 63, 1872 (1998).