Page 460 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Theoretical calculations, structural studies under stable ion conditions, and product 441
and mechanistic studies of reactions in solution have all been applied to understanding
the nature of the intermediates involved in carbocation rearrangements. The energy SECTION 4.4
+
surface for C H in the gas phase has been calculated at the MP4/6-311G ∗∗ level. The Structure and Reactions
3
7
of Carbocation
1- and 2-propyl cations and corner- and edge-protonated cyclopropane structures were Intermediates
compared. The secondary carbocation was found to be the most stable structure. 117
Hydrogen migration was found to occur through a process that involves the corner-
protonated cyclopropane species. Similar conclusions were drawn at the G2 and B3LYP
levels of calculation. 118 Calculations that include an anion change the relative energy
of the 1-propyl cation and the protonated cyclopropane. The 1-propyl cation becomes a
+
stable structure in close proximity to an anion. 119 Relative energies of C H cations
3
7
are shown below.
H
H H
+ + H+ +
CH 3 CH 2 CH 2 CH 3 C HCH 3
MP4/6-311* +19.3 0 8.6 7.3
0
G2 8.2 7.2
B3LYP 0 16.0 12.2
The 2-butyl cation has been extensively investigated both computationally and
experimentally. The 2-butyl cation can be observed under stable ion conditions. C(2)
and C(3) are rapidly interconverted by a hydride shift. The NMR spectrum corre-
sponds to a symmetrical species, which implies either a very rapid hydride shift or a
symmetrical H-bridged structure. A maximum barrier of 2.5 kcal/mol for hydride shift
can be assigned from the NMR data. 120

H very H
fast CH 3 H+ H
CH 3 C CH 2 CH 3 CH 3 CH 2 C CH 3 or
+ + H CH 3

Scrambling of C(3) and C(4) [or C(1) and C(2)] occurs with an E of about
a
7–8 kcal/mol. The scrambling of C(3) and C(4) can occur via an edge-protonated
intermediate. The rearrangement of 2-butyl cation to the t-butyl ion is rather slow,
occurring with an E of 18 kcal/mol. 121
a
E a = 2.5 kcal
+
+
E a = 18 kcal *CH 3 C HCH 2 *CH 3 *CH 3 CH 2 C HCH 3 E a = 18 kcal
(*CH 3 ) 3 *C +
E a = 8 kcal E a = 8 kcal
(*CH 3 ) 3 *C +
E a = 2.5 kcal
+ +
*CH 3 C H*CH 2 CH 3 CH 3 *CH 2 C H*CH 3 E a = 18 kcal
E a = 18 kcal
117 W. Koch, B. Liu, and P. v. R. Schleyer, J. Am. Chem. Soc., 111, 3479 (1989).
118 M. V. Frash, V. B. Kazansky, A. M. Rigby, and P. A. van Santen, J. Phys. Chem. B, 101, 5346 (1997).
119
D. Farcasiu and D. Hancu, J. Am. Chem. Soc., 121, 7173 (1999); D. Farcasiu and D. Hancu, J. Phys.
Chem. A, 101, 8695 (1997).
120 M. Saunders and M. R. Kates, J. Am. Chem. Soc., 100, 7082 (1978).
121
D. M. Brouwer, Recl. Trav. Chim. Pays-Bas, 87, 1435 (1968); D. M. Brouwer and H. Hogeveen, Prog.
Phys. Org. Chem., 9, 179 (1972); M. Boronat, P. Viruela, and A. Corma, J. Phys. Chem., 100, 633
(1996).