Page 657 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

acetaldehyde, the half-life of the exchange reaction is on the order of 1 min under 639
neutral conditions, but is considerably faster in acidic or basic solution. The second-
−1 −1 10
order rate constant for acid-catalyzed hydration of acetaldehyde is about 500M s . SECTION 7.2
Hydration and Addition
of Alcohols to Aldehydes
OH and Ketones
R C O + H 2 ∗ O R ∗ OH R C ∗ O + H O
2
2
2
R
The hydration reaction is the mechanistic prototype for many reactions at carbonyl
centers that involve more complex molecules. 11 Acid catalysis involves either proto-
nation or hydrogen-bonding at the carbonyl oxygen. Both specific and general acid
12
catalysis can be observed. (Review Section 3.7.1.1 for the discussion of specific and
general acid catalysis.)

Specific acid – catalyzed mechanism
+
RCH O + H+ RCH OH
+ slow
RCH OH + H O RCH – OH RCH(OH) 2 + H +
2
+ OH 2
General acid – catalyzed hydration
RCH O + HA RCH O---HA
slow
RCH O---HA + H O RCHOH + A – RCH(OH) 2 + HA
2
+ OH 2

Hydroxide ion addition results in specific base-catalyzed hydration. General base
catalysts function by deprotonating water to give the more nucleophilic hydroxide ion.
Specific base – catalyzed hydration
slow
– –
OH + RCH = O RCH O
OH
RCH O – + H O RCH(OH) 2 + – OH
2
OH
General base – catalyzed hydration
slow
B – + H O + RCH = O RCH O – + B – H
2
OH
RCH O – + H O RCH(OH) 2 + – OH
2
OH
B – H + – OH B – + H O
2

10
P. Greenzaid, Z. Luz, and D. Samuel, J. Am. Chem. Soc., 89, 756 (1967).
11 R. P. Bell, Adv. Phys. Org. Chem., 4, 1 (1966); W. P. Jencks, Chem. Rev., 72, 705 (1972).
12
L. H. Funderburk, L. Aldwin, and W. P. Jencks, J. Am. Chem. Soc., 100, 5444 (1978); R. A. McClelland
and M. Coe, J. Am. Chem. Soc., 105, 2718 (1983).