Glycerol or Glycerine

Glycerol is a trihydric alcohol. Its IUPAC name is propane-1, 2, 3-triol and in industry it is known as glycerine.

Manufacture of Glycerol

1.  From Fats and Oils : On hydrolysis of fats and oils (i.e. glycerides or ester of higher fatty acids) glycerol and fatty acids are formed.

Glycerol is obtained as by-product in soap and candle industry.

2.By Fermentation of sugar :

Alcoholic fermentation of sugar gives 3% glycerol. However, if the fermentation is done in presence of sodium sulphite, the yield can be increased to 25%.

3.  Synthesis (From Propene):

​Properties of Glycerol

Physical

(a)It is a colourless, odourless, viscous and hygroscopic liquid, sweet in taste and non-toxic in nature.

(b)It is soluble in water and ethyl alcohol but insoluble in ether.

(c)The high viscosity and high boiling point of glycerol are due to strong hydrogen bonding.

Chemical :

The molecule of glycerol contains two primary -OH groups and one secondary -OH group. The carbon atoms in glycerol are indicated as α,β and α'.

Primary -OH groups are more reactive than secondary -OH group.

(i) Action of sodium : Only primary alcoholic groups are attacked one by one and secondary alcoholic group is not attacked.

(ii) Action of PCI₅ :

(iii)Action of HCI or HBr :

(iv)Action of HI : Glycerol reacts with HI in two ways :

(a) When glycerol is warmed a small amount of hydrogen iodide, allyl iodide is formed.

(b)When glycerol is heated with excess of HI, the allyl iodide formed first is reduced to propene. Propene in presence of excess of HI forms iso-propyl iodide.

Note : Same products are obtained when glycerol is treated with phosphorus tri-iodide.

(v) Action of nitric acid :

Glycerol trinitrate is an yellow oily, poisonous liquid and causes headache. It explodes violently when heated rapidly or subjected to sudden shock.

       C₃H₅ (ONO₂)₃    →    12CO₂ + 10H₂O + 6N₂ + O₂

It is a safer explosive when absorbed on kieselguhr and known as dynamite. Some other important explosives prepared from glycerol trinitrate are :

(a)Blasting gelatin : A mixture of glycerol trinitrate an cellulose nitrate (gun cotton).

(b)Cordite : It is obtained by mixing glycerol trinitrate with gun cotton and Vaseline. It is smokeless explosive.

(vi)Action of CH₃COOH, acetic anhydride or acetyl chloride : Mono, di and trimesters of glycerol are formed.

(vii)  Action of oxalic acid : Different products are formed under different conditions.

(a)At 110^oC and with excess of oxalic acid, formic acid is formed.                 

(b)At 260^oC, allyl alcohol is formed.

(viii) Dehydration : Glycerol on dehydration by potassium hydrogen sulphate or phosphorus pentaoxide or concentrated sulphuric acid gives acrolein or acraldehyde which has a characteristic bad odour.

         This reaction can be used as a test of glycerol.

(ix) Oxidation : Glycerol gives different oxidation products depending on the nature of oxidizing agent.

Oxidation of Glycerol with ............

(a)Dilute HNO₃ gives mainly glyceric acid.

(b)Conc. HNO₃ gives glyceric acid and tartronic acid.

(c)Bismuth nitrate gives mainly mesoxalic acid.

(d)Fenton's reagent (H₂O₂ + F₂SO₄) or NaOBr or Br₂-water in presence of Na₂CO₃gives glyceraldehydes and dihydroxy acetone.

(e)  Solid KMnO₄, gives oxalic acid and carbon dioxide. The reaction is violent and explosion occurs.
 

Uses of Glycerol

(a) As a sweetening agent in confectionary, beverages and medicines due to nontoxic nature.

(b) As antifreeze in automobile radiators.

(c)In the manufacture of plastics & synthetic fibres.

(d)  In the manufacture of explosives like dynamite and cordite.

(e)  As a lubricant in watches.

(f) In the preparation of good quality of soaps, hand lotions, vanishing creams, shaving creams and tooth pastes.

(g)In the manufacture of various organic compounds like, formic acid, allyl alcohol, acrolein, etc.

(h) As a preservative.

(i) In the preparation of non-drying inks, printing inks and stamp pad inks.
 

Dunstan's test for Glycerol

Phenolphthalein drops in borax solution give pink colour. Addition of glycerol to this solution discharges pink colour in cold which however reappears on heating and again disappears on cooling. This in Dunstan's test for glycerine.

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