Modern inorganic chemistry

GROUP IV 173

common acids, alkalis, oxidising and reducing agents. However, all

hydrocarbons burn in oxygen, the ultimate products being carbon

dioxide and water; this reaction can be used to determine the em-

pirical formula of hydrocarbons. For example,

C 4 H 10 + 6iO 2 -» 4CO 2 + 5H 2 O.

Alkanes also react with halogens to form substitution products.

Alkenes

Every member of this series must contain at least one double bond.

The two carbon atoms making up the double bond are joined to

only three other atoms and they are therefore said to be unsaturated.

^C = C Le. >—: —<

^ H S *- ;• tit , V.'- >

. ' I 4 " St
*^\,: -/X
ethene (ethylene) planar

The carbon atoms of the double bond have a trigonal planar

configuration and free rotation about the C—C bond is prevented

by the n bond. The inability to rotate means that geometrical

isomers can be produced, with substituents a and b, thus:

a b a a

\ / \ /

C=C C=C

/ \ /

b a b b

the trans isomer the ds isomer

The region of high electron density between the doubly bonded
carbon atoms gives alkenes an additional reactivity and in addition
to burning and reacting with halogens, alkenes will add on other
molecules; for example:
H 2 C=CH 2 + HBr -> CH 3 CH 2 Br
and will polymerise in the presence of a suitable catalyst: