Hydrocarbons: Alkanes


Saturated hydrocarbons held together by single covalent bonds.

A compound that contains hydrogen and carbon only

Functional Group
The reactive part of the molecule that largely determines the chemical properties of the molecule (alkanes do not have a functional group as the C-H bond is considered non-polar)

Homologous Series
A group of compounds with the same functional group and similar chemical properties that differ from the next by CH2

Structural Isomers
Molecules with the same molecular formula but a different structural formula



The Alkanes have the general formula of CnH(2n+2) and are referred to as a homologous series.

The first eight alkanes are: Methane, Ethane, Propane, Butane, Pentane, Hexane, Heptane and Octane. Therefore Pentane which has 5 carbon atoms must have 12 hydrogen atoms (C5H(2x5+2)).

You can identify different alkanes using their molecular formula, their structural formula, their displayed formula or their skeletal formula.

For skeletal formula you could imagine that each change in direction is where a carbon atom is located.

Thus far it is likely the only formulas you’ll have come across are molecular formulas and displayed formulas. The reason we use these other two formulas isn’t to try and confuse you! Skeletal formula is useful for drawing large complex hydrocarbons, whilst structural formula is useful for writing chemical equations. Note that any of these formulas can be used to write chemical equations.

The other main reason we don’t just use molecular formulas is because of structural isomers. These are molecules with the same molecular formula but a different structural formula.

Examples of Structural Isomers:
Both of these molecules have the molecular formula C4H10 however as you can see they are very different.

You may have also noticed that brackets have been used. These brackets show that one CH3 group is not part of the main carbon chain.

How to name Alkanes
For this module you will be required to name alkanes. The method used is as follows:
· Look at the number of carbon atoms in the longest chain
· Count the position of any side-chains (counting starts from whichever end gives the lowest side-chain position) also consider the number of side-chains – 2 side-chains is di, 3 is tri and 4 is tetra
· Consider what the side-chain is: if there’s one group it’s methyl, two ethyl, three propyl.
· Look at the functional group (which is –ane for alkanes)


Count the longest carbon chain – four carbons atoms
Consider the position of the side-chains to give the lowest number – 2 from the right (Not 3 from the left)
Consider the number and type of side chains – both on the 2nd Carbon atom and they’re both methyl

2,2 - dimethylbutane

Note how you specify all of the side chains using 2,2. The (-) is also necessary, otherwise you won’t get the marks!

Cycloalkanes are exactly what their name suggests: alkanes in one continuous cycle. They have the general formula CnH2n and have similar chemical properties to ‘regular’ alkanes. An example of a cycloalkane represented using skeletal formula is:


Chemical Properties of alkanes
The following chemical properties apply to all alkanes:
· Insoluble in water because they are non-polar
· Low melting/boiling point (high volatility) due to their weak Van der Waals forces

Strength of the Van der Waals forces
Yes we have to look at Van der Waals forces again! The strength of the Van der Waals forces is dependent on two main factors:
· The length of the longest carbon chain
· The number of branches

The longer carbon chains have stronger Van der Waals forces because there is a greater surface area and the less branched molecules also have higher boiling points due to stronger Van der Waals forces because again there is greater surface contact area.