Chemistry Reference
In-Depth Information
enamine formation
secondary amine
O
N
H
N
H
+
- H
2
O
N
N
N
N
H
+
−
O
H
HO
H
2
O
H
α
β
nucleophilic attack
on to carbonyl
enamine
loss of leaving
group
no proton on N;
therefore, lose proton
from
β
-position
7.8 Nucleophilic substitution on
carbonyl groups: carboxylic acid
derivatives
We have seen that most of the addition reactions
involving aldehydes and ketones are reversible (see
Section 7.1.2). If the anionic intermediate does not
react with an electrophile, then the carbonyl group is
reformed and the original nucleophile is lost, i.e. it
becomes a leaving group.
nucleophilic addition
E
+
Nu
O
Nu
C
O
Nu
C
OE
reversible process,
i.e. Nu can be lost
if there is another leaving group in the molecule, can get
nucleophilic substitution
O
O
C
O
L
L
L
Nu
Nu
Nu
tetrahedral anionic
intermediate
this is really an
addition-elimination
sequence;
O
it may also be considered as
acylation
of the nucleophile
acyl
=
R
However, if there is another leaving group in
this molecule, then this may be lost instead, so
that overall the reaction becomes a
nucleophilic
substitution
, though really it should be regarded
as
carbonyl group is essential for this reactivity. This
is readily appreciated if one compares the reac-
tivity towards water of acetyl chloride and ethyl
chloride.
an
addition - elimination
sequence,
since
the
O
O
H
2
O
occurs readily at
room temperature
H
3
C
Cl
H
3
C
OH
acetyl chloride
H
H
H
H
H
2
O
no reaction at room
temperature; requires base
and high temperature
H
3
C
Cl
H
3
C
OH
ethyl chloride
