Topic 15 · page 15 of 28

Reactions of carboxylic acids

Carboxylic acids are reduced to primary alcohols by lithium tetrahydridoaluminate(III), neutralised by bases to give salts, and converted to acyl chlorides by phosphorus(V) chloride.

15 min read15.12(i)15.12(ii)15.12(iii)

Read first: Naming carboxylic acids

One acid, four reactions 15.12(i)

Ethanoic acid is the model for all carboxylic acids. This page covers three of its reactions. The fourth, with an alcohol, is on the next page.

Ethanoic acid at the centre with four arrows. To sodium ethanoate with sodium hydroxide or a carbonate. To ethanol with lithium tetrahydridoaluminate in dry ether. To ethanoyl chloride with phosphorus pentachloride. To ethyl ethanoate with ethanol, concentrated sulfuric acid and heat.
The four reactions of ethanoic acid in the specification.

Reduction to a primary alcohol 15.12(i)

Lithium tetrahydridoaluminate(III) in dry ether reduces the carboxylic acid to a primary alcohol. It does not stop at the aldehyde, because the aldehyde is reduced faster than the acid.

CHX3COOH+4 [H]→CHX3CHX2OH+HX2O \ce{CH3COOH + 4[H] -> CH3CH2OH + H2O}

The oxidation level goes acid, aldehyde, alcohol. The acid needs four [H], two more than the aldehyde.

Reactions with bases give salts 15.12(ii)

A carboxylic acid is a weak acid. It reacts with the usual bases to form a salt and water. The salt of a carboxylic acid ends in -oate.

Base Equation Observation
sodium hydroxide CHX3COOH+NaOH→CHX3COONa+HX2O\ce{CH3COOH + NaOH -> CH3COONa + H2O} colourless solution; warms slightly
sodium carbonate 2 CHX3COOH+NaX2COX3→2 CHX3COONa+HX2O+COX2\ce{2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2} effervescence
sodium hydrogencarbonate CHX3COOH+NaHCOX3→CHX3COONa+HX2O+COX2\ce{CH3COOH + NaHCO3 -> CH3COONa + H2O + CO2} effervescence
magnesium (a metal) 2 CHX3COOH+Mg→(CHX3COO)X2Mg+HX2\ce{2CH3COOH + Mg -> (CH3COO)2Mg + H2} effervescence of hydrogen

The product with sodium hydroxide is sodium ethanoate, an ionic salt that is soluble in water. Ethanoic acid is released again by adding a stronger acid such as dilute hydrochloric acid.

Test for a carboxylic acid: add sodium carbonate or sodium hydrogencarbonate. Carbon dioxide is given off, which turns limewater cloudy. Alcohols and carbonyl compounds do not give this result, because they are not acidic enough to react with a carbonate.

Reaction with phosphorus(V) chloride 15.12(iii)

Phosphorus(V) chloride, PClX5\ce{PCl5}, replaces the –OH of the acid with –Cl and gives an acyl chloride.

CHX3COOH+PClX5→CHX3COCl+POClX3+HCl \ce{CH3COOH + PCl5 -> CH3COCl + POCl3 + HCl}

The products are ethanoyl chloride, phosphorus trichloride oxide (POCl₃) and hydrogen chloride, seen as misty (steamy) white fumes. The reaction is done with dry apparatus in a fume cupboard, because ethanoyl chloride reacts with water.

Alcohols also give misty fumes of hydrogen chloride with PCl₅, so this is a test for an –OH group but not for a carboxylic acid on its own. Combine it with the carbonate test: only an acid gives both.

PracticalReactions of ethanoic acid

This is part of the further suggested practical on the reactions of acids in 15.12 ii, iii and iv. Wear eye protection. Use a fume cupboard for phosphorus(V) chloride.

  1. Carbonate. Add a spatula of sodium carbonate to 2 cm³ of ethanoic acid. Watch for effervescence and pass the gas through limewater.
  2. Indicator and base. Add universal indicator to 2 cm³ of ethanoic acid and then add sodium hydroxide solution dropwise until the colour is neutral.
  3. Phosphorus(V) chloride. In a fume cupboard, add a very small amount of PCl₅ to a dry test tube containing a few drops of dry ethanoic acid. Look for misty fumes and smell with great care by wafting.

Check yourself

  1. Write an equation for the reaction of propanoic acid with sodium hydroxide and name the product.
  2. Describe a test that distinguishes propanoic acid from propan-1-ol, with the result for each.
  3. State the reagent and conditions for reducing butanoic acid to butan-1-ol, and write the equation using [H].
  4. Write the equation for the reaction of propanoic acid with phosphorus(V) chloride and state what is seen.
  5. Explain why the reaction with phosphorus(V) chloride has to be carried out with dry apparatus.

Answers cover these until you have tried the questions

  1. CHX3CHX2COOH+NaOH→CHX3CHX2COONa+HX2O\ce{CH3CH2COOH + NaOH -> CH3CH2COONa + H2O}. The product is sodium propanoate.
  2. Add sodium carbonate (or hydrogencarbonate) solution to each. Propanoic acid gives effervescence of carbon dioxide, which turns limewater cloudy. Propan-1-ol gives no reaction.
  3. Lithium tetrahydridoaluminate(III) in dry ether (ethoxyethane). CHX3CHX2CHX2COOH+4 [H]→CHX3CHX2CHX2CHX2OH+HX2O\ce{CH3CH2CH2COOH + 4[H] -> CH3CH2CH2CH2OH + H2O}
  4. CHX3CHX2COOH+PClX5→CHX3CHX2COCl+POClX3+HCl\ce{CH3CH2COOH + PCl5 -> CH3CH2COCl + POCl3 + HCl}. Misty white fumes of hydrogen chloride are seen.
  5. PCl₅ and the acyl chloride formed both react with water, so water would use up the reagent and the product, and would produce more fumes of HCl and phosphoric(V) acid.