Alcohols (SQA National 5 Chemistry): Revision Note

The alcohols

What are alcohols?

• Alcohols are a family of organic compounds

  • They are not hydrocarbons because they contain oxygen

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

• Alcohols are a homologous series with four key facts:

1. They contain the -OH (hydroxyl) functional group

• All alcohols contain a hydroxyl group

  • This is an oxygen atom bonded to a hydrogen atom (-OH)

• This is the functional group that gives alcohols their characteristic properties

2. They are represented by a general formula

• The general formula for the alcohol homologous series is C_nH_2n+1OH

  • This formula applies to saturated, straight-chain alcohols

• For example:

  • If an alcohol has 2 carbon atoms, n = 2

  • So, its formula will be C₂H_{(2 x 2) + 1}OH = C₂H₅OH

3. They are used as fuels

• Alcohols are highly flammable and burn with very clean flames, releasing large amounts of energy

• They are used as:

  • Fuels in spirit burners and camping stoves

  • Biofuels for cars

4. They are used as solvents

• Alcohols are excellent solvents because they can dissolve many substances that water cannot

• They are used as solvents in many products, including:

  • Perfumes

  • Deodorants

  • Hand sanitisers

Names & formulae of alcohols

• Like other organic compounds, alcohols are named systematically

• There are three skills for alcohols:

  • Naming alcohols from their structure

  • Drawing alcohols from their name

  • Determining the formula of an alcohol

1. Naming alcohols

• The name of a straight-chain alcohol is made of three parts:

  1. A prefix to show the number of carbon atoms

  2. A number to show which carbon the -OH group is attached to

  3. The ending -ol to show it is an alcohol

• The rules for naming alcohols:

  1. Find the longest continuous chain of carbon atoms

     • This chain must contain the -OH group

     • The -OH group means that the name ends with "-ol"

  2. Number the carbon atoms

     • This is only needed if there are 3 or more carbon atoms

     • Start from the end closest to the -OH group, to give the functional group the lowest possible number

  3. Combine the parts to make the full name

     • Write the main chain prefix

     • Add a hyphen

     • Write the number of the carbon that the -OH group is attached to

     • Add a hyphen

     • Write the "-ol" ending for an alcohol

  4. If there are branches, you name them in the same way as for alkanes

2. Drawing alcohols

• You can work backwards from the name to draw the structure of an alcohol

• The rules for drawing alcohols:

  1. Identify the longest carbon chain

     • Identify the number of carbons in the longest chain

     • For example, "propan" means a 3-carbon chain with carbon-carbon single bonds, C-C

  2. Draw the main carbon chain

  3. Identify the -OH group:

     • Identify the carbon with the -OH group attached

     • For example, "-1-" means the -OH group is attached to carbon 1

  4. Add the -OH group

  5. Add the hydrogens to complete the structure

     • Make sure every carbon atom has exactly four bonds

3. The molecular formula of alcohols

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