Introduction:

Fats and oils are the esters of glycerol having higher fatty acids and are known as glycerides. They are obtained via the combination of propane-1, 2, 3-triol with fatty acid, which are long chain carboxylic acids.

Occurrence and Composition of Fats:

Fats are the major constituents of the storage fat cells in animals and plants and they are one of the significant food reserves of the living organism. Such plant and animal fats can be extracted. Liquid fats are often termed to as oils. 

Carboxylic acids found in the natural fats and oils are mostly unbranched. They can be categorized into two groups: saturated and unsaturated.

Saturated carboxylic acids encompass single bonds in their  hydrocarbon chains only and they are found in animals, whereas unsaturated carboxylic acids have at least one C=C  double in their chains and they are obtained in the plants. Carboxylic acids having more than one C=C double bonds are termed as polyunsaturated.

Saturated carboxylic acids are solid at room temperature. This is due to the reason that the regular nature of their aliphatic chains allows the molecules to be packed in a close, parallel alignment. The Unsaturated carboxylic acids are liquids at room temperature. This is because the double bonds in the hydrocarbon chains interrupt the regular packing.

Therefore the significant difference between oils and fats is that oils are liquids at ordinary temperature whereas fats are solids.

Fig: Approximate composition of common Fats and Oil

Formation of Fats and Oils:

Whenever a molecule of glycerol, that is, propane-1, 2, 3-triol, reacts by a molecule of Stearic acid, glyceryl monostearate is formed.

The product formed might then react with the other molecule of Stearic acid to form glyceryl distearate that further reacts to give glyceryl tristearate.

The glyceryl tristearate made is a solid. It is a fat. If glycerol reacts by other fatty acids to form a liquid product, then it is oil.

However, most of the natural fats and oils are mixed glycerides. They are esters made up from propane- 1, 2, 3-triol and a mixture of different long chain carboxylic acids, like linoleic, Stearic and oleic acids.

Structure of Fats and Oils:

Fats and oils are obtained via the combination of propane-1, 2, 3-triol by fatty acids that are long chain carboxylic acids.

Fig: Structure of Fats and Oils

General Properties of Fats and Oils:

Fig: General Properties of Fats and Oils

General Physical Properties:

1) Oils and fats are the colourless liquids or solids (might be yellow or brown because of impurities).

2) Vegetable oils encompass lower boiling points than animal fats as they usually encompass a higher proportion of unsaturated fatty acids. The more highly unsaturated the acyl group in a triglyceride, the lower its melting point.

3) They are lighter than water.

4) They are immiscible with water.

5) They are freely soluble in the organic solvents like benzene, petroleum ether and so on.

6) They are non-volatile.

7) They decomposes on strong heating giving irritating odor of acrolein

8) They readily form emulsions whenever agitated by water in the presence of soap, gelatin and other emulsifiers.

General Chemical Properties:

1) Drying:

Some oils having glycerides of unsaturated acids by two or three double bonds have the property of slowly absorbing oxygen from the air and then polymerizes to form a hard transparent coating employed in making paints and oil cloth. This method is termed as drying and the oils as drying oil. Illustrations of these oils are linoleic, eleostearic and licanic acids.

Drying occur much more readily if the acids includes conjugated system of double bonds example - in eleostearic and licanic acids dissimilar in acids having non-conjugated double bonds.

b) Rancidification:

Oils and fats, whenever exposed to air and moisture for a long time throughout storage undergo slow decomposition and extend unpleasant smell. The method is termed as Rancidification.

c) Hydrolysis of Fats and Oils:

Oils and fats can be hydrolyzed to glycerol and the fatty acids through dilute acids, alkalis or superheated steam. Hydrolysis might as well be brought about through enzymes. 

Hydrolysis taken out in an alkaline medium is more practical and the reaction is irreversible. This process is termed as saponification. The reaction can be catalyzed through sodium hydroxide. Propane-1, 2, 3-triol and the mixture of sodium salts of carboxylic acids, known as soap, are then formed.

Fig: Hydrolysis of Fats and Oils

Fats and oils ingested to our bodies are hydrolyzed to carboxylic acids and glycerol. Such substances are then employed as fuel in our body, in building cell membranes or are stored up as fatty tissues. Some of the fats as well give necessary carboxylic acids to our body.

d) Hydrogenation:

Oil comprises of more of unsaturated glycerides than fats and so are liquids ordinarily. Whenever hydrogen is passed them under pressure in the presence of finely divided nickel or Raney nickel, the unsaturated glycerides becomes saturated and the oil becomes solid or semi-solid. This method is termed as hydrogenation and the solid oil is termed as margarine. It enhances the colour, odor and taste of the oils.

Fig: Hydrogenation of Fats and Oils

e) Hydrogenolysis:

This is the method of splitting a compound by means of hydrogen. Whenever excess hydrogen is passed via oil or fat under pressure in the presence of copper chromium catalyst, it divides up into glycerol and higher aliphatic alcohols.

Fig: Hydrogenolysis of Fats and Oils

Analysis of Fats and Oils:

The value of specific oil depends on its composition and purity. A few usual physical tests to find out the purity of fats and oil are melting point, specific gravity, refractive index and viscosity.

Several chemical tests are as follows:

1) Acid value: It specifies the amounts of free fatty acids present in oil or fat. It is the number of milligrams of potassium hydroxide needed to neutralize the free organic acids present in 1gm of fat or oil. This is found out via dissolving a weighed quantity of oil or fat in alcohol and titrating against the standard alkali, by employing phenolphthalein as indicator.

A high acid value points out a stale oil or fat.

2) Saponification: It is the number of milligrams of potassium hydroxide needed to fully saponify 1gm of fat or oil or fully neutralize the fatty acids resultant from the complete hydrolysis of 1gm fat or oil.

Saponification gives an idea regarding the molecular weight of fats or oil. The smaller the saponification value, the higher the molecular weight. It is as well helpful in computing the amount of alkali required to transform a definite amount of fat or oil into soap and in identifying the alteration of a fat or oil via one of lower or higher saponification value.

The difference between saponification value and acid value is termed as ester value of the fat or oil.

3) Iodine Value:

One structural difference between the fats and oils is the degree of Unsaturation. The solid animal fats include mostly saturated carboxylic acids whereas vegetable oils have huge amounts of unsaturated carboxylic acids. As unsaturated carboxylic acids can't be synthesized by our body, they are necessary in the diet. Thus, it is helpful to encompass a quantitative measure of the degree of Unsaturation in fats and oils.

A suitable reference for the purpose is the iodine value. Its determination is mainly based on the reaction between iodine and the carbon-carbon double bond in fats and oils. In practice, the more reactive iodine monochloride is employed.

Fig: Iodine Value-fats and oils

Unsaturated fats and oil join with iodine readily, while saturated fats and oils don't. The more unsaturated the fats and the oil is, the more iodine it will react with.

The iodine value of fats or and oil is stated as the number of grams of iodine which reacts by 100g of the fat or oil. The higher the iodine value, the more is the degree of Unsaturation of the fat or oil. It as well gives an idea of the drying character of the fat or oil.

Usually, vegetable oils encompass higher values than animal fats. This exhibits that vegetable oils are more unsaturated. It has been determined that animal fats encompass iodine values less than 70, whereas vegetable oils are more than 70.

4) Reichert-Meissi (R.M.) value:

This measure the volatile fatty acids present as glycerides in the oil or fats. This is defined as the number of ml of 0.1N potassium hydroxide solution needed to neutralize the distillate of 5g of hydrolyzed fat or oil.

5) Acetyl Value:

This is defined as the number of milligrams of potassium hydroxide needed to neutralize acetic acid discharged by the saponification of one gram of fully acetylated oil or fat, the distillate of 5g of hydrolyzed fat or oil.

It computes the alcoholic group present in oil or fat. 

6) Polenske value:

It is stated as the number of ml of 0.1N potassium hydroxide solution needed to neutralize the steam volatile, however water insoluble fatty acids are obtained from the distillate of 5g of hydrolyzed fat or oil.

Uses of Fats and Oil:

1) Used as food.

2) Used in the manufacture of soap, candle, glycerol, margarine, hair creams and so on.

3) Long  chain  alcohols  made up  from  fats  and  oils  are  employed  in the preparation of the synthetic detergents

4) Used in the formation of paints and vanishes.

5) Used in the manufacture of oil cloth and linoleum.

Soaps:

Soaps are the metallic salts of higher fatty acids like Stearic, Palmitic and oleic acids. They are prepared by cold, semi-boiled or boiled methods. Ordinary soap nowadays is a mixture of sodium salts of long-chain fatty acids since the fat from which it is made is a mixture.

Soap might differ in composition and methods of processing; it might be Castile soap, if made from the olive oil; alcohol might be added to make it transparent; air might be beaten into it to make it float; perfumes, dyes and germicides might be added; if a potassium salt rather than a sodium salt, it is soft soap.

In solution, soap is dispersed in the spherical clusters known as micelles, each of which might encompass hundreds of soap molecules. A soap molecule consists of a polar end, -COO^-Na⁺, and a non-polar end, the long carbon chain of 12 to 18 carbons. The polar end is water soluble-hydrophilic whereas the non-polar end is hydrophobic or lipophilic.

Molecules that encompass both polar and non-polar end and are big enough for each and every end to display its own solubility behavior are termed as amphipathic. In solution, the polar ends projects towards the polar solvent-water, whereas the non-polar end seeks for a non-polar environment - that is, the non-polar ends of other soap molecules.

Cleaning action of soap:

The difficulty in cleaning is the fat and grease that make up and contain dirt. Water alone can't dissolve such hydrophobic substances as if oil substance is in contact with water; it tends to coalesce in such a way that there is a water layer and an oil layer. Though, the presence of soap changes the situation. The non-polar ends of soap molecules dissolves in the oil droplet, leaving the carboxylate ends projecting into the surrounding water layer. Repulsion between identical charges keeps the oil droplets from coalescing; a stable emulsion of oil and water makes and can be eliminated from the surface being cleaned.

Hard water includes calcium and magnesium salts that react with soap to make insoluble calcium and magnesium carboxylates.

Synthetic Detergents:

They are known as soapless detergents or syndets. They are more resistant to hard water as compare to soaps. Similar to soaps, they contain both hydrophilic-water soluble and hydrophobic -oil soluble part. They have replaced soap to big extent.

C₁₂H₂₅ → OSO₃Na                C₁₅H₃₁ → COOCNa

Hydrophobic Hydrophilic       Hydrophobic  Hydrophilic

Part                 Part                   Part                Part

Soapless detergent-synthetic    Soap (Sodium palmitate)

detergent

(Sodium lauryl sulphate)

Illustrations are:

C₁₂H₂₅ -O-OSO₂Na           C₁₆H₃₃ -O-SO₂-ONa

Sodium lauryl sulphate      Sodium cetyl sulphate

Synthetic detergents might be anionic surface active, cationic surface active or non-ionic surface active agents. 

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