Chemistry, Third edition

ALKENES

When methane, for example, reacts with chlorine gas in sunlight, chlorine atoms

substitute for hydrogen atoms and chloroalkanesare formed:

h

CH 4 (g)Cl 2 (g)CH 3 Cl(g)HCl(g)

Depending on the amounts of methane and chlorine present, further substitution

can occur to form CH 2 Cl 2 , CHCl 3 and eventually CCl 4. If propane is chlorinated,

then the first substitution may be on an end carbon atom or a middle carbon atom

so that the first product is a mixture of two possible isomers, namely

CH 3 CH 2 CH 2 Cl or CH 3 CHClCH 3

1-chloropropane 2-chloropropane

Because a mixture of products may be produced by the halogenation of alkanes,

comprising different isomers and/or molecules with different degrees of substitu-

tion, the reactions are not really suitable for the laboratory preparation of halogeno-

alkanes. The process is more useful on an industrial scale where separation of

individual compounds can be economically worthwhile.

315

BOX 17.4

Naming halogenoalkanes

The rules for naming halogenoalkanes are very

similar to those for naming alkanes; the name of

halogenoalkane is derived from the parent alkane.

Remember, however, to choose the longest

carbon chain that also contains

the halogen(s)as a starting point.

The halogenoalkane is prefixed fluoro-, chloro-,

bromo-, iodo-, dichloro-, tribromo- etc.

Substituents are again arranged in alphabetical

order:

CHCl 3

trichloromethane

CH 3 CH 2 CHCH 2 CHCH 3

||

CH 3 Br

2-bromo-4-methylhexane

Combustion of alkanes

The enthalpies of combustion for the first six straight-chain alkanes

are given below:

Alkane Formula Enthalpy of combustion

H^ —c/kJ mol^1

Methane CH 4 0  890

Ethane C 2 H 6  1560

Propane C 3 H 8  2220

Butane C 4 H 10  2880

Pentane C 5 H 12  3500

Hexane C 6 H 14  4160

(i) Work out the difference in H—^ cbetween each successive alkane.

DoesH^ —cincrease by a regular amount?

(ii)What is the average increase in H—^ cbetween successive alkanes?

(iii)The answer to (ii)represents the average enthalpy of combustion

for what additional group of atoms?

(iv)Estimate the enthalpy of combustion for heptane, C 7 H 16.

Exercise 17H

Alkenes

Another family of hydrocarbons is the alkenes. Alkene molecules contain a double

bondbetween carbon atoms. They have the general formula CnH 2 nand their names

end in ‘ene’ (Table 17.2). The first three are gases at 25 °C.

Shape of the ethene molecule

In ethene, each carbon atom lies at the centre of a triangle. The corners of each triangle

are the two attached hydrogen atoms and the other carbon atom. The bond angles are

approximately 120 °:

C

H

H

H

H

116 °

C

122 °

17.2