Page 756 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Cl Cl Cl 739
+ –
+ SbCl 5 SbCl 6
SECTION 8.3
Cl Cl Cl Cl Aromaticity in Charged
Ref. 117
Rings
Cl
+
+ SbCl 5 SbCl 6 –
Ref. 118

The 1,2,3-tri-t-butylcyclopropenium cation is so stable that the perchlorate salt can be
recrystallized from water. 119 An X-ray study of triphenylcyclopropenium perchlorate
has verified the existence of the carbocation as a discrete ion. 120 Quantitative estimation
of the stability of the unsubstituted cyclopropenium ion can be made in terms of its
pK + value of −7 4, which is intermediate between such highly stabilized ions as
R
triphenylmethyl cation and the bis-(4-methoxyphenyl)methyl cation. 121 (See Section
4.4.1 for the definition of pK +). An HF/6-31G* MO calculation on the following
R
isodesmic reaction:

+
+ CH CH CH 2 + CH 2 CHCH 2 +
3
yields a H of +38 2kcal/mol, whereas experimental data on the heats of formation
of the various species give H =+31kcal/mol. 122 Both values imply that the cyclo-
propenium ion is much more stable than the allyl cation. G2 calculations indicate total
aromatic stabilization of 59.1 kcal/mol based on the reaction 123

+ +
+ +

A radical-based homodesmotic reaction gives a value of 30.4 kcal/mol, which compares
with 29.1 kcal/mol for benzene by the same approach. 124 The gas phase heterolytic bond
dissociation energy to form cyclopropenium ion from cyclopropene is 225 kcal/mol.
This compares with 256 kcal/mol for formation of the allyl cation from propene and
268 kcal/mol for the 1-propyl cation from propane. 125 It is clear that the cyclopropenyl
cation is highly stabilized.
In contrast, the less strained four -electron cyclopentadienyl cation is quite
unstable, being calculated to have a negative stabilization of 56.7 kcal/mol. 126 The

117 S. W. Tobey and R. West, J. Am. Chem. Soc., 86, 1459 (1964); R. West, A. Sado, and S. W. Tobey, J.
Am. Chem. Soc., 88, 2488 (1966).
118
R. Breslow, J. T. Groves, and G. Ryan, J. Am. Chem. Soc., 89, 5048 (1967).
119 J. Ciabattoni and E. C. Nathan, III, J. Am. Chem. Soc., 91, 4766 (1969).
120 M. Sundaralingam and L. H. Jensen, J. Am. Chem. Soc., 88, 198 (1966).
121
R. Breslow and J. T. Groves, J. Am. Chem. Soc., 92, 984 (1970).
122 L. Radom, P. C. Hariharan, J. A. Pople, and P. v. R. Schleyer, J. Am. Chem. Soc., 98, 10 (1976).
123
M. N. Glukhovtsev, S. Laiter, and A. Pross, J. Phys. Chem., 100, 17801 (1996).
124 C. H. Suresh and N. Koga, J. Org. Chem., 67, 1965 (2002).
125 F. P. Lossing and J. L. Holmes, J. Am. Chem. Soc., 106, 6917 (1984).
126
P. v. R. Schleyer, P. K. Freeman, H. Jiao, and B. Goldfuss, Angew. Chem. Int. Ed. Engl., 34, 337
(1995); B. Reidl and P. v. R. Schleyer, J. Comput. Chem., 19, 1402 (1998).