Lone pairs acyl halides

The electron-withdrawing effect of the halogen, coupled with that of the carbonyl oxygen, leads to a very electron-deficient carbon, and this is not effectively counteracted by the lone pairs on halogens such as chlorine. Consequently, the carbonyl carbon atom is very sensitive to nucleophilic addition to form a tetrahedral intermediate. The collapse of the tetrahedral intermediate with the expulsion of the halide ion, which is a good leaving group, enhances the reactivity of the acyl halides (Scheme 3.64a). The direct fission of the acyl halide C-X bond leads to the formation of an electrophilic acylium ion (Scheme 3.64b). 

The acylium ion (RCO ), which may be generated from an acyl halide and a Lewis acid catalyst, also achieves some stabilization from oxygen lone pairs, and the carbocation remains localized on the carbonyl group. Consequently, Friedel-Crafts acylation is not accompanied by the rearrangements that affect alkylations (Scheme 4.7). 

The extent of resonance can be observed directly in the structures of carboxylic acid derivatives. In the progression from acyl halides to esters and amides, the C-L bond becomes progressively shorter, owing to increased double-bond character (Table 20-1). The NMR spectra of amides reveal that rotation about this bond has become restricted. For example, W,N-dimethylformamide at room temperature exhibits two singlets for the two methyl groups, because rotation about the C-N bond is very slow on the NMR time scale. The evidence points to considerable tt overlap between the lone pair on nitrogen and the carbonyl carbon, as a result of the increased importance of the dipolar resonance form in amides. The measured barrier to this rotation is about 21 kcal moF (88 kJ moF ). 

These reactions occur at about 100°C using aluminum chloride as a catalyst. This accepts a lone pair of electrons from the halogen atom on the haloalkane or acyl halide, which results in a positive charge on the adjoining carbon atom. The reaction is then ELECTROPHILIC SUBSTITUTION. It is also possible to use alkenes (for alkylation) and acid anhydrides (for acylation). It is named for the French chemist Charles Friedel (1832-99) and the US chemist James Crafts (1832-99). 

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