Summary notes for AQA A-Level Chemistry Unit 3.3.9 - Carboxylic acids and derivatives (A-level only) by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: Carboxylic Acids, Esters, Carboxylic Acid Derivatives, Acylation
Carboxylic Acids and Derivatives
Carboxylic Acids
Carboxylic acids have a functional group known as the carboxyl group shown as COOH in structural
formulae + are named using the -oic acid suf x.
• Carboxylic acids are weak acids as they are only partially dissociated in solution (i.e. only a small
fraction of the molecules are ionised in solution).
Carboxylic acids can form salts in their reactions. The metal salt of a carboxylic acid = a carboxylate.
• The sodium salt of ethanoic acid: is sodium ethanoate:
- ∴ -oic ➜ metal -oate
Physical Properties
Solubility in water
Short chain carboxylic acids are very soluble in water. This is because the highly polar C=O + -OH can
form H-bonds w/ water. As the no. of C atoms in the chain ↑, the solubility ↓ due to the longer non-polar
hydrocarbon chain.
Boiling point
The b.p. of carboxylic acids is higher than corresponding alcohols.
• E.g. propan-1-ol + ethanoic acid…
- Have the same relative Mr + ∴ the same no. of electrons, resulting in similar Van der Waals
forces between the molecules - both have H-bonds between their molecules.
- The higher b.p. of the carboxylic acid is because the H-bonding occurs between 2 molecules of
the acid to form a DIMER. This doubles the size of the molecule + ↑ the van der Waals’ forces
between the dimers resulting in a higher b.p.
Acid Reactions of Carboxylic Acids
The carboxylic acids react w/ bases, alkalis + reactive metals to form salts as other acids do.
With carbonates
Observations: there will be effervescence + the solid carbonate will be used up, producing a colourless
solution.
Acid + carbonate → salt + water + carbon dioxide
• Ethanoic acid w/ sodium carbonate: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2
• Ethanoic acid w/ sodium hydrogen carbonate: CH3COOH + NaHCO3 → CH3COONa + H2O + CO2
With metals
Observations: there will be effervescence + the solid metal will be used up producing a colourless
solution.
Acid + metal → salt + hydrogen
• Ethanoic acid w/ magnesium: 2CH3COOH + Mg → (CH3COO)2Mg + H2
With bases
Observations: there is a release of heat + colourless solution remains.
Acid + base → salt + water
• Ethanoic acid w/ sodium hydroxide: CH3COOH + NaOH → CH3COONa + H2O
fi
, Section 3 : Organic Chemistry
With ammonia
Observations: there is a release of heat + the colourless solution remains.
Acid + ammonia → ammonium salt
• Ethanoic acid w/ ammonia: CH3COOH + NH3 → CH3COONH4 (ammonium ethanoate)
Esters
Esters have a functional group known as the ester group or ester linkage shown as -COO- in structural
formulae.
• Esters have the general structure:
- R is from the acid
- R’ is from the alcohol.
• Esters are named as an alkyl carboxylate. When naming, the alkyl part of the ester comes from the
alcohol; the carboxylate part comes from the carboxylic acid. (Remember it by thinking: alcohol
comes before carboxylic acid alphabetically.)
Carboxylic acids + alcohols react, in the presence of heat + a strong acid catalyst (e.g.H2SO4), to give
esters. This reaction = esteri cation. The formation of the ester above can be represented by the eq.
- Water is eliminated from the reactants in the reaction ∴ condensation reaction.
- Reversible reaction
• E.g. methanoic acid + propan-1-ol ⇌ propyl methanoate + water
methanoic acid + butan-2-ol ⇌ 2-butyl methanoate + water
Common Uses of Esters
1. Plasticisers
- Plasticisers are additives mixed into polymers to improve their exibility.
2. Solvents
- Solvents for paints, glues, printing inks + nail polish remover as esters = polar liquids.
- Common solvent due to its low cost and low toxicity.
3. Perfumes
- As they have pleasant smells.
4. Food avourings
- Esters are responsible for the smell + avour of many fruits + owers.
- They are used as arti cial avourings.
Hydrolysis of Esters
Hydrolysis is the reverse of an esteri cation reaction. The ester is split by the action of water into the
carboxylic acid + alcohol.
• There are 2 types of ester hydrolysis reactions - acid hydrolysis + alkaline hydrolysis…
Acid Hydrolysis
Catalyst: dilute hydrochloric acid
Conditions: heat under re ux
Products: carboxylic acid + alcohol
• This is a reversible reaction, giving
lower yields.
E.g.
fl fi fl fi fifl fl fl
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