Page 366 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

K 347
fast slow
B + H + BH + + C B C + H +
SECTION 3.7
+
+
+
Rate = k BH C = k C K B H = k H B C Catalysis
2
2
obs
In the actual experiments, since buffers are used to control pH, the points are extrapo-
lated to zero buffer concentration by making measurements at several buffer concen-
trations (but at the same pH). Such plots are linear if the reaction is subject to specific
acid base catalysis.
Several situations can lead to the observation of general acid catalysis. General
acid catalysis can occur as a result of hydrogen bonding between the reactant R and
a proton donor D–H to form a reactive complex D–H···R , which then undergoes
reaction with a reactant Z:
D–H+R D–H···R
slow
D–H···R +Z −→ D–H+R–Z
Under these circumstances, a distinct contribution to the overall rate is found for each
potential hydrogen bond donor D–H. General acid catalysis is also observed when a
rate-determining proton transfer occurs from acids other than the solvated proton:
slow
+
R +HA−→ RH+A −
fast
RH −→ product
+
1
2
Each acid HA ,HA , etc., makes a contribution to the overall rate of the reaction.
General acid catalysis is also observed if a prior equilibrium between the reactant
and the acid is followed by rate-controlling proton transfer. Each individual conjugate
base A appears in the overall rate expression:
−
+
R +HA RH+A −
slow
+ −
RH+A −→ product +HA

Note that specific acid catalysis describes a situation where the reactant is in
equilibrium with regard to proton transfer and proton transfer is not rate-determining.
On the other hand, each case that leads to general acid catalysis involves proton
transfer in the rate-determining step. Because of these differences, the study of rates
as a function of pH and buffer concentrations can permit conclusions about the nature
of proton transfer processes and their relationship to the rate-determining step in a
reaction.
The details of proton transfer processes can also be probed by examination of
solvent isotope effects by comparing the rates of a reaction in H O versus D O. The
2 2
solvent isotope effect can be either normal or inverse, depending on the nature of the
+
+
proton transfer process. D O is a stronger acid than H O . As a result, reactants in
3
3
D O solution are somewhat more extensively protonated than in H O at identical acid
2
2
concentrations. A reaction that involves a rapid equilibrium protonation proceeds faster
in D O than in H O because of the higher concentration of the protonated reactant. On
2
2
the other hand, if proton transfer is part of the rate-determining step, the reaction will
be faster in H O than in D O because of the normal primary kinetic isotope effect of
2 2
the type of reaction.