Man's physical universe

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COVALENT REACTIONS 613

enter into. Such a compound is called unsaturated, because it can form

compounds by addition as well as by substitution, as will be mentioned

later.

These unsaturated compounds contained in cracked gasoline are an

actual advantage, because they raise its antiknock value.

A large nitrogen-fixation plant in California obtains carbon and

hydrogen by cracking the hydrocarbons in natural gas. The hydrogen

thus obtained is combined with the nitrogen of the air, which is

separated by liquefying and boiling the air, to produce ammonia.

A type of process which resembles cracking is covalent hydrolysis.

Hydrolysis requires water and is usually activated by hydronium or

hydroxyl ions. Covalent hydrolysis differs from the cracking of hydrocarbons

in that the former is a reaction with water to form smaller

saturated molecules, while the latter is a splitting of saturated molecules

to form unsaturated molecules. Thus carbohydrates, such as

cellulose and starch, may be split to form smaller carbohydrate molecules

such as the simple sugars; and proteins may be split in a stepwise

fashion into a series of products, ending in simple amino acids.

Digestion

is largely a process of splitting large molecules into smaller molecules

that can be absorbed and used in

molecules.

Chain Hydrocarbons May Also Be Polymerized.

the body to build new large

Polymerization is the opposite of cracking; that is, it is the building

of large molecules from small molecules. Gasoline and other hydrocarbon

products may be prepared by the polymerization of gaseous hydrocarbons.

It is thought that carbohydrates are produced in nature by a polymerization

of -formaldehyde, CH2O. Thus six molecules of formaldehyde

would polymerize to form C6H12O6, a simple sugar. Still larger

molecules of starches and celluloses would be formed by a continuation

of the process.

Rubber may be formed by the polymerization of hydrocarbons

found in turpentine.

Plastics are produced by polymerization reactions.

Unsaturated Hydrocarbons and Even Carbon Itself

May Be Hydrogenated.

Hydrogenation is a process in which hydrogen is added to carbon

or its unsaturated compounds. Thus vegetable oils, such as cottonseed,

peanut, and olive oils, and animal oils, such as fish oil, may be changed

to solid fats by hydrogenation. This is possible because the oils are

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