Glycerol: Preparation, Properties, Uses and Test
Glycerol is also known as glycerine and is a tri-hydric alcohol. The name glycerol or glycerine was derived from the word glyceros which means sweet. Glycerol is mainly obtained from different plants and animals. Its IUPAC name is 1,2,3,-propanetriol.
It is odourless and Colourless hygroscopic liquid which is soluble in water. The molecular formula of glycerol is C3H8O3, molar mass is 92.094 g·mol−1, density is 1.261 g/cm3, melting point is 17.8 °C, and the Boiling point is 290 °C. Glycerol is used mainly in Personal Care Products, pharmaceuticals, and the food industry. Dustan's test is used to test the glycerol.
Preparation of Glycerol
Glycerol can be prepared industrially by the following methods.
By the Hydrolysis of Oils and Fats
Natural oils and fats are the triesters of glycerol with higher organic acids, mainly palmitic acid, CH3(CH2)14C00H, stearic acid, CH3(CH2)16C00H, and oleic acid, CH3(CH2)7CH=CH(CH2)7COOH. Upon hydrolysis with alkali, the fats and oils produce glycerol and the salt of the above acids which are called soaps.
The hydrolysis of fats is carried originally either for soap manufacture or for the production of stearic acid needed in the candle industry, and glycerol is obtained as a by-product. This is still a commercial source of glycerol.
Glycerol from Soap Manufacture
In soap manufacture, the fat is hydrolysed by boiling with alkali solution. The free fatty acids produced by hydrolysis react with the alkali to form solid soap white the glycerol is left in solution (Spent lye).
After the removal of soap, the spent lye containing 3 to 5 per cent glycerol is treated for its recovery. The impurities present in it are precipitated by adding alum and the filtrate concentrated by evaporation. Since glycerol oses much below its boiling point, the evaporation is carried under vacuum, The concentrated solution which now contains about 80 per cent glycerol is treated with animal charcoal and purified by
distillation with super-heated steam under reduced pressure. The distiliate is then evaporated in vacuo unit the glycerol has specific gravity of 1.26.
Glycerol from Candle Industry
Its making candles from wax, the latter is mixed with a little stearic acid so that it may not soften on heating The stearic acid needed for the purpose is obtained by the hydrolysis of solid fats carried by heating with water (or steam) under pressure using sulphuric acid as a catalyst. The solid stearic acid is removed and the remaining solution (Sweet Water) after neutralisation is treated for the recovery of glycerol as described before.
By the Fermentation of Sugar
Glycerol is also obtained by the fermentation of molasses or sugar in the presence of sodium sulphite.
C6H12O6 → C3H8O3 + CH3–CHO + CO2
Sodium sulphite upon hydrolysis yields an alkaline medium which favours the above course of fermentation. The yield of glycerol is as high as 20-25 per cent. This method was used by Germany during war when there was acute shortage of fats.
Synthetic Methods
In USA, over half the annual production of glycerol is now made by synthetic methods. In the following two methods propylene obtained from petroleum: the starting material.
Via Allyl Chloride: Propylene is chlorinated at 400-600° to give allyl chloride which upon treatment with hypochlorous acid gives glyceryl dichlorohydrin. The dichlorohydrin when reacted with lime forms epichlorohydtin which when bydrolysed with sodium hydroxide yields glycerol.
Via Acrolein: Propylene is oxidised with oxygen in the presence of CuO catalyst at 350° to produce acrolein. This is then reduced to allyl alcohol by isopropyl alcohol in the presence of MgO + ZnO catalyst at 400. The ally[ alcohol upon addition of hydrogen peroxide in the presence of tungsten oxide (WO3) as catalyst yields glycerol.
In a country like India where petroleum is not found in abundance, propylene required for the above syntheses may also be obtained from acetone of the wood distillation industry.
Properties of Glycerol
Physical Properties of Glycerol
Glycerol is a colorless, odorless, and thick liquid and sweet in taste. Boiling point is 290°C but on heating it starts decomposing before this temperature. It is miscible in ethanol and water while immiscible in ether. On cooling it forms transparent crystal whose melting point is 180°C. It is heavier than water and its relative density is 1.261 g/cm3.
Chemical Properties of Glycerol
Glycerol molecule is made of two primary alcoholic groups and one secondary alcoholic group, and it undergoes many of the reactions to be expected of these types of alcohols. The two primary alcoholic groups are more reactive than the secondary alcoholic group.
Reaction with Sodium
When glycerol is treated with sodium at room temperature, one a-hydroxyl group is attacked to form monosodium glycerolate. At higher temperatures the second α-hydroxyl is also attacked to give disodium glycerolate. β-hydroxyl group, however, does not react at all.
Reaction with Hydrogen Chloride
When hydrogen chloride is passed into glycerol at 110°, both α- and β-glycerol monochiorohydrins are obtained. Upon continued action of hydrogen chloride at 110o glycerol; α,α'-dichlorohydrinand glycerol α- and β-dichlorohydrin are produced.
Reaction with Phosphorus Pentachloride
Glycerol reacts with phosphorus pentachloride to form glyceryl trichioride, all the three OH groups in it being replaced by Cl atoms.
Reaction with Organic Acids
When glycerol is treated with monocarboxylic acids, mono-, di- or tri-esters may be obtained depending on the amount of acid used. With acetic acid and acetic anhydride, glycerol forms three esters.
Reaction with Nitric Acid
Glycerol reacts with nitric acid in the presence of sulphuric acid at temperatures below 25o C to form glyceryl trinitrate, commonly known as nitroglycerine.
Reaction with Hydrogen Iodide
When glycerol is warmed with a small amount of hydrogen iodide, it gives glyceryl triodide. The triodide being unstable at once splits out a molecule of iodine to form allyl iodide.
When glycerol is heated with a large amount of hydrogen iodide, allyl iodide first produced as above is reduced to propylene. In the presence of excess of hydrogen iodide, propylene adds a molecule of hydrogen iodide giving isopropyl iodide.
Reaction with Oxalic acid
Glycerol reacts with oxalic acid in two ways.
Formation of Formic Acid: When heated with crystalline oxalic acid to 110o C, glycerol forms glycerol monoformate which upon hydrolysis gives formic acid and glycerol is regenerated.
Formation of Allyl alcohol: When glycerol is heated with oxalic acid to 230o C, glycerol dioxalaté is produced. It at once splits out two molecules of carbon dioxide and forms ally] alcohol.
Oxidation of Glycerol
The two primary alcohol groups in glycerol are capable of being oxidised to titealdehyde and then the carboxyl group, while the secondary alcohol group can be oxidised to the carbonyl group. Thus glycerol can give rise to a variety of oxidation products depending on the nature of the oxidising agent used.
Thus dilute nitric acid converts glycerol into glyceric acid and tartonic acid, while concentrated nitric acid oxidises it to mainly glyceric acid. Bismuth nitrate produces mainly mesoxalic acid, Sodium hypobromite, or hydrogen peroxide and ferrous sulphate (Fenton's reagent) oxidises glycerol to a mixture of glyceraldehyde and dihydroxyacetone.
Glycerol undergoes oxidative cleavage by periodic acid giving formaldehyde and formic acid.
When glycerol is dropped on solid potassium permanganate, it is ignited by the heat of reaction and burns with a blue flame.
Dehydration of Glycerol
When heated alone or with potassium hydrogensuiphate, glycerol eliminates two molecules of water to form acrolein.
Formation of Glyptal Resins
Glycerol reacts with phthalic anhydride forming polyesters known as glyptals. Each of the three OR groups in glycerol forms an ester linkage With the anhydride, giving a thermosetting polymer (plastic) used for making synthetic fibers.
Uses of Glycerol
Important uses of glycerol are given below-
1. As a sweetening agent in beverages and confectionary.
2. In the preparation of high class toilet soaps and cosmetics.
3. In preserving tobacco from drying out.
4. As antifreeze in automobile radiators.
5. For lubricating fine machinery such as watches and clocks.
6. As a preservative for fruits and other eatables which require to be kept moist.
7. In making printing inks and inks for stamp pads.
8. In the preparation of nitroglycerine.
9. As a starting material for several allyl and propyl derivatives.
10. For the manufacture of glyptal plastics used for artificial fibers.
Tests of Glycerol
Acrolein Test
When glycerol is heated with potassium hydrogen sulfate, a pungent odour of acrolein (CH2=CH–CHO) is produced. The presence of glycerol is confirmed by the characteristic smell of acrolein.
Dunstan's test or Borax Test
Glycerol mix with borax solution and a few drops of phenolphthalein indicator, the color of mixture turns into pink. The solution turns colorless when cooled, but the color returns when heated. The presence of glycerol is indicated by the pink coloration, which is due to the formation of a borax-glycerol complex.
Formic Acid Test
When glycerol is heated with oxalic acid, formic acid is produced which indicates the presence of glycerol.
Schiff's Test
When a few drops of Schiff's reagent is added to the glycerol solution, glycerol reduces the Schiff's reagent and the solution turns pink or magenta which confirms the presence of glycerol.
Iodine Test
Glycerol mixed with iodine and a base, such as sodium hydroxide, the solution turns yellow or brown. The color change indicates the presence of glycerol, as it forms a complex with iodine and the base.
Source: Advanced Organic Chemistry: B.S.Bahl and Arun Bahl Download pdf
