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Amines (A-level only)

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Question 16

Benzoic acid can be prepared from ethyl benzoate. Ethyl benzoate is first hydrolysed in alkaline conditions as shown:

C6H5COOCH2CH3+NaOH→C6H5COONa+CH3CH2OH \text{C}_6\text{H}_5\text{COOCH}_2\text{CH}_3 + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{COONa} + \text{CH}_3\text{CH}_2\text{OH} C6​H5​COOCH2​CH3​+NaOH→C6​H5​COONa+CH3​CH2​OH

A student used the following method:

  1. Add 6.0 cm3 of ethyl benzoate (density=1.05 g cm−3\text{density} = 1.05\text{ g cm}^{-3}density=1.05 g cm−3, Mr=150.0M_r = 150.0Mr​=150.0) to 25.0 cm3 of aqueous 2.5 mol dm-3 sodium hydroxide in a round-bottomed flask.
  2. Add a few anti-bumping granules and attach a condenser to the flask. Heat the mixture under reflux for 45 minutes. Allow the mixture to cool to room temperature.
  3. Pour 35.0 cm3 of 2.0 mol dm-3 hydrochloric acid into the cooled mixture.
  4. Filter off the precipitate of benzoic acid under reduced pressure.

Questions:

a.

Suggest how the anti-bumping granules prevent bumping during reflux.

[2]
b.

Show, by calculation, that an excess of sodium hydroxide is used in this reaction.

[4]
c.

Suggest why an electric heater is used rather than a Bunsen burner in this hydrolysis.

[2]
d.

Write an equation for the reaction between sodium benzoate and hydrochloric acid.

[1]
e.

In a similar experiment, another student used 0.038 mol of ethyl benzoate and obtained 4.90 g of benzoic acid (Mr=122.0M_r = 122.0Mr​=122.0). Calculate the percentage yield of benzoic acid and suggest why the calculated yield is greater than 100%.

[3]

Amines (A-level only) Questions

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