A-level Chemistry/WJEC/Module 2/Hydrocarbons

Alkanes are saturated hydrocarbons. This means that they contain only carbon and hydrogen atoms and they contain no carbon-carbon double bonds.

The alkane homologous series has the general formula of ${\displaystyle C_{n}H_{2n+2}}$. An exception is the cycloalkanes which lose two hydrogens so that the carbon can form another C-C bond; cycloalkanes have the general formula of ${\displaystyle C_{n}H_{2n}}$

There are a few things you will need to know about their physical properties in an exam:

As the relative molecular mass, or number of carbon atoms in an alkane increases, so does its boiling or melting point:

As you should be aware, alkanes are held together by induced dipole-induced dipole (ID-ID) forces. The larger a molecule is, the more electrons it has. This means it can form larger dipoles and its ID-ID forces will be larger. It will therefore take more energy to break the bonds and so the boiling and melting point will be higher.

Some alkanes can be branched instead of straight-chained. Boiling point decreases as a molecule becomes more branched. This is because branched molecules cannot pack so tightly together, so their ID-ID forces must act over larger distances (intensity of the ID-ID forces decreases) and thus require less energy to break. For example, hexane has five isomers:

The alkanes are fairly unreactive, as the C-H bond is non-polar. However they undergo three reactions.

Take the alkane ${\displaystyle C_{4}H_{1}0}$.

It will undergo combustion to form ${\displaystyle CO_{2}+H_{2}O}$

It will undergo substitution reactions (Say with ${\displaystyle Cl_{2}}$) to create ${\displaystyle C_{4}H_{9}Cl+HCl}$

It will also undergo cracking reactions to create a smaller alkane and at least one alkene, in this case, ${\displaystyle C_{2}H_{6}+C_{2}H_{4}}$

The most important of these being combustion, alkanes are very volatile & burn easily when in the presence of plentiful amounts of oxygen & thus are major fuels.

Longer chain molecules have higher boiling points and are more difficult to ignite. These long chain molecules can be broken into shorter chain (more useful) molecules through catalytic cracking. Any alkane from ${\displaystyle C_{4}}$ to ${\displaystyle C_{50}}$ can be cracked. As it is catalytic cracking it requires a catalyst, previously this catalyst was either ${\displaystyle SiO_{2}}$ or ${\displaystyle Al_{2}O_{3}}$, but now zeolite is preferred. These reactions also need high temperatures (773 K or 450 °C is usually used).

The products from cracking can vary. For example you can create several moles of one alkene and hydrogen from cracking an alkane if done right. However you can also create a mixture of alkenes & alkanes or just a mixture of alkenes.

Example: ${\displaystyle C_{8}H_{18}\rightarrow \;4C_{2}H_{4}+H_{2}}$

Under conditions of 450 °C and with an ${\displaystyle Al_{2}O_{3}}$ catalyst present.

Alkanes are very unreactive due to the non-polar C-H bond, so most reactions are not possible. However, radical substitutions are possible due to the reactivity a radicals. Radical substitution involves a halogen and UV light to initiate homolytic fission.

An example with methane.

Initiation reaction:

${\displaystyle Cl_{2}\rightarrow \;Cl^{\bullet }+Cl^{\bullet }}$

Propagation:

${\displaystyle Cl^{\bullet }+CH_{4}\rightarrow HCl+CH_{3}^{\bullet }}$

This sets into motion the chain reaction, where one free radical reacts with a stable species to form another stable species and a free radical.

The next propagation steps:

${\displaystyle CH_{3}^{\bullet }+Cl_{2}\rightarrow CH_{3}Cl+Cl^{\bullet }}$

Or

${\displaystyle CH_{3}^{\bullet }+CH_{3}Cl\rightarrow C_{2}H_{6}+Cl^{\bullet }}$

This goes on until 2 radicals meet and combine to form a stable species. Once all the radicals have done this, the reaction terminates, hence why it is the termination step.

In this reaction we have 3 possible termination steps:

${\displaystyle CH_{3}^{\bullet }+Cl^{\bullet }\rightarrow CH_{3}Cl}$

${\displaystyle Cl^{\bullet }+Cl^{\bullet }\rightarrow Cl_{2}}$

${\displaystyle CH_{3}^{\bullet }+CH_{3}^{\bullet }\rightarrow C_{2}H_{6}}$

Henceforth, the reaction ends.

Note that a free-radical hydrogen is never produced.

As we know from above, alkanes burn readily in oxygen and give off carbon dioxide, which is a weak greenhouse gas and thus a contributor to global warming.

Alkanes must burn in excess oxygen for complete combustion and the production of carbon dioxide:

CH₄ + 2 O₂ -> CO₂ + 2 H₂O

Incomplete combustion comes from a reduced supply of oxygen and produces water and carbon monoxide:

CH₄ + 1½ O₂ -> CO + 2 H₂O

Carbon monoxide is poisonous when inhaled. It binds irreversibly to haemoglobin in red blood cells and makes carboxyhaemoglobin. The red blood cells can no longer carry oxygen and this can cause suffocation if enough cells are carrying carboxyhaemoglobin.

UK regulations ensure that all gas equipment must be annually serviced and that all gas installations require adequate ventilation.