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Reactions of unsaturated compounds

In general, multiple bonds are more reactive (chemically active) than are single carbon-car­bon bonds. Compounds containing multiple bonds may undergo polymerization, cycliza-tion, and addition reactions. Polymerization is the most important of these. Hydrocarbon polymers (large, chainlike molecules) include plastics, resins, and both natural and synthetic rubbers. In a cyclization reaction, a straight chain curls around to form a ring. Several im­portant solvents are made in this way. Addi­tion reactions break one of the bonds in a mul­tiple bond and add two new groups onto the molecule, one at each end of the broken bond. It is this reactivity of the multiple bond, as well as the many synthetic possibilities, that makes these compounds so important in the chemi­cal industry.

One consequence of multiple bonding is that the two linked carbon atoms are not able to rotate freely, as they would if joined by a



Car tirescontain a large proportion—more than 25 per cent (by weight) in some cases—of carbon black. Car­bon black greatly increases a tire's elasticity, strength, and resistance to wear. The carbon black is produced by burning ethyne in a lim­ited supply of air.


Organic chemistry: Unsaturated aliphatic hydrocarbons 73



 

Ethyne (acetylene C8H2) Propyne (C3H„)     1-butyne(C1H,.i     2-butyne (C4H6)
    H \   H\ H I   \ /H
H—C=C- -H H^c_c~c- H -H H-C-H7 -c—cs=c- I H -H H7l> H —C—C—C—H XH



single bond. If an alkene has two different atoms or groups attached to each carbon atom, it can exist in two physically different forms. The form would depend on whether the groups are on the same side of the mole­cule or on opposite sides.

Uses of alkenes

Simple alkenes are produced from some types of natural gas. Alkenes can also be produced by cracking naphtha, a straw-colored liquid obtained during the refining of crude oil. Cracking is the process used in the petro­chemical industry to break big molecules down into smaller ones. Naphtha, a mixture of saturated hydrocarbons, undergoes thermal cracking at 1000-1200° F. (about 540-650° C) in the absence of air. This breaks it down into un­saturated hydrocarbons such as ethene, pro-pene, and butadiene. Although natural gas is primarily methane, some sources (notably in the U.S.) contain ethene, which can be con­verted into almost pure ethene.

Ethene is a colorless, highly inflammable gas with a sweet odor and taste. It is the single most important organic chemical feedstock. A feedstock is a principal material in chemical processes that produce petroleum products. Ethene is also an important natural product. It is involved in the ripening of fruits. About half of the ethene (ethylene) produced industrially is used to make the plastic polyethylene. Many other plastics, synthetic rubbers, and resins can also be derived from it. Ethene is used in making antifreeze, cosmetics, paints, lacquers, and acetic acid, a major ingredient of vinegar. Ethyne can also be made from ethene.



Propene (propylene) is a colorless, inflam­mable gas that can be used to produce isopro-pyl alcohol, from which acetone is made. Ace­tone is an important organic solvent for dissolving other substances. Cumene, pro­duced from propene and benzene, is used to make phenol. Phenol (carbolic acid) is used in making antiseptics and disinfectants. Propyl­ene glycol (another alkene product) is a sol­vent for fats, oils, resins, perfumes, colors and dyes, soft drink syrups, and flavor extracts. Bu­tadiene is important in the manufacture of syn­thetic rubbers and certain types of resins. These are extremely hard, durable, and resist­ant to fire.

Higher alkenes can be produced by thermal cracking of waxes, which are saturated hydro­carbons, or by building up a long chain from ethene. They are used to make detergents and lubricants.


Date: 2015-12-11; view: 245


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