Chemistry, Third edition

ALKANES

the top. Each group of compounds collected is called a fraction. Each fraction is still a

complicated mixture, since it contains alkanes of a range of carbon numbers and

isomers of each carbon number. In addition to alkanes, crude oil contains some

cycloalkanesandaromatic compounds– the proportion of each present depends

upon the source of the oil. Aromatic compounds will be discussed later. Most synthetic

chemicals produced by industry originate from the alkanes derived from crude oil:

plastics, pharmaceuticals, adhesives and synthetic fibres are just a few examples.

Natural gas is used to produce hydrogen and carbon monoxide by reacting it with

steam:

CH 4 (g)H 2 O(g)\===\CO(g)3H 2 (g)

synthesis gas

The resulting gas mixture is called synthesis gas. The hydrogen from the mixture is

used to manufacture ammonia in the Haber–Bosch process (see p. 281).

Coal

Coal is formed by the decomposition of plants under pressure. Some coals contain

2–6% sulfur. Burning these coals has led to pollution of the atmosphere and acid rain.

When coal is heated in the absence of air, a process called destructive distillation,

it yields three products:

1.coal gas– mainly CH 4 and H 2 with some toxic carbon monoxide;

2.coal tar– this contains a wide range of organic materials which can be separated

by distillation;

3.coke– a useful fuel and used in the making of steel.

At present, most organic chemicals are derived from petroleum – petroleum

refining is cheaper and cleaner than refining coal. As the cost of petroleum rises and

reserves fall, however, conversion of coal into useful organic chemicals becomes a

more attractive prospect.

Cracking

The very long-chain alkanes are less useful than the smaller alkanes. For example, there

is much more of a demand for petrol than for lubricating oil. Long-chain alkanes

313

Fig. 17.2A fractionating column.

BOX 17.1

Cycloalkanes

Molecules such as

CH 3 CH 2 CH 2 CH 3 have their

carbon atoms connected in

a chain. Carbon atoms can

also be joined together in

rings, in which case a cyclic

molecule is formed. Cyclic

molecules are often

represented by polygons –

the corners of the polygon

represent a carbon atom

together with the hydrogens

joined to it. Some examples

are:

CH 2

CH 2

CH 2

H 2 C

H 2 C

Cyclopentane

or

CH 2

H 2 C CH 2

H 2 C

Cyclobutane

or

CH 2

CH 2 CH 2

Cyclopropane

or

Cycloalkanes

(i) Can you write a general

molecular formula for

the cycloalkanes?

(ii)Cyclopentane is a much

more stable and less

reactive compound than

cyclopropane. Can you

give a reason why?

(HintMake models of

both molecules.)

(iii)What are the molecular

and structural formulae

ofn-hexane and

cyclohexane?

Exercise 17G

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