Phenylacetic acid, C6H5CH2COOH\text{C}_6\text{H}_5\text{CH}_2\text{COOH}C6H5CH2COOH, is a naturally occurring carboxylic acid used in the synthesis of various pharmaceuticals and perfumes.
A student prepares phenylacetic acid in three steps as outlined below.
Step 1 The student mixes 6.50 cm36.50\text{ cm}^36.50 cm3 of 2-phenylethanol, C6H5CH2CH2OH\text{C}_6\text{H}_5\text{CH}_2\text{CH}_2\text{OH}C6H5CH2CH2OH (density =1.02 g cm−3= 1.02\text{ g cm}^{-3}=1.02 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 phenylacetic acid appear.
Step 3 The student recrystallises the impure crystals to obtain 2.45 g2.45\text{ g}2.45 g of pure phenylacetic 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 phenylacetic acid appear.
Write the overall equation for the preparation of phenylacetic acid from 2-phenylethanol. Use [O][\text{O}][O] for the oxidising agent.
Calculate the percentage yield of phenylacetic acid. Give your answer to 3 significant figures.
In Step 3, describe how the student can recrystallise the impure crystals to obtain pure phenylacetic acid.
