750 CHAPTER 18 Carbonyl Compounds II
The mechanism for enamine formation is exactly the same as that for imine forma-
tion, until the last step of the reaction. When a primary amine reacts with an aldehyde
or a ketone, the protonated imine loses a proton from nitrogen in the last step of the re-
action, forming a neutral imine. However, when the amine is secondary, the positively
charged nitrogen is not bonded to a hydrogen. A stable neutral molecule is obtained by
removing a proton from the -carbon of the compound derived from the carbonyl
compound. An enamine is the result.In aqueous acidic solutions, an enamine is hydrolyzed back to the carbonyl compound
and secondary amine, a reaction that is similar to the acid-catalyzed hydrolysis of an
imine back to the carbonyl compound and a primary amine.PROBLEM 19a. Write the mechanism for the following reactions:- the acid-catalyzed hydrolysis of an imine to a carbonyl compound and a primary amine
- the acid-catalyzed hydrolysis of an enamine to a carbonyl compound and a sec-
ondary amine
b. How do these mechanisms differ?
PROBLEM 20Give the products of the following reactions. (A catalytic amount of acid is present in each
reaction.)
a. c.
b. d.Formation of Imine Derivatives
Compounds such as hydroxylamine hydrazine and semicar-
bazide are similar to primary amines in that they all have an
group. Thus, like primary amines, they react with aldehydes and ketones to form(NH 2 NHCONH 2 ) NH 2(NH 2 OH), (NH 2 NH 2 ),cyclopentanone+diethylamine acetophenone+cyclohexylaminecyclopentanone+ethylamine acetophenone+hexylamineHCl
O +
++ H 2 O
N
H HNa−H+
H+an enaminethis intermediate cannot lose
a proton from N, so it loses
a proton from an α-carbonN–protonated
carbinolamineO–protonated
carbinolamineelimination
of waterneutral tetrahedral
intermediate
a carbinolaminemechanism for enamine formationONH+ R+ H 2 OO
NRHB+ HB+R
HRR
HRRR+ +N−NRRNOHBBOHN
R
HRRR
+ +OH
NH