Page 183 - Mechanism and Theory in Organic Chemistry

Reaction coordinate
Figure 4.1 Proposed reaction coordinate diagram for the SN2 reaction.

activated complex of this single-step reaction, and therefore both enter into the
rate equation.
The decomposition of benzhydryltrimethylammonium hydroxide (Equation
4.5), on the other hand, according to Ingold, proceeds by initial slow formation of
the relatively stable diphenylmethyl carbocation5 and subsequent fast attack on
the carbocation by hydroxide. Because hydroxide is not part of the activated
complex of the slow step of this reaction, it does not enter into the rate equation.
o_r
Hughes and Ingold, in 1935, went on to postulate - - -- tha: these me~ha~nis~ms,
-
-
t

e
a combination of them in w role in the
de- d-tkdeaving-~al f~r aliphatic nucleophilic~bsti-
a11
tutions:
Broadly, then, if the substitution site is primary, and therefore access to it is
not hindered sterically, the nucleoph~appr~aches~t~~~~~_b~_dpnat~n -- of its
electron pair, forms -- -- a - - partial - bond - to cdan while the leaving-group-carbon
bond - begins to break (Equation 4.7). At the transition state, both bonds partially
-
transition state
exist, although bond making and bond breaking need not have occurred to the
same extent. When the reaction is over, the role of the Lewis bases is revxrsed
. .
f j v : V ~ts lone pakb-ut _X-is not. Thus Y is
one unit less and X one u i v t l y WVPlychias called the SE2 (sub-
stitution-nucleophilic-bimole~cul_a~~ meckangm.
The reaction coordinate diagram that corresponds to this proposed
mechanism for the direct displacement reaction is shown in Figure 4.1. Although

We use the term carbocation to refer to any cationic carbon species. For more about the nomen-
clature of carbocations, see Chapter 6, p. 288.
J. L. Gleave, E. D. Hughes, and C. K. Ingold, J. Chem. Soc., 236 (1935).