Chemistry Reference
In-Depth Information
Gaseous alkanes, such as methane, will burn in a
good supply of air, forming carbon dioxide and water
as well as plenty of heat energy.
methane
Structural isomerism
Sometimes it is possible to write more than one
structural formula to represent a molecular formula.
The structural formula of a compound shows how the
atoms are joined together by the covalent bonds. For
example, there are two different compounds with the
molecular formula C
4
H
10
. The structural formulae
of these two substances along with their names and
physical properties are shown in Figure 14.4.
+
→
+
+
oxygen
carbon
dioxide
CO
2
(
g
)
water
energy
+
→
+
CH
4
(
g
)
2O
2
(
g
)
2H
2
O(
g
)
The gaseous alkanes are some of the most useful
fuels. Methane, better known as natural gas, is used
for cooking as well as for heating offi ces, schools and
homes (Figure 14.5a). Propane and butane burn
with very hot fl ames and they are sold as liquefi ed
petroleum gas (LPG). In rural areas where there
is no supply of natural gas, central heating systems
can be run on propane gas (Figure 14.5b). Butane,
sometimes mixed with propane, is used in portable
blowlamps and in gas lighters.
H
H
H
H
melting point
138
C
H
C
C
C
C
H
boiling point
0
C
H
H
H
H
a
butane
H
H
H
melting point
159
C
C
H
H
C
C
boiling point
12
C
H
H
H
H
C
H
b
2-methylpropane
Figure 14.4
The isomers of C
4
H
10
.
Compounds such as those in Figure 14.4 are
known as
isomers
. Isomers are substances which
have the same molecular formula but different
structural formulae. The different structures of the
compounds shown in Figure 14.4 have different
melting and boiling points. Molecule
b
contains a
branched chain and has a lower melting point than
molecule
a
, which has no branched chain. All the
alkane molecules with four or more carbon atoms
possess isomers. Perhaps now you can see why there
are so many different organic compounds!
a
This is burning methane.
b
Central heating systems can be run on propane.
Figure 14.5
Question
1
Draw the structural formulae for the isomers of C
5
H
12
.
Another useful reaction worth noting is that
between the alkanes and the halogens. For example,
methane and chlorine react in the presence of
sunlight (or ultraviolet light). The ultraviolet light
splits the chlorine molecules into atoms. When this
type of reaction takes place, these atoms are called
free radicals
and they are very reactive.
●
The chemical behaviour
of alkanes
Alkanes are rather unreactive compounds. For
example, they are generally not affected by alkalis,
acids or many other substances. Their most important
property is that they burn easily.
sunlight
chlorine gas
chlorine atoms
(free radicals)
Cl
2
(
g
)
2Cl(
g
)
