O Level Notes : Chemistry - Macromolecules
Hello students. This is a tricky chapter so I suggest you all pay attention. In this topic we study about the macromolecules that the organic compounds we studied about in the previous topic form.
You will recall that we learnt a little about polymerization. It is the joining together of small molecules, referred to as monomers, to form giant macromolecules called polymers. Let us learn more about these polymers in this chapter.
Synthetic Polymers:
Synthetic or man-made polymers can be made by two types of polymerizations: addition polymerization and condensation polymerization.
Addition polymerization is the linking together of unsaturated monomers to form a polymer, without losing any atoms or molecules. Each polymer is only made of one kind of monomer molecule. This type of polymerization is suitable with alkenes and alkynes.
In this type of polymerization, the double bond or the triple bond of the original molecule breaks and forms the monomer that then combines with other monomers to form a long chain. The conditions for this polymerization have to be very exact, and are different for every type of polymer to be made. The temperature and pressure usually has to be very high, and a catalyst is used.
An example of synthetic polymer made by addition polymerization is polyethene, also known as plastic. Addition polymers are good insulators of heat and electricity, and are resistant to chemical attack. For example, Perspex is a polymer made to use car windscreens, because it is transparent and less easy to break compared to glass. Polyvinyl chloride (PVC), another polymer, is used to make pipes, raincoats, thin gloves and flooring mats.
Condensation polymerization is the linking together of monomers with the elimination of a simple molecule like water. Each polymer may contain two kinds of monomer molecule.
There are two main groups of condensation polymers: the polyamides and the polyesters.
Nylon is an example of a synthetic polyamide. It is made from the monomers dicarboxylic acid and diamine.
Besides nylon, Terylene is another example of a condensation polymer. Terylene is made from the monomers benzene-1, 4-dicarboxylic acid, and ethane-1,2-diol.
Clothes made from synthetic fibres are shrink-proof and crease proof. They are also easier to wash and dry. Examples of items made from nylon and Terylene are curtains, parachutes, fishing lines and sleeping bags.
Synthetic polymers, or plastics, are now used in place of natural materials such as wood, metal, cotton and leather because they are relatively cheaper, easily molded into various shapes, light, tough, waterproof, and resistant to decay and rusting. However, there are also many disadvantages of these plastics in respect to the environment. When plastics burn, fires can spread very quickly and poisonous gases are produced, causing air pollution. Plastics are non-biodegradable; they cannot be decomposed by bacteria in the soil. As we cannot burn them either, we must dump them somewhere, so they take up place and cause land pollution.
Natural Macromolecules:
The three main classes of food are proteins, fats and carbohydrates. All of these are natural polymers or macromolecules, where small monomer units are linked to form a giant molecule.
Food which contains only carbon, hydrogen and oxygen are either carbohydrates or fats. Food which contains carbon, hydrogen, oxygen and sulphur or nitrogen are proteins.
All natural macromolecules are biodegradable and are broken down during digestion using biological catalysts called enzymes. These catalyse the reaction of protein, fats and carbohydrates with water. This reaction is called hydrolysis.
Proteins: the monomers in proteins are linked by amide or peptide linkage. This is in fact the same amide linkage that nylon possesses but proteins are formed from monomer units of amino acids. To break proteins up, the enzyme pepsin is used in acidic medium. Proteins break up to form amino acids.
Fats: they are linked by the same ester linkage that is in Terylene, but with different monomer units. To break fats up, we boil them with sodium hydroxide, and the enzyme lipase is used. They break up to form fatty acids and glycerol.
Carbohydrates: the monomers in carbohydrates are linked by oxygen linkage. To break these up, we boil them with dilute hydrochloric acid, and the enzyme amylase is used. They break up to form simple sugars, for example glucose, maltose, etc.
This is all for this chapter. Thank you students.