Ethanoic Acid: Basics
Carboxylic acids
- These are a homologous series of organic compounds that all contain the same functional group: –COOH
- They are colourless liquids which are weakly acidic and have typical acidic properties
- They react with alkaline solutions, turn blue litmus red and form salts called ethanoates
The first three carboxylic acids
Exam Tip
The carbon atom in the -COOH functional group is counted as part of the molecule and not just the functional group. Eg. CH3CH2CH2COOH has 4 carbon atoms so is called butanoic acid, not propanoic acid.Extended Only
Ethanoic Acid
- Ethanoic acid is a typically weak acid and dissociates slightly in water, producing a mildly acidic solution
- The equilibrium lies far to the left during ionization:
CH3COOH ⇌ H+ + CH3COO-
- Ethanoic acid reacts with the more reactive metals, hydroxides and carbonates
Reactions of ethanoic acid
- In the reaction with metals a metal salt and hydrogen gas are produced
- For example in reaction with magnesium the salt magnesium ethanoate is formed:
2CH3COOH + Mg → (CH3COO)2Mg + H2
- In the reaction with hydroxides a salt and water are formed in a neutralisation reaction
- For example in reaction with potassium hydroxide the salt potassium ethanoate is formed:
CH3COOH + KOH → CH3COOK + H2O
- In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced
- For example in reaction with potassium carbonate the salt potassium ethanoate is formed:
2CH3COOH + K2CO3 → 2CH3COOK + H2O + CO2
Extended Only
Ethanoic Acid & Esterification Reactions
Making carboxylic acids
Oxidation by fermentation
- The microbial oxidation of ethanol will produce a weak solution of vinegar (ethanoic acid).
- This occurs when a bottle of wine is opened as bacteria in the air (acetobacter) will use atmospheric oxygen from air to oxidise the ethanol in the wine:
C2H5OH + O2 → CH3COOH + H2O
- The acidic, vinegary taste of wine which has been left open for several days is due to the presence of ethanoic acid
Oxidation with potassium manganate (VII)
- Alcohols can also be oxidised to carboxylic acids by heating with acidified potassium manganate (VII)
- The heating is performed under reflux which involves heating the reaction mixture in a vessel with a condenser attached to the top
- The condenser prevents the volatile alcohol from escaping the reaction vessel as alcohols have low boiling points
Diagram showing the experimental setup for the oxidation with K2MnO4 using reflux apparatus
Making esters
- Alcohols and carboxylic acids react to make esters in esterification reactions
- Esters are compounds with the functional group R-COO-R
- Esters are sweet-smelling oily liquids used in food flavourings and perfumes
- Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate:
CH3COOH + C2H5OH → CH3COOC2H5 + H2O
Diagram showing the formation of ethyl ethanoate
Naming esters
- An ester is made from an alcohol and carboxylic acid
- The first part of the name indicates the length of the carbon chain in the alcohol, and it ends with the letters ‘- yl’
- The second part of the name indicates the length of the carbon chain in the carboxylic acid, and it ends with the letters ‘- oate’
- g. the ester formed from pentanol and butanoic acid is called pentyl butanoate
Diagram showing the origin of each carbon chain in ester
Examples of esters