Reactions and characterisation of aliphatic carboxylic acids

Test the solution so obtained for unsaturation by adding cold 1 per cent, potassium permanganate solution a drop at a time. The immediate disappearance of the purple colour and the formation of a brown turbidity indicates the presence of a double bond Baeyer a test). It must be noted that many substances, not unsaturated, decolourise warm acid or neutral potassium permanganate solution. 

Test a small quantity of the aqueous solution or extract of the carboxylic acid with litmus or with Universal indicator paper. 

Amides, anilides and p toluidides. The dry acid is first converted by excess of thionyl chloride into the acid chloride  

The by-products are both gaseous and the excess of thiouyl chloride (b.p. 78°) may be readily removed by distillation. Interaction of the acid chloride with ammonia solution, aniline or p-toluidiiie yields the amide, anilide or p-toluidide respectively  

Stopper the side arm of a 25 or 50 ml. distilling flask and fit a vertical water condenser into the neck. Place 0-5-1 -0 g. of the dry acid (finely powdered if it is a solid) into the flask, add 2-5-5 0 ml. of redistilled thionyl chloride and reflux gently for 30 minutes it is advisable to place a plug of cotton wool in the top of the condenser to exclude moisture. Rearrange the condenser and distil off the excess of thionyl chloride t (b.p. 78°). The residue in the flask consists of the acid chloride and can be converted into any of the derivatives given below. 

RCOC1 + 2NHS — RCONH2 + NH4C1 RCOC1 + 2R NH2 — RCONHR + R NH2,HC1 

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