A Preview of Carbonyl Compounds

Carbonyl compounds are everywhere in nature. The majority of biologically important molecules contain carbonyl groups, as do most pharmaceutical agents and many of the synthetic chemicals that touch our eveiyday lives. Acetic acid, the chief component of vinegar acetaminophen, the active ingredient in many over-the-counter headache remedies and Dacron, the polyester material used in clothing, all contain different kinds of carbonyl groups. 

There are many different kinds of carbonyl compounds, depending on what groups are bonded to the C=0 unit. The chemistry of all carbonyl groups is similar, however, regardless of their exact structure. 

All contain an acyl group, R—c- bonded to another residue. The R substituent of the acyl group may be alkyl, aryl, alkenyl, or alkynyl the other substituent to which tlie acyl fragment is bonded may be a carbon, hydrogen, oxygen, halogen, nitrogen, sulfur, or other atom. 

The -R and -H in these compounds can t act as leaving groups in substitution reactions. 

In the next five chapters, we ll discuss the chemistry of the carbonyl group, C=0 (pronounced car-bo-iieel). Although there are many different kinds of carbonyl compounds and many different reactions, there are only a few fundamental principles that tie the entire field together. The purpose of this brief preview is not to show details of specific reactions but rather to provide a framework for learning carbonyl-group chemistry. Read through this preview now, and return to it on occasion to remind yourself of the larger picture. 

The -OH, -X, -OR, -SR, -NH2, -OCOR, and -0P03 in these compounds can act as leaving groups in nucleophilic substitution reactions. 

The Ccirbon-oxygen double bond of a carbonyl group is similar in many respects to the carbon-carbon double bond of an alkenc. The carbonyl carbon atom is sp -hybridized and forms three a bonds. The fourth valence electron remains in a carbon p orbital and forms a tt bond to oxygen by overlap with an oxygen p orbital. The oxygen atom also has two nonbonding pairs of electrons, which occupy its remaining two orbitals. 

Both in the laboratory and in living organisms, the reactions of carbonyl compounds take place by one of four general mechanisms nucleophilic addition, nucleophilic acyl substitution, alpha substitution, and carbonyl condensation. These 

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As we saw in A Preview of Carbonyl Compounds, the most general reaction of aldehydes and ketones is the nucleophilic addition reaction. A nucleophile, Nu-, approaches along the C=0 bond from an angle of about 75° to the plane of the carbonyl group and adds to the electrophilic C=0 carbon atom. At the same time, rehybridization of the carbonyl carbon from sp2 to sp3 occurs, an electron pair from the C=0 bond moves toward the electronegative oxygen atom, and a tetrahedral alkoxide ion intermediate is produced (Figure 19.1). 

As a general rule, nucleophilic addition reactions are characteristic only of aldehydes and ketones, not of carboxylic acid derivatives. The reason for the difference is structural. As discussed previously in A Preview of Carbonyl Compounds and shown in Figure 19.14, the tetrahedral intermediate produced by addition of a nucleophile to a carboxylic acid derivative can eliminate a leaving group, leading to a net nucleophilic acyl substitution reaction. The tetrahedral intermediate... 

The chemistry of all acid derivatives is similar and is dominated by single reaction—the nucleophilic acyl substitution reaction that saw briefly in A Preview of Carbonyl Compounds ... 

We said in A Preview of Carbonyl Compounds (p. 743) that much of the chemistry of carbonyl compounds can be explained by just four fundamental reaction types nucleophilic additions, nucleophilic acyl substitutions, a substitutions, and carbonyl condensations. Having studied the characteristics of nucleophilic addition reactions and nucleophilic acyl substitution reactions in the past three chapters, let s now look in more detail at the third major carbonyl-group process —the u(-substitution reaction. 

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