18.2. Esters
A subsection of Chemistry, 9701, through 18. Carboxylic acids and derivatives
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Esterases are enzymes that hydrolyse esters. R C + H2O + HO–R' O O R' R C O O H Enzymes can be quite specific in the structures of the substrates they act upon. For example, an esterase isolated from the mould Aspergillus niger will hydrolyse phenyl ethanoate, CH3CO2C6H5, but not its isomer methyl benzoate, C6H5CO2CH3. Outline how enzymes catalyse reactions, and explain their specificity. Use diagrams in your answer where appropriate. Sample bottles of each of the isomers phenylethanoate and methylbenzoate have lost their labels and so have been named isomer A and isomer B. The carbon-13 NMR spectra of isomers A and B contain the following peaks. isomer A isomer B δ 52 δ 26 δ 128 δ 122 δ 129 δ 126 δ 130 δ 129 δ 133 δ 151 δ 167 δ 169 The identity of the compound responsible for each spectrum can be deduced by studying the chemical shifts (δ) of the peaks in the spectra. Use the Data Booklet to assign the correct peaks to the labelled carbon atoms in the structures of the isomers below. Write each value next to the relevant carbon atom and hence deduce the identity of each isomer. C CH3 O O methyl benzoate is isomer O CH3 C O phenyl ethanoate is isomer These two isomers are difficult to distinguish chemically. Describe a method of converting them to suitable products in step 1 which can then be tested in step 2. You should state the reagents and conditions for each step, and any observations you would make. step 1 step 2
9701_s16_qp_42
THEORY
2016
Paper 4, Variant 2
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