Physical Properties and Structures of Acyl Compounds

Acyl compounds are quite polar and have boiling points substantially higher than those of the alkanes. Amides, like carboxylic acids, form hydrogen-bonded dimers and oligomers, and are exceptionally high boiling. Table 18.1 gives some physical properties of the acyl compounds related to acetic acid. 

TABLE 18.1 Some Properties of Acyl Compounds Related to Acetic Acid 

The amoimt of resonance stabilization varies in these derivatives of carboxylic acids. The acid halides are the least stabflized, followed by anhydrides, the esters and acids, and finally amides, which are the most stabilized. Now two questions arise How do we know the relative amoimt of resonance stabilization, and what factors contribute to the order The first question must wait until the next section on spectroscopy, but we can deal with the second question right here. 

First of all, the positive charge in the polar resonance form is better accommodated by the relatively electropositive nitrogen of amides than by the more electronegative oxygen of esters and acids, or the chlorine of acid chlorides. 

Therefore, the polar resonance form contributes most in amides, and they are the most stabilized. Second, in acid chlorides, resonance stabilization requires overlap between a 7 p orbital on chlorine and the carbonyl Jt orbital made up of 2p orbitals. In the other acid derivatives the overlap is between 2p orbitals. 

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