Chemistry, Third edition

ALKANES

The last is a stick formula, where each end represents a carbon joined to three hydro-

gens and each point a carbon joined to two hydrogens.

Shapes of alkane molecules

Although we often draw the methane molecule as being flat, the shape of the

molecule is actually tetrahedral, as drawn in Fig. 17.1.

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Isomers of pentane

Draw the structures of the three isomers of pentane. It may help to make models of the

isomers. If you do not have a molecular modelling kit, use straws to represent bonds and

plasticine balls, of different colours, to represent atoms of hydrogen and carbon.

Exercise 17C

Fig. 17.1The shape of the methane molecule.

Shapes of the

alkanes

Draw the three-dimensional

structure of ethane (C 2 H 6 ).

Exercise 17B

Space-filling model of

methane.

Isomers

There are two possible structures for C 4 H 10 :

They have a different structure, although they both have the same molecular for-

mula. One is called ‘normal’ butane, abbreviated to n-butane; the other may be

called isobutane or 2-methylpropane – the latter name describes the structure of the

compound, as you will see later. These compounds are isomersof one another. They

arenot the same compoundsand have different melting points, boiling points and

solubilities.Isomers are compounds which have the same molecular formula, but

different molecular structures. After butane, the longer the carbon chain of an

alkane, the more structural isomersare possible for a particular molecular formula.

For example, there are 75 decanes (C 10 H 22 ) and over three-hundred-thousand

eicosanes (C 20 H 42 )!

H C

H

H

C

H

H

C

H

H

C

H

H

HC

H

H

C

C

H

H

HH

C

H

H

H H

n–Butane Isobutane

(2–methylpropane)