Glycerol
Glycerol
Glycerol or
propane-1,2,3-triol is the only important trihydric alcohol, b.p. 290oC.
It occurs in almost all animal and vegetable oils and fats as the glyceryl
esters of mainly palmitic, stearic and oleic acids.
Manufacture of glycerol
Glycerol is
obtained in large quantities as a by-product in the manufacture of soap, and
this is still a commercial source of glycerol. It is prepared synthetically as
follows:
An
alternative route that is used is:
Reaction of glycerol
1. When glycerol is treated with sodium, one α-bydroxyl group is readily attacked and the other α-group less readily; the β-hydroxyl groups is not attacked at all:
HOCH2CHOHCH2OH (Na) ⟶ NaOCH2CHOHCH2OH
(Na) ⟶ NaOCH2CHOHCH2ONa
2. On passing
hydrogen chloride into glycerol at 110oC until there is the
theoretical increase in weight corresponding to the esterification of one
hydroxyl group, both α- and β-glycerol
monochlorohydrin are formed, the former predominating (66 per cent).
Continued action of hydrogen chloride at 110oC, using 25 per cent of
acid in excess required by theory for the esterification of two hydroxyl
groups, produces glycerol α,α-dichlorohydrin (α-dichlorohydrin) and glycerol α,β-dichlorohydrin (β-dichlorohydrin), t he former predominating (55-57 per
cent); some other products are also formed:
HOCH2CHOHCH2OH (HCl) ⟶ ClCH2CHOHCH2OH +
HOCH2CHClCH2OH
(HCl) ⟶ ClCH2CHOHCH2Cl
+ ClCH2CHClCH2OH
When either
of these dichlorohydrins or glycerol itself is treated with phosphorus
penta-chloride, glycerol trichlorohydirin (1,2,3-trichloropropane) is obtained.
This is a liquid, b.p. 156-158oC.
3. when
glycerol is treated with monocarboxylic acids, esters are obtained which may be
mono-, di- or tri-esters, according to the amount of acid used; high
temperature and an excess of acid favour the formation of the tri-ester.
Use of glycerol
Glycerol is
used as antifreeze, for making explosives, and because of its hygroscopic
properties, as a moistening agent for tobacco, shaving soaps, etc.
Glycerol
contains one secondary and two primary alcoholic groups, and it undergoes many
of the reactions to be expected of these types of alcohols. The carbon atoms in
glycerol are indicated as shown: αCH2OH βCHO αHorC YH2OH
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