Catalytic hydrogenation by bonded hydrogen

3 -(/7-Chlorophenyl)propionic acid A solution of 4-chlorocinnamic acid (5 g) in tetralin (180 ml) containing palladium black (1 g) is heated at the boiling point for 1.5 h in a Kjeldahl flask under an air condenser. The side chain is then completely reduced. The catalyst is filtered off, the acid components are removed from the filtrate in sodium carbonate solution, adhering tetralin is removed from the alkaline solution by ether, the alkaline solution is acidified by hydrochloric acid, and the 3-(p-chlorophenyl)propionic acid that separates is taken up in ether. On evaporation, the pure 3-(p-chlorophenyl)propionic acid of m.p. 123° is obtained. 

The tetralin used is purified by extraction with dilute hydrochloric acid, washing, and drying with calcium chloride and is then distilled. 

However, conversion of the acetylenic into the olefinic bond makes special preparative demands. This half-reduction can be effected by chemical reagents as well as by catalytically activated hydrogen the stable fra .s-ethylenic stereoisomers are usually obtained by the first method, whereas the second of these methods gives predominantly the metastable cis-isomers of higher energy content. 

A review of hydrogenations leading to total or partial saturation can be obtained from relevant monographs.252 

There is a relatively meagre choice of methods suitable for partial reduction of acetylene and its derivatives. In individual cases partial reduction can be effected with sodium and alcohols, zinc and glacial acetic acid, or the zinc-copper couple (cf. a review by Campbell and Campbell253). Partial reduction of the triple bond in acetylene itself was described as early as 1916 by Traube and Passarge254 who used chromium(n) chloride. A variation of this process, in which chromium(n) sulfate is used in water or aqueous dimethylformamide, has been used recently with considerable success for preparation of trans-olefins from various compounds containing C=C bonds 255 formation of by-products or conversion into m-ethylenes was not observed with this method. 

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