4-Methylbenzoic acid, CH3C6H4COOH\text{CH}_3\text{C}_6\text{H}_4\text{COOH}CH3C6H4COOH, is used as a chemical intermediate.
A student prepares 4-methylbenzoic acid as outlined below.
Step 1 The student mixes 4.50 cm34.50\text{ cm}^34.50 cm3 of 4-methylbenzyl alcohol, CH3C6H4CH2OH\text{CH}_3\text{C}_6\text{H}_4\text{CH}_2\text{OH}CH3C6H4CH2OH (density =1.03 g cm−3= 1.03\text{ g cm}^{-3}=1.03 g cm−3), with sodium carbonate and aqueous potassium manganate(VII) as an oxidising agent. The mixture is heated under reflux.
Step 2 The resulting mixture is cooled and then acidified with concentrated HCl\text{HCl}HCl. Impure crystals of 4-methylbenzoic acid appear.
Step 3 The student recrystallises the impure crystals to obtain 1.85 g1.85\text{ g}1.85 g of pure 4-methylbenzoic acid.
In Step 1, sodium carbonate, Na2CO3\text{Na}_2\text{CO}_3Na2CO3, makes the reaction mixture alkaline. Write an ionic equation to show how carbonate ions form an alkaline solution in water.
In Step 2, explain why the mixture must be acidified so that crystals of 4-methylbenzoic acid appear.
Write the overall equation for the preparation of 4-methylbenzoic acid from 4-methylbenzyl alcohol. Use [O][\text{O}][O] for the oxidising agent.
Calculate the percentage yield of 4-methylbenzoic acid. Give your answer to 3 significant figures.
In Step 3, describe how the student can recrystallise the impure crystals to obtain pure 4-methylbenzoic acid.
