Chemistry, Third edition

322 17 · ORGANIC CHEMISTRY: HYDROCARBONS

BOX 17.7

Analysis of organic compounds

Often, one of the the first steps in the identification of an unknown organic compound is to

submit the compound for quantitative elemental analysis. This type of analysis will determine the

percentage by mass of the elements present in the compound. From the results of the analysis,

the empirical formula of the compound can be calculated. To find out the relative amounts of

carbon and hydrogen in a hydrocarbon, a weighed sample of the hydrocarbon is passed through a

tube packed with copper(II) oxide at a temperature of about 700 °C. The copper(II) oxide oxidizes

the carbon in the hydrocarbon to carbon dioxide and the hydrogen to steam:

‘CH 2 ’ from organic compound 3CuO3CuCO 2 H 2 O

The gases pass out of the combustion tube and through two weighed tubes containing first a

drying agent, to absorb the water, and then a strong base (such as sodium hydroxide) to

absorb the carbon dioxide formed. The increase in the mass of each tube gives the mass of

each product from the combustion of the hydrocarbon.

For example, a sample of methane with a mass of 7.25 mg produced 19.90 mg carbon dioxide

and 16.17 mg of water.

Mass of carbon present = mass of carbon dioxide 

M(C)

M(CO 2 )

= 19.90 

12

= 5.4 mg

44

Mass of hydrogen present = mass of water 

M(2H)

M(H 2 O)

= 16.17 

2

= 1.8 mg

18

The percentage composition of the methane is therefore

Carbon:

5.4

100 = 75%; Hydrogen:

1.8

7.25 7.25100 = 25%

The empirical formula of an unknown compound can be worked out from its percentage

composition using the calculations in Unit 8.

Note that it is possible, using various methods, to analyse for other elements in an organic

compound, such as nitrogen, sulfur and halogens.

Aromatic hydrocarbons

The families of hydrocarbons studied so far, those with linear chains of carbon as the

‘backbone’ of the compounds, are classed as aliphatichydrocarbons. There is

another class of hydrocarbons – aromatichydrocarbons. These are compounds that

contain a benzene ring, or behave chemically like benzene. Benzene has the molecu-

lar formula C 6 H 6 , and its structural formula was first suggested by Kekulé in 1865:

C

H

H

H H

HH

C

C C

C

C

17.4

Quantitative

analysis of

compounds

(i)A 7.02 mg sample of a

hydrocarbon X gave

21.99 mg of carbon

dioxide and 8.95 mg of

water on combustion.

Calculate the

percentage composition

of the elements in the

hydrocarbon and

determine its empirical

formula.

(ii)X has molecular mass

42. X reacts with excess
    hydrogen in the
    presence of a nickel
    catalyst to form Y. X
    decolorises bromine
    water to form a
    compound Z. Write
    structural formulae for X,
    Y and Z.

Exercise 17O