P-Cyanobenzoic acid

Problem 18.35 Using C Hj, C HjCHj, via diazonium salts and any other needed reagents, prepare (a) o-chlorotoluene, (h) m-chlorotoluene, (c) 1,3,5-tribromobenzene, (d) m-bromochlorobenzene, (c) p-iodotoluene, (/) p-dinitrobenzene and (g) p-cyanobenzoic acid. Do not repeat the synthesis of intermediate products. <... 

Peradds. Swem s group found this acid to be superior to sulfuric acid as the solvent and catalyst for the conversion of carboxylic acids into peroxy acids. It can be used even with some acids containing acid-labile groups. It has good solvent power for carboxylic acids, but the peroxy acid usually precipitates from the medium as it is formed. Peracids are obtained in 75-98% yield from /-butyl-, o- and p-nitro-, and p-cyanobenzoic acid, terephthalic acid, lauric acid, palmitic acid, stearic acid, 12-hydroxystearic acid, and a-bromostearic acid. The method failed with m- and p-methoxybenzoic acid. 

Pincock and his co-workers have studied the photochemical fragmentation reactions of the esters (31). This system has an in-built electron accepting sensitiser. When (31a-c) are irradiated in methanol the principal reaction is fission to yield the styrene (32) and p-cyanobenzoic acid. The other products formed from the reactions are the styrene addition products (33)-(35). The authors propose that the Norrish T) e II process in this instance involves a proton transfer and this occurs within the zwitterionic biradical formed as the primary intermediate on electron transfer. Further proof of the authenticity of this mechanism was obtained by irradiation of the deuteriated derivatives (31 d, e). The results of a study of the photochemical decomposition of benzyl phenylacetate, as a suspension in water over Ti02, have been reported. Bond fission is the result of irradiation of (36) in cyclohexane/ethyl acetate. A Norrish Type II hydrogen abstraction occurs with the elimination of the enone moiety. This affords a path to the CD ring system (37) of vitamin D. 

The lower yield of trimerized product from the o-nitrobenzo-nitrile compared to p-nitrobenzonitrile can be accounted for by steric effects. The very hi yield of trimerized product from the p-cyanobenzoic acid mi t have resulted from a reaction in which the p-cyanobenzoic acid itself served as a CO-catalyst. 

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