Page 246 - Mechanism and Theory in Organic Chemistry

Carbocations 235

fluoric or fluorosulfuric acid. A substance of very low basicity such as SO,,
S02C1F, or SO,F, serves as diluent when required. As we have seen in Section
3.2 (p. 134), these solvent systems are considerably more acidic than concentrated
sulfuric acid as measured by the H, acidity function.74 Olah and his co-workers
have made extensive contributions to this field.75 The ready availability of solu-
tions of many types of carbocations has made possible spectroscopic observations
of a greatly expanded variety of structures. Nuclear magnetic resonance, both
proton and 13C, has been fruitful and has yielded information not only about
structure but also about rearrangement processes; other methods, particularly
infrared and Raman spectroscopy, have proved informative as well. X-Ray
photoelectron spectroscopy (electron spectroscopy for chemical analysis, ESCA),
which measures binding energies of 1s electrons of the carbon atoms, yields in-
formation about delocalization of charge within the ion.76

Structures of Carbocations
The, salient fea_t.ure of the structure oae cgbenium ions-is- their preferenm-fnr
cLplanaritY-of ~ the cationic carbon. and the .three attached ato-ma..Struc;tural
theqry.in its2implest fqmLth_p~incip1e of mi-nimum e1,ectr~n-pair interadon
(see Section 1.1, p. 8), predicts a planar spZhybridized structure. Analogy with
the boranes is in agreement : Planar77 (CH,) ,B is isoelectronic with (CH,) ,C + ,
which ought therefore to be planar also. More sophisticated theoretical computa-
tions agree with these simple argument^.^^ We have seen in Section 5.2 that the
indirect evidence from rates of formation of bridgehead ions supports the idea of
preferred planarity. Spectroscopic investigation of ions in strong acid solutions
furnishes more direct evidence. Olah and his collaborators have recorded the
infrared and Raman spectra of (CH,),C+ ; the close similarity to spectra of
(CH,) ,B confirms planarity of the ion.7g
In allylic systems, favorable overlap of the p orbitals of the n system should
require a coplanar arrangement of the three sp2 carbons and their five substituent
atoms; evidence that such a structure is indeed preferred comes, for example,
from proton magnetic resonance observations that demonstrate barriers to bond
rotation in the isomeric dimethylallyl ions 21, 22, and 23. These ions form stereo-
specifically from the three dimethylcyclopropyl chlorides (Section 12.2), and
barriers to rotation about the partial double bonds are sufficiently high to prevent
interconversion at low temperature. At - 10°C, 21, the least stable isomer,

74 R. J. Gillespie and T. E. Peel, Aduan. Phys. Org. Chem., 9, 1 (1 97 1). See Figure 3.3, p. 136.
76 For reviews see (a) G. A. Olah and J. A. Olah in Carbonium Ions, Olah and Schleyer, Eds., Vol. 11,
p. 715; (b) R. J. Gillespie, Accts. Chem. Res., 1, 202 (1968).
See the following reviews in Olah and Schleyer, Eds., Carbonium Ions, Vol. I : (a) electronic spectra,
G. A. Olah, C. U. Pittman, Jr., and M. C. R. Symons, p. 153; (b) vibrational spectra, J. C. Evans,
p. 223; (c) NMR spectra, G. K. Fraenkel and D. G. Farnum, p. 237; a review of applications of all
the spectroscopic techniques to carbocation structures and reactions is (d) G. A. Olah, Angew.
Chem. Int. Ed., 12, 173 (1973).
77 H. A. Levy and L. 0. Brockway, J. Amer. Chem. Soc., 59, 2085 (1937).
78 See, for example: (a) J. E. Williams, Jr., R. Sustmann, L. C. Allen, and P. v. R. Schleyer, J.
Amer. Chem. Soc., 91, 1037 (1969); (b) L. Radom, J. A. Pople, V. Buss, and P. v. R. Schleyer, J.
Amr. Chem. Soc., 94, 31 1 (1972); (c) L. Radom, P. C. Hariharan, J. A. Pople, and P. v. R. Schleyer,
J. Amr. Chem. Soc., 95,6531 (1973) ; for a review, see (d) V. Buss, P. v. R. Schleyer, and L. C. Allen,
Top. Stereochem., 7, 253 (1973).
70 G. A. Olah, J. R. DeMember, A. Commeyras, and J. L. Bribes, J. Amer. Chem. Soc., 93,459 (1971).