Chemistry Reference
In-Depth Information
Figure 7.2 Comparison of S
N
2 and S
N
2
0
reactions as explained using arrow pushing.
Scheme 7.16 Addition of nucleophiles to
a
,
b
-unsaturated carbonyl groups as explained using
arrow pushing.
In Section 7.3.1, the addition of nucleophiles to carbonyls was directly compared to S
N
2
reactions. In recognition of these mechanistic similarities, one may anticipate that nucleo-
philes can similarly add to
a
,
b
-unsaturated carbonyl systems. Such additions are, in fact,
common and, as such, are illustrated in Scheme 7.16 using arrow pushing. As shown, the
nucleophile initially adds to the double bond with delocalization of the negative charge into
the carbonyl group generating an enolate anion. Once treated with acid, the enolate anion
becomes protonated and forms an enol. Enols, being high-energy species, readily isomerize
and regenerate the carbonyl functionality.
If, as shown in Scheme 7.17, the atoms of an
a
,
b
-unsaturated carbonyl are
numbered with 1 representing the oxygen, 2 representing the carbonyl carbon atom, and
3 and 4 sequentially representing the adjacent two olefinic carbon atoms, we notice
that addition of a nucleophile in the manner illustrated in Scheme 7.16 results in the
introduction of a new substituent at atom 4. Therefore, this type of addition is known as
a 1,4-addition.
While 1,4-additions to carbonyls are common, it is important to recognize that the same
a
,
b
-unsaturated carbonyl systems are also subject to 1,2-additions. Fortunately, these
two types of additions are highly dependent upon the form of the nucleophiles used.
