A Preview

REACTIONS THAT LEAD TO AMINES A REVIEW AND A PREVIEW... 

Reactions That Lead to Amines A Review and a Preview... 

As each functional grotip is discussed in future chapters, mass-spectral fragmentations characteristic of that group will be described. As a preview, though, we Jl point out some distinguishing features of several common functionaj groups. 

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 a single reaction—the nucleophilic acyl substitution reaction that we saw briefly in A Preview ofC ubonyl Compowuls. 

We said in A Preview ofCnrbonyl Compounds that much of the chemistry of carbonyl compounds can be explained by just four fundamental reaction types nucleophilic additions, nucleophilic acyl substitutions, o substitutions, and carbonyl condensations. Having studied the first two of these reactions in the past three chapters, let s now look in more detail at the third major carbonyl-group process—the a-substitution reaction. 

I An overview chapter, A Preview of Carbonyl Chemistry, follows Chapter 18 and highlights the author s belief that studying organic chemistry requires both summarizing and Looking ahead. 

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