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a nucleophile to add, preferentially, to the secondary site generating the reaction outcome
presented in Scheme 7.8. Thus, in general, Markovnikov's rule states that when an acid is
added across a double bond, the conjugate base adds to the more substituted carbon atom.
7.3 ADDITIONS TO CARBONYLS
Olefins, in the absence of attached polarizing groups, generally react as described above
with reactivity mediated through the nucleophilicity of the double bond. However, repla-
cing one of the olefinic carbon atoms with oxygen results in formation of a polar carbonyl.
As shown in Figure 7.1, the polarity is described through placement of a partial negative
charge on the oxygen and a partial positive charge on the carbon. Discussions describing
the polarity of carbonyls (and other functional groups), based on the electronegativities
of the various atoms involved, were presented in Chapter 1. Addition reactions involving
carbonyls are discussed in the following paragraphs.
7.3.1 1,2-Additions
Because of the inherent polarity associated with carbonyl groups, nucleophiles are drawn to
the carbonyl carbon atoms in much the same way that nucleophiles participate in S
N
2 reac-
tions. This mechanism, alluded to in several problems presented in previous chapters, is
illustrated in Scheme 7.10 using arrow pushing. As shown, a bonding pair of electrons
joining the carbonyl oxygen atom to its associated carbon atom acts as the leaving
group, placing a full negative charge on the oxygen atom. Generally, this type of reaction
Figure 7.1 While unsubstituted olefins are not polar, carbonyls are polar.
Scheme 7.10 Nucleophiles can add to carbonyls to form alcohols.
