Composition and structure of fat molecules.
Esters can be formed from a variety of carboxylic acids and alcohols. Of greatest importance are those formed by the trihydric alcohol glycerol and higher carboxylic acids. The latter include, for example, stearic acid of the composition C 17 H 35 COOH and oleic acid of the composition C 17 H 33 COOH.
The first is saturated acid, the second is unsaturated. Its hydrocarbon radical has a double bond between carbon atoms, so the oleic acid molecule has two fewer hydrogen atoms:
Stearic acid Oleic acid
Esters of carboxylic acids and glycerol are called fats. If the formula
carboxylic acid write in general form , then the formation of fat
can be represented by the esterification reaction equation:
Glycerol carboxylic acid Fat
The chemical nature of fats began to be studied in the first half of the 19th century. The synthesis of tristearin fat was first carried out by the French chemist M. Berthelot in 1854.
Physical properties of fats. The composition and structure of hydrocarbon radicals affect the properties of fats. Here is how, for example, their melting points change:
|Radical||Name of fat||T pl , °С|
|– C 17 H 35||Tristearin|
|– C 17 H 35||Triolein||– 4|
As you can see, fat formed by saturated acid is solid under normal conditions, unsaturated fat is liquid. The composition of liquid vegetable oils (sunflower, corn, olive, etc.) includes residues of predominantly unsaturated acids, while solid animal fats (beef, mutton, etc.) contain residues of saturated acids.
Fats are lighter than water and insoluble in it, but soluble in organic solvents.
Fats, along with proteins and carbohydrates, belong to biologically active substances. They are part of the cells of plant and animal organisms and are a source of energy for them. As a result of oxidation, 1 g of fat is released
37.7 kJ of energy, twice as much as when oxidizing 1 g of protein or carbohydrate.
The main amount of fats consumed by humans is found in meat, fish, dairy and grain products. In the case when more energy enters the human body with food than it uses, fatty substances are formed that are deposited in the tissues of the body. Thus, it accumulates energy.
In accordance with the data of modern medicine, excessive consumption of fats formed by saturated acids, that is, animal fats, can lead to the accumulation of substances that impede the flow of blood in the arteries, in particular those that supply blood to the brain. Fats formed by unsaturated acids, that is, vegetable oils, are recognized as more useful for consumption. For example, sunflower oil contains 91% unsaturated carboxylic acids.
Chemical properties of fats.
In liquid fat molecules, unlike solid fats, there are double carbon-carbon bonds. As you already know, an addition reaction, in particular hydrogen, is possible at the place of the double bond. As a result of this reaction, an unsaturated compound turns into a limiting one, and liquid fat into a solid one.
The process of hardening (hydrogenation) of fats underlies the production of margarine (from the Greek word meaning pearl). Unhydrogenated fats go rancid, oxidized at the double bonds, and they develop an unpleasant odor and taste. Hydrogenation of fats slows down these processes, in addition, it makes it possible to obtain more valuable solid fats from cheaper vegetable oils.
Fats as esters undergo hydrolysis.
Fats are hydrolyzed with the formation of trihydric alcohol glycerol and carboxylic acids.
If tristearin is hydrolyzed in the presence of alkali, a salt of stearic acid is formed, known as a soap base:
Since soap is formed as a result of alkaline hydrolysis of fat, the reaction is called fat saponification.
Sodium salts of higher carboxylic acids are the main component of solid soap, potassium salts – liquid soap.
To obtain soap from fat in industry, instead of alkali, soda Na 2 CO 3 is used. The soap obtained directly from this reaction is called sound soap and is known as laundry soap. Toilet soap differs from household soap in the presence of additives: dyes, flavors, antiseptics, etc.
The main component of solid soap is a mixture of soluble salts of higher fatty acids. Usually these are sodium, less often potassium and ammonium salts of such acids as stearic, palmitic, myristic, lauric and oleic.
One of the options for the chemical composition of solid soap is C17H35COONa (liquid is C17H35COOK).
The washing action of soap is a complex physical and chemical process. Soap is an intermediary between polar water molecules and non-polar particles of contaminants that are insoluble in water. If we denote the hydrocarbon radical by the letter R, then the composition of the soap can be expressed by the formula R – COONa. By chemical nature, soap is a salt, an ionic compound. In addition to the polar group -COONa, it contains a non-polar radical R, which can include 12-17 carbon atoms. During washing, the molecules are oriented on the contaminated surface in such a way that the polar groups are facing the polar water molecules, and the non-polar hydrocarbon radicals are facing the non-polar pollution particles. The latter, as it were, are surrounded by soap molecules and are easily washed off the surface with water.
In hard water, insoluble magnesium and calcium salts of carboxylic acids are formed, so soap loses its washing effect, and salts settle on the surface of the product:
2C 17 H 35 COONa + MgSO 4 → (C 17 H 35 COO) 2 Mg↓ + Na 2 SO 4
Synthetic detergents, with all the variety of their chemical composition, have a soap-like structure of molecules, in which there is a polar part soluble in water and an insoluble hydrocarbon radical. But they, unlike soap, are salts of a different chemical nature and do not form insoluble compounds in hard water. This is the advantage of synthetic detergents over ordinary soap.
Soaps and synthetic detergents belong to the so-called surfactants. Their widespread use is often associated with environmental pollution, in particular water bodies. The fact is that phosphates are added to synthetic detergents, which in water bodies turn into substances that feed microorganisms, the rapid reproduction of which can lead to swamping of water bodies. Therefore, modern surfactants must be chemically or biologically degraded after use into safe substances that do not pollute effluents.