17 · ORGANIC CHEMISTRY: HYDROCARBONScan be broken into smaller molecules, using a process called cracking. In thermal
cracking, alkanes are heated to a high temperature. Smaller alkanes, hydrogen and
smallalkenes, such as ethene, are produced. Alkenes are members of another family
of organic compounds, which are discussed later on in this unit; they are useful starting
materials for the synthesis of many organic chemicals. Catalytic crackinginvolves
heating higher boiling point petroleum fractions in the presence of a silica–alumina
catalyst. Shorter alkanes which are suitable for use in petrol are made.Chemical properties of alkanes
Alkanes are termed saturatedbecause their molecules contain the maximum number
of hydrogen atoms and other atoms or groups cannot ‘add on’ to alkane molecules.- Combustion
Alkanes are not particularly reactive; they do not readily react with dilute acids,
alkalis, or oxidizing agents such as potassium permanganate and sodium dichro-
mate. They do, however, burn in a plentiful supply of oxygen to form carbon dioxide
and water:
CH 4 (g)2O 2 (g)CO 2 (g)2H 2 O(l)The reaction is very exothermic, which is why alkanes are used as fuels. Another
advantage of using alkanes as fuels is that ‘clean’ products are formed when they
burn. The heavier alkanes, however, tend to burn in air with a sooty flame because
there is insufficient oxygen in the air for all the carbon in the alkane to be converted
to carbon dioxide (complete combustion), so some carbon and carbon monoxide
are formed (incomplete combustion).- Halogenation
In ultraviolet light, chlorine or bromine react with alkanes to form chloroalkanes or
bromoalkanes (the general term ishalogenoalkanes). The reaction with halogens is
calledhalogenationand, since it takes place in the presence of light, it is termed a
photochemicalreaction.314
BOX 17.2
Quality of petrol
Long-chain hydrocarbons tend to burn unevenly in car engines and so cause a
rattling noise (knocking). Branched alkanes, produced from straight-chain alkanes
by catalytic cracking, are added to the fuel to prevent knocking because they burn
more evenly.The alkane 2,2,4-trimethylpentane has good antiknock properties, whereas the
antiknock properties of n-heptane are poor.Theoctane number of petrol is a measure of its quality – an octane number of 0,
for a particular petrol, means that it burns as well as pure heptane. If the octane
number is 100, then the fuel is equivalent to pure 2,2,4-trimethylpentane. Many
petrols have an octane number of 70; they burn as would a mixture of 70% 2,2,4-
trimethylpentane and 30% heptane.The burning of fuel in a car engine produces a variety of pollutants, such as carbon
monoxide, unburned hydrocarbons and nitrogen oxides. Catalytic converters change
these compounds into more environmentally friendly compounds. Although
tetraethyllead (Pb(C 2 H 5 ) 4 ) has been used as an additive which can be used to
improve the antiknock properties of fuel (leaded petrol), it stops the platinum
catalyst in a catalytic converter from functioning. Leaded petrol should therefore not
be used in a car that contains a catalytic converter.BOX 17.3
Putting a smell in
dangerous gasesCrude propane is also known as LP
(liquid petroleum) gas. It is generally
used in rural areas that are not served
by gas pipelines. LP contains about
80% propane. It can be stored as a
liquid under normal temperatures,
provided it is stored under pressure.
The liquid propane occupies a much
smaller volume than the gaseous
propane. When pressurized tanks
containing LP are opened, the gaseous
fuel is released.
Propane is extremely flammable and
has no taste, colour or smell. To warn
users of an accidental leak, it is mixed
with a strong-smelling substance such
asethyl mercaptan, C 2 H 5 SH (found in
the aroma of skunks and rotting meat).