Page 282 - Mechanism and Theory in Organic Chemistry

1,2-Shifts in Carbenium Ions 269

H3C CH3 H3C CH3 H3C CH,
I I H + I I I I CH -
CH3-C-C-CH, _--' CH3-C-C-CH, CH3-C-C-CH3 2
(6.2)
I I I I I
HO OH HO O+ HO +
/ \
1 H H
CH3-C-C-CH3 ,
CH3
CH3
I
I
+
-H+
CH3-C-C-CH3
1 I II I
q3 CH, 0 CH3
T&name"pmacoln+" if uw-givU_the general type 9f3s5
on
arxqement exemplified_.by_Equatian 6-2,in whish ~-gu11lgs the
g2-diol -bereplaced&y& h)L$regen, _c?_r aryl groups.
In the pinacol rearrangement the driving force to migration is the formation
of a carbonyl group. The driving force to migration in solvolyses and similar
reactions is usually the formation of a more stable carbocation. Since the energy
differences between a tertiary and a secondary and between a secondary and a
primary carbocation are ca. 16 kcal mole-l each,3 rearrangements converting a
less to a more highly substituted carbocation are exothermic. Thus, for example,
reaction of neopentyl iodide with silver nitrate gives entirely rearranged products
(Equation 6.3).4
CH3 CH3 CH3
I 1-x 1 Ha0
CH3-C-CH2-I > CH3-C-CH2 + CH3-+-CH2CH3 (6.3)
I 1 H+

Similarly, the dehydration of 1-butanol leads to 2-butenes (Equation 6.4).5,6

Vinyl cations are less stable than their aliphatic counterparts. Therefore
solvolysis of 1-methyl-2,2-diphenylethenyl triflate (trifluoromethylsulfonate)

(a) F. D. Lossing and G. P. Semeluk, Can. J. Chem., 48,955 (1970); (b) L. Radom, J. A. Pople, and
P. v. R. Schleyer, J. Amer. Chem. SOC., 94, 5935 (1972).
' F. C. Whitmore, E. L. Wittle, and A. H. Popkin, J. Amer. Chem. SOC., 61, 1586 (1939).
F. C. Whitmore, J. Amer. Chem. SOC., 54, 3274 (1932).
Although the reactions, shown in Equations 6.3 and 6.4, and in some equations and schemes
found later in this chapter depict a primary carbocation as an intermediate, it is not certain whether
these highly unstable species exist in solution. Both reactions may involve migration concerted with
departure of the leaving group. See (a) P. Ausloos, R. E. Rebbert, L. W. Sieck, and T. 0. Tiernan,
J. Amer. Chem. Soc., 94, 8939 (1972) and references therein; (b) P. C. Hariharan, L. Radom, J. A.
Pople, and P. v. R. Schleyer, J. Amer. Chm. SOC., 96, 599 (1974).