Section 17.5 How Class I Carbonyl Compounds React 681
A compound that has an carbon
bonded to an oxygen atom generally
will be unstable if the carbon is
bonded to another electronegative
atom.
sp^3
sp^3
The weaker the base, the better it is as a
leaving group.
17.5 How Class I Carbonyl Compounds React
The reactivity of carbonyl compounds resides in the polarity of the carbonyl group;
oxygen is more electronegative than carbon. The carbonyl carbon, therefore, is an
electrophile, so we can safely predict that it will be attacked by nucleophiles.
When a nucleophile attacks the carbonyl carbon of a carboxylic acid derivative, the
carbon–oxygen bond breaks and an intermediate is formed. The intermediate is
called a tetrahedral intermediatebecause the trigonal carbon in the reactant has
become a tetrahedral carbon in the intermediate. Generally, a compound that
has an carbon bonded to an oxygen atom will be unstable if the carbon is bond-
ed to another electronegative atom. The tetrahedral intermediate, therefore, is unstable
because Y and Z are both electronegative atoms. A lone pair on the oxygen reforms the
bond, and either or is expelled with its bonding electrons.
Whether or is expelled depends on their relative basicities. The weaker base
is expelled preferentially, making this another example of the principle we first saw in
Section 10.3:The weaker the base, the better it is as a leaving group. Because a weak
base does not share its electrons as well as a strong base does, a weaker base forms a
weaker bond—one that is easier to break. If is a much weaker base than
will be expelled. In such a case, and the reaction can be written as follows:
In this case, no new product is formed. The nucleophile attacks the carbonyl carbon, but
the tetrahedral intermediate expels the attacking nucleophile and reforms the reactants.
On the other hand, if is a much weaker base than will be expelled and a
new product will be formed. In this case, and the reaction can be written
as follows:
C
Z
+ Y−
R
C
Z
R Y
k 2
k− 2
C
Y
+ Z−
−
R
O O O
Y−^ is a weaker
base than Z−
k 1
k− 1
k 2 Wk- 1 ,
Y- Z-,Y-
C
Y
k 1
k− 1
+ Z−
−
R
O
C
Z
R Y
O Z
− (^) is a weaker
base than Y−
k- 1 Wk 2 ,
Z- Y-,Z-
Y- Z-
Z
C
Y
- Z
−
sp^2 sp^3 sp^2
R R
CY+
−
O
C
Z
a tetrahedral intermediate
R Y
k 2
k− 2
−
O O
k 1
k− 1
p Y- 1 k 22 Z- 1 k- 12
sp^3 sp^3
(sp^3 )
(sp^2 )
p
O
δ−
δ+
C
R Y