Carbocation Stability

Although generations of organic chemists have used rates of nucleophilic substitution reactions as a means of judging the stability of carbocations, this approach is fraught with error.

Two methodologies exist for more directly measuring carbocation stabilities.

Your textbook gives brief collections of data obtained in these ways. The table below contains a more extensive selection. It also includes for a few structures the 13C chemical shift of the carbocation carbon in hyperacid media, which is very nearly invariant with structure.

Carbocation DHf, kcal/mol DHrxn, kcal/mol d, 13C, ppm
CH3+ 261 not formed not formed
CH3CH2+ ** 216.2 not formed not formed
CH3CH2CH2+ ** 208 .... ....
(CH3)2CH+ 191.8 .... 320.6
** 176.7 .... ....
(CH3)3C+ 162.1 -35.5 335.2
154.6 -40.3 335.4
152.5 -36.7 336.4
(CH3CH2)3C+ 147.7 .... ....
226 .... ....
200 .... ....
** 183.8 .... ....
171 .... ....
164.2 -38.3 ....
152.7 -37.8 ....
** 187 .... ....
172 .... ....
256 .... ....
211 .... ....
** 213 .... ....
200 .... ....
188 -40.3 ....
.... -39.3 199.4
213.6 -37.5 229.3
.... -49.0 ....

Structures marked with a ** are in fact not the simple structures represented. See the next page for an explanation.


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