Alcohols (Oxford AQA IGCSE Combined Science Double Award)

The Alcohol Functional Group

• All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecule that is responsible for their characteristic reactions

• Alcohols are a homologous series of compounds that have the general formula C_nH_2n+1OH

• They differ by one -CH₂ in the molecular formulae from one member to the next

The -OH group in alcohols

The first three alcohols

• The names and structures of the first threealcohols are shown below

• In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘ol’

Table to show the name, formula and displayed formula of the first four alcohols

Name	Formula	Displayed formula
Methanol	CH3OH
Ethanol	C2H5OH
Propanol	C3H7OH

Uses of Alcohols

• Alcohols are colourless liquids that dissolve in water to form neutral solutions

• The first four alcohols are commonly used as fuels

• School laboratories use ethanol in spirit burners as it burns cleanly and without strong odours

• Methanol and ethanol are also used extensively as solvents

• This is because they can dissolve many substances that water cannot such as fats and oils, but can also dissolve most of the substances that water can

Diagram to show the process of dissolving

• Alcohol is also used to make alcoholic beverages

  • Ethanol is the main alcohol used

• Alcohols react with sodium metal to produce hydrogen gas and a metal salt

• The word equation for the reaction of methanol with sodium is:

sodium + methanol → sodium methoxide + hydrogen

Oxidation of Ethanol

• Microbial oxidation occurs when a substance is oxidised by microbes such as bacteria or yeast

• When ethanol undergoes oxidation, ethanoic acid is formed

  • Ethanoic acid is the main acid in vinegar

• The bacteria Acetobacter causes this reaction to take place

• Oxidising agents such as acidified potassium dichromate can be used to form carboxylic acids from alcohols

• We can use [O] in an equation to represent oxidation

  • Oxidation is also the removal of hydrogen from the ethanol, this is why water is formed

• The equation for this reaction is:

C₂H₅OH + 2[O] → CH₃COOH + H₂O

Ethanol being oxidised to ethanoic acid