Glycerine or glycerol is a very useful trihydric alcohol. It is trihydric alcohol as it has three -OH groups attached to it. It is found in most vegetable and animal oils and fats, which are the triesters of glycerol with long-chain fatty acids. Therefore, oils and fats are triacylglycerols and are commonly called triglycerides. The structural formula of glycerine is C3H8O3. In the IUPAC system, the formula of glycerol is Propane 1,2,3-triol.
Preparation of Glycerine
The following methods can prepare glycerine:
- Saponification of oils and fats: Glycerine is mainly derived by reacting oils and fats with caustic soda(NaOH) solution. Since soaps are the sodium salts of fatty acids, the hydrolysis reaction of oils and fats with NaOH is called saponification. Soaps are prepared in factories using this reaction. Therefore, glycerine is obtained as a by-product from the soap factories. Glycerol is found in many natural fats in the form of glycerol fatty acids called triglycerides.
- From propylene: Glycerine is also prepared from propylene commercially. Propylene used for this preparation is derived by cracking petroleum. The cracking of petroleum is a process by which the big molecules of petroleum are broken into smaller particles with the help of heat, pressure, and catalysts. Catalyst is a substance used in chemical reactions that increases its speed but doesn’t get used up.
- Synthesis from its basic constituents: Glycerine can be synthesized from its basic constituents, that is, C, H, and O.
Physical properties of Glycerine
- Glycerine is a liquid that has no colour or smell. Its consistency is dense, and it has a sweet taste. It is hygroscopic, that is, it loves water.
- Because glycerine has three -OH (hydroxyl) groups, it undergoes considerable intermolecular H-bonding. Therefore, the glycerol molecules are deeply bonded, and thus glycerine has a viscous consistency. It is denser than ethylene glycol, with only two -OH (hydroxyl) groups per molecule.
- Because glycerine has three -OH (hydroxyl) groups, it combines with water by forming hydrogen bonds. Therefore, glycerol is highly soluble in water. Further, because it is a polar compound, it is not soluble in nonpolar and mildly polar solvents like carbon tetrachloride, chloroform, ether, etc.
- Due to considerable hydrogen bonding within the molecule, glycerol has a significantly high boiling point, higher than the boiling point of ethylene glycol. The properties of ethylene glycol are similar to glycerol as both are sweet, dense, and used to prevent automobile parts from freezing.
- It boils at a temperature of 563 K with some decomposition. Decomposition is a process by which big molecules are converted into smaller fragments, usually in the presence of energy.
- It can easily mix with water and alcohol in any ratio. However, it is not soluble in ether and chloroform.
- Glycerine is non-toxic. Human beings even use glycerine to moisturize lips and skin.
Chemical properties of Glycerine
Glycerol or glycerine has two primary and one secondary alcoholic group ( the primary alcoholic group is one where -OH is bonded to a carbon atom that is not bonded to any other carbon atom, while the secondary alcoholic group is one where -OH is bonded to a carbon atom which is bonded with several other carbon atoms). It undergoes reactions characteristic of both these groups. However, because there is a strong interaction between the three -OH groups, glycerine also undergoes some special reactions characteristic of polyhydric alcohols. Some important chemical properties of glycerine are:
- It reacts with sodium metal. Sodium easily reacts with one primary alcoholic group of glycerol at room temperature and less readily with the second primary alcoholic group at 373 K with the release of hydrogen gas.
- It reacts with halogen acids such as hydrogen chloride to produce different products. Glycerol and dry HCl (1:1) react at 383 K to form a combination of alpha- and beta- glycerol monochlorohydrin. However, if we pass more HCl gas at 383 K, glycerol produces a combination of alpha- dichlorohydrin and beta- dichlorohydrin. Glycerol gives similar reactions when reacted with hydrogen bromide. With hydrogen iodide, different types of products are obtained under different conditions.
- It reacts with concentrated nitric acid or sulphuric acid to give several products. This reaction is called nitration. When glycerol is mixed slowly with concentrated sulphuric acid and concentrated nitric acid at 283-298 K, glyceryl trinitrate is obtained. It is also called trinitroglycerin or Noble’s oil. Glyceryl trinitrate is an oily liquid which lacks colour. It explodes aggressively if we try to heat it.
- It reacts with potassium hydrogen sulphate. If we heat glycerol in the presence of anhydrous potassium hydrogen sulphate, zinc chloride or concentrated sulphuric acid, it undergoes dehydration to form acrolein— an alpha, beta-unsaturated aldehyde. Dehydration is when a reactant loses one or more water molecules during a chemical reaction.
- Oxidation: Glycerol can form many products on oxidation. However, the actual product obtained depends specifically on the oxidising agent used. With dilute HNO3, a combination of glyceric acid and tartronic acid is produced. With concentrated HNO3, mostly glyceric acid is obtained. With bismuth nitrate, only mesoxalic acid is the only product formed. Bromine water, sodium hypobromite and Fenton’s reagent are mild oxidising agents. They form a mixture of glyceraldehyde and dihydroxyacetone reactions with glycerol. This mixture is called glycerose.
- With periodic acid, glycerol undergoes oxidative breakdown to form formaldehyde and formic acid. During this oxidation, periodic acid itself is reduced to iodic acid.
- Reaction with acidified potassium permanganate: When heated with a solution of acidified potassium permanganate, glycerol gets oxidised to form oxalic acid, carbon dioxide and water.
- Reactions with phosphorus halides: All the three hydroxyls (-OH) groups of glycerol react when treated with PCl5 or PCl3 to form 1, 2, 3- trichloropropane. With PBr3, glyceryl tribromide is formed, whereas PI3 behaves exactly like HI; that is, allyl iodide is obtained with a small amount of PI3.
- Reactions with monocarboxylic acids: Glycerol reacts with monocarboxylic acids to form mono-, di-, and tri- esters. This reaction depends upon the quantity of acid used and the temperature conditions. A high volume of acid and a high temperature leads to the formation of triesters. For example, acetic acid, glycerol monoacetate, diacetate and triacetate are formed.
- Acetylation: Glycerol gives rise to glycerol triacetate on reacting with acetyl chloride.
- Reaction with oxalic acid:
- When oxalic acid is heated with glycerol at 383 K, it gives rise to glyceryl mono-oxalate, which drops one molecule of CO2 to form glycerol monoformate. Glycerol monoformate then undergoes hydrolysis and produces formic acid.
- At 503 K, oxalic acid reacts with glycerol and forms glyceryl dioxalate, which drops two carbon dioxide molecules to form allyl alcohol.
Uses of Glycerine
- It is used in the preparation of nitroglycerine. Nitroglycerine is in turn used to manufacture dynamite. Dynamite is a solution of glyceryl dinitrate and glyceryl trinitrate absorbed on a special type of earth called Kieselguhr. Alfred Nobel first discovered it, after whose name the Nobel prizes were instituted. Dynamite is used in the production of bombs and to make tunnels by breaking up rocks. It is also used in building roads and other structures.
- Nitroglycerine produced by reacting glycerol with a mixture of HNO3 and sulphuric acid is used to treat a medical condition called angina pectoris. It is also used to treat high blood pressure after an operation and to decrease the patient’s blood pressure during an operation. Decreasing the patient’s blood pressure during operation decreases the chances of perioperative bleeding.
- It is used to prevent automobile radiators from freezing. This use of glycerol is based on the fact that it lowers the freezing point of water, preventing it from freezing in low temperatures.
- It is used to produce medicines like cough syrups, lotions, toothpaste, mouthwashes etc.
- It is used to produce glyptal, which is used to manufacture surface coatings like paints and lacquers. Glyptal is also formed from ethylene glycol and phthalic acid.
- It is used to manufacture non-drying printing inks, stamp colours, shoe polishes etc.
- It is used to manufacture expensive soaps and cosmetics since it does not let them dry due to its hygroscopic property.
- It is used as a preservative for fruits and other food items and helps to keep them moist and fresh. This is also due to its hygroscopic nature. Glycerol takes up moisture from the air and keeps the food moist and fresh. A mixture of glycerol and water is used to preserve plant leaves.
- It is used as a moisturiser for cracked lips and heels.
- It is used as a sweetener in the food and pharmaceutical industry.
- It is used to lubricate the internal parts of watches and clocks.
- It is used to prepare certain organic compounds like acrolein, allyl alcohol, formic acid etc.
- It is used as a laxative to prevent constipation.
- Glycerol can rapidly decrease the internal pressure of the eye. So, it is used in the manufacture of eye drops used to treat different ophthalmic diseases.
- It is also used as a thickening agent.
