Chemistry Reference
In-Depth Information
Most addition reactions actually involve both steps,
but the order in which these occur depends on the
nature of the reagent and the reaction conditions.
Under
the carbonyl first, and the reaction is completed
by abstraction of an electrophile, a proton, from
the solvent.
basic
conditions,
the
nucleophile
attacks
Nu
O
HOH
Nu
OH
proton abstracted
from solvent
Under acidic conditions, electrophilic addition
occurs first, namely protonation of the carbonyl and
formation of the conjugate acid. The conjugate acid,
with a full positive charge, is now a more reactive
electrophile
group, which only has partial charge separation
due to polarization. As a result, addition can now
occur with less-reactive nucleophiles, and typically
these are uncharged and attack via their lone pair
electrons.
than
the
original
uncharged
carbonyl
protonation leads to
conjugate acid
H
O
O
H
HNu
O
H
H
Nu
OH
Nu
OH
nucleophilic attack
on to conjugate acid
loss of proton
from nucleophile
In most of the reactions that we shall encounter
there will be attack of a charged nucleophile on
to an uncharged carbonyl or, alternatively, attack
of lone pair electrons in an uncharged nucleophile
onto a charged conjugate acid. An uncharged nucle-
ophile tends to be insufficiently reactive for addi-
tion reactions to occur with an uncharged carbonyl.
At the other extreme, the combination of charged
nucleophile and charged carbonyl is not usually
favourable. Since negatively charged nucleophiles are
also bases, an acidic environment will not permit their
involvement.
The most significant change in these reactions is
the formation of the carbon - nucleophile bond; so, in
both types of mechanism, the reaction is termed a
nucleophilic addition
. It should be noted that the
polarization in the carbonyl group leads to nucle-
ophilic addition, whereas the lack of polarization
in the C
=
C double bond of an alkene leads to
electrophilic addition reactions (see Chapter 8). Car-
bonyl groups in carboxylic acid derivatives undergo
a similar type of reactivity to nucleophiles, but the
presence of a leaving group in these compounds
leads to substitution reactions rather than addition
(see Section 7.8).
7.1.1 Aldehydes are more reactive than
ketones
The reactions undergone by
aldehydes
and
ketones
are essentially the same, but aldehydes are more
reactive than ketones. There are two rational reasons
for this. Alkyl groups have an electron-donating
inductive effect (see Section 4.3.3) and the presence
of two such groups in ketones against just one in
aldehydes means the magnitude of
is reduced
in ketones. Put another way, the carbonyl group in
aldehydes is more electrophilic than that in ketones. It
should also be noted that aromatic aldehydes, such as
benzaldehyde, are less reactive than alkyl aldehydes.
This is because the aromatic ring allows electron
delocalization via a resonance effect that also reduces
the positive charge on the carbonyl carbon.
δ+
