Chemistry, Third edition

308 17 · ORGANIC CHEMISTRY: HYDROCARBONS

from combustion reactions they are not particularly chemically reactive. They are

found in crude oil and natural gas and are used extensively as fuels, or as starting

materials for a wide variety of organic compounds. The first four members of the

alkane family, which are all gases at 25 °C, are shown in Table 17.1. The alkanes

belong to a larger classification of compounds, the hydrocarbons. Hydrocarbons

are organic compounds that contain the elements carbon and hydrogen only. The

alkanes have the general formula CnH 2 n 2 wheren, the number of carbon atoms, is

1, 2, 3, 4, 5 and so on. A family of compounds whose formulae differ by –CH 2 is

called a homologous series.

After butane, the names of higher alkanes are taken from the Latin or Greek pre-

fix for the number of carbon atoms in one molecule of the alkane:

Name Number of carbon atoms

Pentane 5

Hexane 6

Heptane 7

Octane 8

Nonane 9

Decane 10

Undecane 11

Dodecane 12

Writing the structural formulae of the higher alkanes can take up a great deal of

space, so we often write condensed structural formulae. For example, the struc-

tural formula of pentane can be written in four different ways. These are shown

below:

HHHHH

|||||

H—C—C—C—C—C—Hor CH 3 CH 2 CH 2 CH 2 CH 3 or CH 3 (CH 2 ) 3 CH 3 or

|||||

HHHHH

Table 17.1The first four alkanes

Name Molecular formula Structural formula

Methane (natural gas) CH 4 H

|

H—C—H

|

H

Ethane C 2 H 6 HH

||

H—C—C—H

||

HH

Propane (camping gas) C 3 H 8 HHH

|||

H—C—C—C—H

|||

HHH

HHHH

||||

Butane (lighter fuel) C 4 H 10 H—C—C—C—C—H

||||

HHHH

Alkanes

(i) The alkanes are

covalent compounds.

Write Lewis structures

for ethane and

propane.

(ii)Write the molecular and

structural formulae for

octane.

(iii)What is the molecular

formula for dodecane,

which has 12 carbon

atoms?

Exercise 17A