6.1.3: Carboxylic acids and esters
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Carboxylic acid functional group
R — C — O — OH
Ester functional group
R — C — O — O —- R
Physical properties of carboxylic acids
• High MBP ∵ can form hydrogen bonds between molecules • Soluble in water: solubility decreases w/ increasing chain length ∵ greater proportion of chain = hydrophobic • Permanent dipole ⟶ δ⁻ on the oxygens, δ⁺ on the C and H
Chemical properties of carboxylic acids: reaction with base
Acid + Base ⟶ Salt + Water e.g. CH₃COOH + NaOH ⟶ CH₃COONa + H₂O
Chemical properties of carboxylic acids: reaction with carbonate
Acid + Carbonate ⟶ Salt + Water + Carbon dioxide e.g. CH₃COOH + NaHCO₃ ⟶ CH₃COONa + H₂O + CO₂
Chemical properties of carboxylic acids: reaction with PCl₅
CH₃COOH + PCl₅ ⟶ CH₃COCl + POCl₃ + HCl
Chemical properties of carboxylic acids: reduction
CH₃COOH –[H]–> CH₃CH₂OH Where [H] is LiAlH₄
Why use LiAlH₄ rather than NaBH₄ to reduce carboxylic acids?
• LiAlH₄ is more aggressive than NaBH₄ • This is necessary because you need to go back two steps to reduce a carboxylic acid to an alcohol 1) Carboxylic acid reduced to aldehyde 2) Aldehyde reduced to primary alcohol
Chemical properties of carboxylic acids: reaction with NH₃
CH₃COOH + NH₃ (aq) ⟶ CH₃CONH₂ + H₂O
Give the two ways esters can be synthesised?
• Alcohol + Carboxylic acid ⇌ Ester + Water (H₂SO₄ catalyst, heat under reflux) • Alcohol + Acid anhydride ⟶ Ester + Carboxylic acid
Which method of synthesising esters is better?
• Alcohol + Acid anhydride ∵ This gives a better yield (not under reflux) ∵ No catalyst required
An acid anhydride is the condensation reaction product of
Two carboxylic acids (which are dehydrated in the reaction, hence water molecule produced)
What is the test for an OH group?
• Add PCl₅ * If white fumes of gaseous HCl are given off, OH group is present
Uses of esters
• Perfumes, fragrances * Solvents
How to obtain a carboxylic acid from an ester?
• Hydrolysis ⟶ Hydrolysis is either with an acid or base catalyst hence “acid-catalysed hydrolysis” or “base-catalysed hydrolysis”
Conditions for acid-catalysed ester hydrolysis
• Dilute but strong acid (therefore water and protons provided) ⟶ H₂SO₄ good ∵ diprotic • Heat under reflux
Downside of acid-catalysed ester hydrolysis
• An equilibrium is established ∴ some ester will always remain unhydrolysed
Advantage of base-catalysed ester hydrolysis
• Hydrolysis goes to completion * More efficient process
Base-catalysed ester hydrolysis conditions
• Excess NaOH (aq) * Heat under reflux
Base-Catalysed ester hydrolysis produces
The carboxylate salt of the carboxylic acid (+ alcohol)
How to obtain a carboxylic acid from its carboxylate salt?
Acidification e.g. C₆H₅COO⁻Na⁺ + HCl (aq) ⟶ C₆H₅COOH + NaCl (aq)
Acid-catalysed ester hydrolysis equation
Ester + Water ⇌H₂SO₄⇌ Carboxylic acid + Alcohol
Base-catalysed ester hydrolysis equation
Ester + Base(aq) ⇌ Carboxylate salt + Alcohol
What is the mechanism for the reaction between acyl chloride and water
Addition-elimination