Page 371 - Mechanism and Theory in Organic Chemistry

Table 7.9
X
I
+
CH3-C-CH2CH3 AcOH pH3 + H3C\ /H + CH,=C-CH2CH3
/C=C
\
+ 'H CH3 4
I
I
cis trans terminal
X Percent cis Percent trans Percent terminal
C1 68 9 23
0 Ac 53 2 45
NHNH2 40 0 60
SOURCE: D. J. Cram and M. R. V. Sahyun, J. Amer. Chem. Soc., 85, 1257 (1963). Reprinted by per-
mission of the American Chemical Society.

in glacial acetic acid gives the products shown in Table 7.9.4s the leaving groyp
bgcomes more basic, Saytzeff elimination becomes lessd less im~rtant
and Hofmann product besins to nrPrlnrmnate. The change in product compo-
>tion can be explained by a consideration of Hammond's postulate. The more
basic the counter-ion that pulls off the proton, the more the transition state
for the product-determining step will look like carbocation and the less double-
bond character it will have. Then the orientation of the double bond in the pro-
duct depends more on the relative acidity of the two kinds of protons than on the
relative stabilities of the possible double bonds. In solution the ymethyl group
renders the P-methylene hydrogens less acidic than the P-methyl hydrogens; thus
the more basic the counter-ion the more terminal olefin results.
Carbanion Mechanisms71
If, instead of a good leaving group as is required for the El reaction, a com-
pound has a poor leaving group but a highly acidic proton, elimination may take
place through the consecutive reactions shown in Equations 7.25 and 7.26. These
are usually called ElcB reactions but, depending on the relative magnitudes of
B- + H-C-C-X ,
I
I
I
I
kl
+
BH
-C-C-X
I / k-1 I '1
the rate constants and on the degree of separation between BH and the anion,
Equations 7.25 and 7.26 actually describe four different mechanisms. Table 7.10
shows Bordwell's summary of the characteristics of these mechanisms.
If k, is much greater than both k-l and k, of Equations 7.25 and 7.2&
that is, if the /3 hydrogen is very acidic but the leaving group is poor-then if
sufficient base is present, formation of the anion will be almost complete before

71 D. J. McLennan, Quart. Rev., 21, 490 (1967).