Glycerol: Versatile Renewable Chemical Properties

Glycerol, or glycerine, is the basic oleochemical obtained from oils and fats along with fatty acids during the hydrolysis or fat splitting process. It is a three-carbon trihydric alcohol and the simplest triol. Glycerol is found in all natural fats and oils in the form of long-chain fatty acid esters, which are also called as glycerides. It is also an important intermediate in the metabolism of living organisms. Although it is produced today from naturally occurring oils and fats, it was also be synthesized through other chemical routes on commercial scale, till several years ago. 

Glycerine synonyms – Glycerol, Propane-1,2,3-triol, 1,2,3-Trihydroxypropane

CAS Registry No. [56-81-5]

Structure –  CH2OH – CHOH – CH2OH

Generally, the term glycerol is usually used to denote the pure chemical compound, i.e., 1,2,3-propanetriol., the term glycerine applies to the purified commercial grade normally having more than 95% purity. However, in industry jargon, both these terms are used interchangeably.

SOURCES AND YIELDS
Glycerol occurs naturally in combined form in all vegetable and animal fats and oils, usually as a triglyceride with fatty acids such as stearic, oleic, palmitic and lauric acids. Vegetable oils like coconut, palm kernel, cottonseed, soybean and olive oil yield larger amounts of glycerol than animal fats like lard and tallow.

Glycerol also occurs as triglycerides in all animal and vegetable cells in the form of lipids such as lecithin and cephalins. These complex fats differ from simple fats in that they invariably contain a phosphoric acid residue in place of one fatty acid residue.

Glycerol is recovered as a byproduct when oils and fats are saponified during soap manufacturing process, or in the direct fat splitting process for the production of fatty acids. It is also obtained during the transesterification or interesterification or methanolysis process for the manufacture of biodiesel.

The most important industrial synthetic route uses propylene as the starting material. However, as large amounts of glycerol are available from oils and fats, either during the production of fatty acids or through biodiesel manufacture, the need for synthetic glycerol has reduced significantly.

Coconut and palm kernel oils containing a high percentage (70 to 80 %) of C6 to C14 fatty acids yield larger amounts of glycerol than do fats and oils containing mostly C16 and C18 fatty acids, such as animal fats (like tallow and lard), cottonseed, soybean and palm oil.

The proportion of glycerol by weight in vegetable oils varies from 8 to 14%. The yield of glycerol through various industrial processes is about 10% by weight of oils and fats.

PHYSICAL PROPERTIES
Glycerol is the simplest, three-carbon triol, i.e., containing three hydroxyl groups attached to three carbon atoms. It is a sweet tasting liquid. Glycerol is about 0.6 times as sweet as cane sugar. When in pure state, glycerol is a colorless, odorless, syrupy liquid, viscous at room temperature.

Physical properties of glycerol are listed in the following table –

TABLE 1: PHYSICAL PROPERTIES OF GLYCEROL

Glycerol is seldom seen in the crystallized state because of its tendency to supercool and its pronounced freezing point depression when mixed with water. A mixture of 66.7% glycerol and 33.3% water forms a eutectic mixture with a freezing point of -46.5 deg. C. A solid, glassy state is observed from -70 to -110 deg. C. 

Glycerol is hygroscopic, i.e., it absorbs moisture from air. It also absorbs hydrogen sulfide, hydrogen cyanide and sulfur dioxide.

Glycerol is completely soluble in water and alcohol, slightly soluble in diethyl ether, ethyl acetate, and dioxane. It is also completely miscible with methanol, propanol, butanol, pentanol, phenol, propanediols, amines and nitrogen-containing heterocyclic compounds, like pyridine and quinoline.

Glycerol is insoluble in hydrocarbons owing to its three hydroxyl groups. It is also insoluble in long-chain aliphatic alcohols, fatty oils, and halogenated solvents like chloroform.

These physical properties of glycerol make it useful in many different applications.

CHEMICAL PROPERTIES
Glycerol is a chemically reactive molecule that undergoes all the usual reactions of alcohols. The two terminal primary hydroxyl (-OH) groups are more reactive than the internal secondary hydroxyl group in the glycerol molecule. It forms esters, ethers, halides, amines, aldehydes and such unsaturated compounds as acrolein. Under neutral or alkaline conditions, glycerol can be heated to 275 deg. C without formation of acrolein. By contrast, in the presence of small amounts of a strong mineral acid; the odour of acrolein (odour threshold 0.2 to 0.4 ppm) is already quite perceptible at 160 deg. C; by 200 deg. C, the evolution of acrolein is vigorous. Therefore, reactions of glycerol are best carried out under neutral or alkaline conditions. At 180 deg. C, alkaline glycerol begins to dehydrate, forming ether-linked polyglycerols. As an alcohol, glycerol also has the ability to for salts such as sodium glyceroxide. 

Glycerol is easily oxidized – the terminal carbon atoms to aldehyde or carboxyl groups and the central carbon atom to a carbonyl group.

Being hygroscopic, glycerol readily absorbs water at room temperature and, when dilute, it is attacked by microorganisms. 

Industrially important reaction products of glycerine are summarized below – 

1. Mono-, di- and tri-esters of inorganic and organic acids.
2. Mono-, di- and tri-glycerides of fatty acids formed by transesterification (or interesterification) of triglycerides present in oils and fats.
3. Aliphatic and aromatic esters, formed by reaction with alkylating or arylating agents, respectively.
4. Polyglycerols, formed by the intermolecular elimination of water (dehydration) with alkaline catalysts.
5. Cyclic 1,2- or 1,3-acetals or ketals, formed by reaction with aldehydes or ketones, respectively.
6. Mono- and di- glycerides, formed by the action of alkali or metal alcoholates. 

HAZARDS AND TOXICITY
Glycerol in contact with strong oxidizing agents such as chromium trioxide, potassium chlorate, or potassium permanganate, may produce an explosion.

Glycerol is not harmful to health. Ingestion even of large amounts of glycerol causes no harm to humans. Glycerol has been generally recognized as safe (GRAS) since 1959, as a miscellaneous or general purpose food additive, permitted in most countries around the world. It is also permitted in certain food packaging materials. Glycerol is considered as a non-toxic chemical.

Glycerol has antimicrobial and antiviral properties also.

Slight irritation of the skin or mucous membranes is possible on contact with undiluted glycerol because the strongly hygroscopic glycerol draws water from the skin.

Oral LD50 levels have been determined in mouse at 470 mg/kg and in guinea pig at 7750 mg/kg. LD50 levels in rats is reported to be more than 20 ml/kg orally, and 4.4 ml/kg intravenously.

Several studies have shown that large quantities of both synthetic and natural glycerol can be administered orally to experimental animals and humans without the appearance of adverse effects. Intravenous administration of solutions containing 5% glycerol to animals and humans has been found to cause no toxic or otherwise undesirable effects.

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