Page 238 - Mechanism and Theory in Organic Chemistry

Effects of Structure and Solvent 227

Whereas substitution of hydrogen directly attached to the cationic center by
alkyl or aryl has a large stabilizing effect, substituents at more remote positions
are more subtle in their influence. We might expect, on the basis of the ability
of alkyl groups to stabilize charge, that cation 6 should be more stable than cation
5. This prediction is indeed correct for the isolated ions in the gas phase. In

solution the order is reversed, and 5 is more stable.46
Arnett has observed that heats of reaction for carbocation formation in
highly acidic media (Equation 5.12) correlate well with solvolysis rates.47 This
RX-HA + R+ + HAX- (5.12)
result supports the idea that ion and solvolysis transition state are closely related
structurally, and that we can safely use ideas about ion stability to predict
solvolysis rates and vice versa.
W-hen atoms with unshared pairs of electrons are bonded to the reaction
--
two effects must be ~onsider~~_the.i_n.duct_ve effect, which is -usually
--
electron-withdrawing, and thxlectron-donating coniugative effect (7). For thk
--
--
.
more basic atoms, Oy Ny S, conjugation is domjnn!,as the relative solvolysis
\ + \ +
C-E: - C=E
/ /
7
rates of 8 and 9 show. Insulation from the conjugative influence by one CH,
C2H5--0-CH2-Cl CH3CH2CH2CH2-Cl C2H5-0-CH2CHz-CI
8 9 10
Relative rate: log 1 0.2
group (10) leaves the inductive effect to cause a rate depres~ion.~~ Mechanisms
are unlikely to be limiting in 9 and 10, but should be more nearly so in 8; the
relative rates therefore probably have only qualitative significance. With a-
the two effkts are more nearly balanced. Fluorine is ~ ~ m -
tqative and d-ecreases the sate~&de~~itS_un.sha_rddectrnns; chlorine
and increase rates, but much less than does oxygen (kx/k, ratios 10 to 500),
presumably because of the less effective overlap with carbon of the large 3p or 4p
orbitals that contain the unshared pairs in these atoms.49 Chemical shifts in 13C
nuclear magnetic resonance spectra independently demonstrate the decreasing
effectiveness of donation of electron density by conjugation to an adjacent
positive carbon in the order F > C1 > Br.50
Another important consequence of structural change, first observed and

48 J. W. Larsen, P. A. Bouis, M. W. Grant, and C. A. Lane, J. Amer. Chem. Soc., 93, 2067 (1971).
" E. M. Arnett and J. W. Larsen, in Carbonium Ions, G. A. Olah and P. v. R. Schleyer, Eds., Vol. I,
p. 441.
'13 A. Streitwieser, Jr., Solvolytic: Disfilacement Reactions, pp. 102-103.
4Q See note 48.
G. A. Olah, Y. K. Mo, and Y. Halpern, J. Amer. Chem. Soc., 94, 3551 (1972).