Alkanes

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Nomenclature of Alkanes

Alkanes are saturated hydrocarbons.

Alkanes are classified according to the number of carbons on the longest straight part of their chain and will always end in -ane.

The following are their prefixes:

Meth - One carbon only

Eth - Two carbons

Prop - Three carbons

But - Four carbons

To remember which order these come in, just remember the mnemonic:

Monkeys Eat Peanut Butter

So a hydrocarbon with the formula \(C_3H_8\), drawn below, is known as propane.

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Catalytic cracking

Catalytic cracking is the breaking down of long chain hydrocarbon molecules into smaller, more useful molecules using heat and a catalyst.

Crude oil usually undergoes catalytic cracking as part of its refining process.

Vapour containing the unwanted fractions is passed over a very hot catalyst surface and this breaks up the long molecules into smaller fragments, which can then be condensed into useful fuels.

Alternatively, the long chains are mixed with steam and heated to a very high temperature so that thermal decomposition reactions then occur.

These will have shorter chains than the original compounds and hence will have lower boiling points and will burn more easily.

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Homologous series

Alkanes are a homologous series - they have similar properties and a general formula.

The general formula is \(C_nH_{2n+2}\).

Alkanes have similar chemical properties - they all react with excess oxygen to form carbon dioxide and water.

For example, propane (found in camping gas used to heat barbeques) reacts with excess oxygen:

\(C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O\)