Reactions and characterisation of polyhydric alcohols

Some characteristic reactions of polyhydric alcohols are given below  

This reaction is due to the combination of two da hydroxyl groups of the compound with the boric acid to form a much stronger monobasic acid  

Add 1 drop (0 05 ml.) of concentrated nitric acid to 2 0 ml. of a 0 5 per cent, aqueous solution of paraperiodic acid (HjIO,) contained in a small test-tube and shake well. Then introduce 1 op or a small crystal of the compound. Shake the mixture for 15 seconds and add 1-2 drops of 5 per cent, aqueous silver nitrate. The immediate production of a white precipitate (silver iodate) constitutes a positive test and indicates that the organic compound has been oxidised by the periodic acid. The test is based upon the fact that silver iodate is sparingly soluble in dilute nitric acid whereas silver periodate is very soluble if too much nitric acid is present, the silver iodate will not precipitate. 

An alternative procedure for the above test is as follows. Mix 2-3 ml. of 2 per cent, aqueous paraperiodic acid solution with 1 drop of dilute sulphuric acid (ca. 2 5N) and add 20-30 mg. of the compound. Shake the mixture for 5 minutes, and then pass sulphur dioxide through the solution until it acquires a pale yellow colour (to remove the excess of periodic acid and also iodic acid formed in the reaction). Add 1-2 ml. of Schiff s reagent (Section 111,70) the production of a violet colour constitutes a positive test. 

Benzoates. The preparation of benzoates of polyhydric alcohols may be illustrated by reference to glycerol. They are usually crystaUine solids. 

CHOHR + HI04 — RCHO + RCHO + H20 + HIOa RCHOH.COR + HIOt — RCHO + R COOH + HIOs 

CHOHR + HIO4 — ECHO + R CHO + HjO + HIO, RCHOH.COR + HIO4 —3- RCHO -f- R COOH HIO, 

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The experimental procedure to be followed depends upon the products of hydrolysis. If the alcohol and aldehyde are both soluble in water, the reaction product is divided into two parts. One portion is used for the characterisation of the aldehyde by the preparation of a suitable derivative (e.g. the 2,4-dinitro-phenylhydrazone, semicarbazone or dimethone, see Aldehydes and ketones, Section 9.6.13, below). The other portion is employed for the preparation of a 3,5-dinitrobenzoate, etc. (see Alcohols and polyhydric alcohols, Section 9.6.4, p. 1241) it is advisable first to concentrate the alcohol by distillation or to attempt to salt out the alcohol by the addition of solid potassium carbonate. If one of the hydrolysis products is insoluble in the reaction mixture, it is separated and characterised. If both the aldehyde and the alcohol are insoluble, they are removed from the aqueous layer separation is generally most simply effected with sodium metabisulphite solution (compare Expt 5.82), but fractional distillation may sometimes be employed. 

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