Section 24.3 Acid Catalysis 1005Specific-acid and general-acid catalysts speed up a reaction in the same way—by
donating a proton in order to make bond making and bond breaking easier. The two
types of acid catalysis differ only in the extent to which the proton is transferred in the
transition state of the slow step of the reaction. In a specific-acid-catalyzed reaction,
the transition state has a fully transferred proton, whereas in a general-acid-catalyzed
reaction, the transition state has a partially transferred proton (Figure 24.3).
In the examples that follow, notice the difference in the extent of proton transfer
that has occurred when the nucleophile attacks the reactant. In specific-acid-catalyzed
attack by water on a carbonyl group, the nucleophile attacks a fully protonated car-
bonyl group. In general-acid-catalyzed attack by water on a carbonyl group, the car-
bonyl group becomes protonated as the nucleophile attacks it.
In specific-acid-catalyzed collapse of a tetrahedral intermediate, a fully protonated
leaving group is eliminated, whereas in general-acid-catalyzed collapse of a tetrahe-
dral intermediate, the leaving group picks up a proton as the group is eliminated.
A specific-acid catalyst must be an acid that is strong enough to protonate the reac-
tant fully before the slow step begins. A general-acid catalyst can be a weaker acid be-
cause it only partially transfers a proton in the transition state of the slow step.
PROBLEM 3The following reaction occurs by a general-acid-catalyzed mechanism:Propose a mechanism for this reaction.CH 2HOCH 3OHB+BOHCH 3 OHgeneral-acid-catalyzed elimination of the leaving group
CH 3 C OCH 3OHOH+
CH 3C+OHHB+CH 3 OHspecific-acid-catalyzed elimination of the leaving group
CH 3 C OCH 3 +OHOHOHOHH+ CH 3 C OH +OHCH 3 OCH 3
HC++H 2 Ogeneral-acid-catalyzed attack by water+
CH 3 OCH 3CHB+ B
OCH 3 C OCH 3OH
HOH+H 2 Ospecific-acid-catalyzed attack by water
+
CH 3 OCH 3COH+ + CH 3 C OCH 3OH
HOHCH 3 OCH 3COH++