Glycols

Glycols, in organic chemistry, the generic name given to the aliphatic dihydric alcohols. These compounds may be obtained by heating the alkylen iodides or bromides (e.g. ethylene dibromide) with silver acetate or with potassium acetate and alcohol, the esters so produced being then hydrolysed with caustic alkalis, thus:

C2H4Br2 \+ 2 C2H3O2·Ag -> C2H4(O·C2H3O)2 -> C2H4(OH)2 \+ 2 K·C2H3O2;

by the direct union of water with the alkylen oxides; by oxidation of the olefines with cold potassium permanganate solution (G. Wagner, Ber., 1888, 21, p. 1231), or by the action of nitrous acid on the diamines.

Glycols may be classified as primary, containing two −CH2OH groups; primary-secondary, containing the grouping −CH(OH)·CH2OH; secondary, with the grouping −CH(OH)·CH(OH)−; and tertiary, with the grouping >C(OH)·(OH)C<. The secondary glycols are prepared by the action of alcoholic potash on aldehydes, thus:

3(CH3)2CH·CHO + KHO = (CH3)2CHCO2K + (CH3)2CH·CH(OH)·CH(OH)·CH(CH3)2.

The tertiary glycols are known as pinacones and are formed on the reduction of ketones with sodium amalgam.

The glycols are somewhat thick liquids, of high boiling point, the pinacones only being crystalline solids; they are readily soluble in water and alcohol, but are insoluble in ether. By the action of dehydrating agents they are converted into aldehydes or ketones. In their general behaviour towards oxidizing agents the primary glycols behave very similarly to the ordinary primary alcohols (q.v.), but the secondary and tertiary glycols break down, yielding compounds with a smaller carbon content.

Ethylene glycol, C2H4(OH)2, was first prepared by A. Wurtz (Ann. chim., 1859 [3], 55, p. 400) from ethylene dibromide and silver acetate. It is a somewhat pleasant smelling liquid, boiling at 197° to 197.5° C. and having a specific gravity of 1.125 (0°). On fusion with solid potash at 250° C. it completely decomposes, giving potassium oxalate and hydrogen,

C2H6O2 \+ 2KHO = K2C2O4 \+ 4H2.

Two propylene glycols, C3H8O2, are known, viz. α-propylene glycol, CH3·CH(OH)·CH2OH, a liquid boiling at 188° to 189°, and obtained by heating glycerin with sodium hydroxide and distilling the mixture; and trimethylene glycol, CH2OH·CH2·CH2OH, a liquid boiling at 214° C. and prepared by boiling trimethylene bromide with potash solution (A. Zander, Ann., 1882, 214, p. 178).

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