Page 236 - Mechanism and Theory in Organic Chemistry

Effects of Structure and Solvent 225
he stated that "the faster an SNl . . . reaction, the more ionic its transition
state."34
Since Hoffmann's work appeared, the koTs/kBr ratio has come into use as
one of the tools for measuring transition state charge separation. Nevertheless,
we might profitably examine the argument more closely. In Figures 5.2 and 5.3
the Hammond postulate is the guide for constructing reaction coordinate dia-
grams of S,2 and S,l_pghways in which some feature oTthe substrate structure
or mntcring greup mcleophi!icity- cauw-a Ehang< of rate, As
we have seen in Section 4.3, Hoffmann's proposal is -cord with the rathale
rovided by these diagram 'gun~5$ : The reactions with
he~~lcy-vee hfnarrxhgk earlier -___ transition _--- states with
--
-
less charge development nn-the leaving-gzoup; these are the cases with small
ko,,/kBr ratios. The faster S,l_r_e-ac&i_sns (Figure 5.3) should-also-be-tho_se with
ex
with
large
the
ones
are
erimentaI_lyl
these
less charge development -----.-- (curve al- ->--.I----
-
--
koTs/kBr ratios.
Using-dependence of rate on solvent, a technique we discuss further in
Section 5.4, Bingham and Schleyer were unable to detect any significant varia-
tion in transition state charge separation in a series of bridgehead derivatives of
varying rea~tivity.~~ They also pointed out a difficulty arising from Hoffmann's
having based his conclusions on data drawn from reactions of tertiary, secondary,
and even primary substrates. Hoffmann had assumed that reactions carried out
in solvents favorable to ion formation (such as water or formic acid) would be
limiting SN1 processes, and had concluded that the differences in koTs/kBr that he
observed with different substrates arose primarily from different amounts of
charge separation at the transition state. It now appears that at least the primary
substrates, and probably also the secondary ones, solvolize with substantial
assistance by nucleophilic attack of solvent. Bingham and Schleyer have pro-
posed that the more bulky tosylate is subject to greater crowding in a tertiary
substrate than is bromide, and t these steric effects are largely responsible for
the variations in koTs/kBr ratio.3 8 ur conclusion is that, as a measure of transition
state charge separation in limiting S,1 reactions, the koTs/kBr ratio is of little
us@e shall return to the problem of location of the transition state along the
reaction coordinate in Section 5.4.
A_n imzortant neutral leaving group is N2. Diazotization of alkyl amines
(Equation 5.11) leads to the unstable alkyl diazonium ions, which immediately
lose nitrogen, leaving czrbocations.
C-NH, + HONO -
R R1
t I +
X- - N, + X- (5.11)
$2+
\
~'2 R3
Interpretation of the reactions of these ions has proved difficult. Product distribu-
tions and stereochemistry differ from those typical of solvolysis ;37 the large energy

34 H. M. R. Hoffmann, J. Chem. Soc., 6753, 6762 (1965).
35 R. C. Bingham and P. v. R. Schleyer, J. Amer. Chem. Soc., 93, 3189 (1971).
36 See note 35 and J. Slutsky, R. C. Bingham, P. v. R. %hieyer, W. C. Dickason, and H. C. Brown,
J. Am. Chem. Soc., 96, 1969 (1974).
37 For further discussion see Section 6.1.