Chemistry Reference
In-Depth Information
H
3
C
H
3
C
d
+
d-
d+
d-
aldehydes are more
reactive than ketones
CO
CO
H
3
C
H
the aldehyde carbon is more
electrophilic than the ketone carbon
etc.
aromatic aldehydes are less
reactive than aliphatic aldehydes
d+
d-
O
O
O
H
H
H
aromatic compounds are less reactive;
the aromatic ring delocalizes positive
charge away from carbonyl carbon
into a tetrahedral
sp
3
system in the product (bond
angle 109
◦
) creating more steric crowding, i.e. the
groups are brought closer together.
The second feature is a steric consideration. During
nucleophilic addition, the planar
sp
2
system of the
carbonyl compound (bond angle 120
◦
) is converted
note formation of
chiral centre
R
Nu
NuH
the addition reaction
increases steric crowding
109º
120º
O
O
R
R′
R′
formation of new bond creates
more steric crowding
7.1.2 Nucleophiles and leaving groups:
reversible addition reactions
This crowding is more severe with two alkyl
substituents (from ketones) than with one alkyl and
the much smaller hydrogen (from aldehydes). A
consequence of this change is that the planar aldehyde
or ketone can be attacked from either side of the plane
with essentially equal probability. If the substituents
are all different, then this will result in the creation
of a chiral centre; but, since both enantiomers will
be formed in equal amounts, the product will be
an optically inactive racemate (assuming no other
chiral
In principle, all carbonyl addition reactions could
be reversible; but, in practice, many are essentially
irreversible. Let us consider mechanisms for the
reverse of the nucleophilic addition reactions given
above. For the base-catalysed reaction, we would
invoke the following mechanism:
centres
are
present
in
the
R
groups);
see
Section 3.4.1.
Nu
O
Nu
O
Nu
O
H
OH
nucleophile as
leaving group
carbonyl formation
with loss of leaving
group
base removes
proton
