Alkanes (SQA National 5 Chemistry): Revision Note

The alkanes

What are alkanes?

• Alkanes are the simplest family of hydrocarbons

  • For a full table of the first alkanes and their structures, see the Full & shortened structural formulae

• Alkanes are a homologous series with four key facts:

1. They are saturated hydrocarbons

• This means they contain only carbon and hydrogen atoms

• All the carbon-carbon bonds in their structure are single bonds (C-C)

• Since all bonds are single, alkanes contain the maximum possible number of hydrogen atoms for their size

2. They are represented by a general formula

• The general formula for the alkane homologous series is C_nH_2n+2

  • Where 'n' stands for the number of carbon atoms.

• For example:

  • If an alkane has 3 carbon atoms, n = 3

  • So, its formula will be C₃H_{(2 x 3) + 2} = C₃H₈

3. They are commonly used as fuels

• Alkanes are the main components of fossil fuels like:

  • Natural gas (methane)

  • Petrol (a mixture including octane)

• They are excellent fuels

  • They are burnt in oxygen, known as combustion

  • This is a highly exothermic reaction, releasing large amounts of energy

• Complete combustion of alkanes produces only carbon dioxide and water

4. They are insoluble in water

• Alkanes are non-polar molecules

• They do not mix with water, which is polar

Names & formulae of alkanes

• Alkanes are named systematically

• This means they follow a clear set of rules to indicate their structure

• There are three skills for alkanes:

  • Naming alkanes from their structure

  • Drawing alkanes from their name

  • Determining the formula of an alkane

1. Naming alkanes

Straight-chain alkanes

• Straight-chain alkanes are the simplest type of alkane

• Their name has two parts:

  • A prefix to show the number of carbon atoms

  • The ending -ane because it's an alkane

Prefixes for carbon chain length

Branched-chain alkanes

• Branched-chain alkanes are more complex than straight-chain alkanes

• Their name has two sections:

  1. The branch

     • The branch name, which tells you the size of the branch

     • A number associated with the branch, which tells you where the branch is

  2. The main chain

     • This is the same as the process for a straight-chain alkane

     • The prefix shows the number of carbon atoms in the longest carbon chain

     • It keeps the -ane ending

• Naming branches:

  • Branches are named using the same prefixes as straight-chain alkanes

  • But, the ending is changed from "-ane" to "-yl"

• The rules for naming branched alkanes:

  1. Find the longest continuous chain of carbon atoms

     • This gives you the name of the main chain

     • For example, if the longest chain is 5 carbons, the name will end in "-pentane"

  2. Identify the branch number

     • Number the carbon atoms in the longest chain

       • Numbering should be done so that any branches have the lowest possible numbers

     • State the number of the carbon atom that has the branch

  3. Identify and name the branch

     • Use the "-yl" ending

     • For example, a branch of 2 carbons will be "ethyl"

  4. Combine the parts to make the full name

     • Write the branch number

     • Add a hyphen

     • Write the branch name

     • Write the longest chain name, without a space

2. Drawing alkanes

Straight-chain alkanes

• To draw the structural formula of a straight-chain alkane from its systematic name:

  • Use the name to deduce the number of carbon atoms

  • For example, "-butane" means a 4-carbon chain

• Then, draw the carbon chain and add the hydrogen atoms

Branched-chain alkanes

• To draw the structural formula of a branched-chain alkane from its systematic name:

  • Break the name down into its parts

  • Use these as instructions

• The rules for drawing branched alkanes:

  1. Identify the main carbon chain

     • This will be at the end of the chemical name

     • For example, "-hexane" means a 6-carbon chain

  2. Draw the main carbon chain

     • Do not include the hydrogens

  3. Number the carbon atoms from left to right

  4. Identify the branch and its position

     • This will be at the start of the chemical name

     • For example, "2-ethyl-" means a 2-carbon branch on carbon 2

  5. Draw the branch on the correct carbon atom

  6. Add the hydrogens to complete the structure

     • Make sure every carbon atom has exactly four bonds

3. The molecular formula of alkanes

• The general formula of alkanes can be used to determine the molecular formula