A-LevelChemistryCarboxylic acids and derivatives9701/21 · Oct/Nov 2018 · Q417 Marks

The structure of glycolic acid is shown. [Figure 4.0 - Structure of glycolic acid: H-C(H)(OH)-COOH] OH glycolic acid

(a)Complete the table to show what you would observe when an aqueous solution of glycolic acid is added separately to each of the reagents. If a reaction occurs, state the functional group of glycolic acid that is responsible for the reaction. reagent | observation with glycolic acid | does a reaction occur? ✓ /x | functional group ---|---|---|--- Na2CO3(aq) | | | 2,4-DNPH | | | acidified Cr2O72- | | |[4]
(b)(i)Two reaction sequences to make glycolic acid are shown. sequence A HCHO HCN and NaCN --(reaction 1)--> X --(reaction 2)--> CH2(OH)CO2H sequence B CH3CO2H Br2 --(reaction 3)--> CH2BrCO2H --(reaction 4)--> CH2(OH)CO2H Draw the structure of X.[1]
(b)(ii)Name the reagent for reaction 2.[1]
(b)(iii)Name the mechanism of reaction 3.[1]
(b)(iv)Suggest the essential condition for reaction 3.[1]
(b)(v)Reaction 4 occurs via an SN2 mechanism. Complete the diagram for the mechanism for reaction 4. Include all relevant charges, partial charges, curly arrows and lone pairs. [Figure 4.1 - Partial SN2 mechanism showing ¯OH approaching H-C(H)Br-COOH][2]
(c)(i)Glycolic acid can also be made by reacting glyoxylic acid with NaBH4. [Figure 4.2 - Glyoxylic acid (O=CH-COOH) reacting with NaBH4 to form glycolic acid (HO-CH2-COOH)] State the role of NaBH4 in this reaction.[1]
(c)(ii)Write an equation for this reaction using molecular formulae. Use [H] to represent NaBH4.[2]
(d)(i)When glycolic acid is heated in the presence of a sulfuric acid catalyst, a new compound, Y, C4H4O4, is formed. The equation for the reaction is given. 2CH2(OH)CO₂H → C4H4O4 + 2H2O glycolic acid Y The infra-red spectrum of Y is shown. [Figure 4.3 - IR spectrum of Y] State how this spectrum differs from an infra-red spectrum of glycolic acid. Explain your answer with particular reference to the peaks within the range 1500–4000 cm⁻¹.[2]
(d)(ii)Suggest a structure for Y.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

[4] [Table of answers]: Na2CO3(aq) | effervescence / fizzing / bubbling | ✓ | COOH / carboxylic acid; 2,4-DNPH | no visible reaction owtte | x | (no group required); acidified Cr2O72- | orange to green | ✓ | -OH / alcohol; 1 mark for each in column 2 (obs); 1 mark for COOH and OH
[1] [Structure: H-C(H)(OH)-C≡N]
[1] hydrochloric / sulfuric / nitric / phosphoric acid
[1] free-radical substitution
[1] UV (light) / sunlight
[2] [Diagram with arrows and dipoles]; M1 lone pair on ¯OH AND curly arrow from lone pair to C of C-Br; M2 correct dipole on C-Br AND curly arrow from bond to Br
[1] reducing agent / reductant
[2] C2H2O3 + 2[H] → C2H4O3; M1 for correct molecular formulae C2H2O3 and C2H4O3; M2 for balancing
[2] EITHER Glycolic acid would have: M1 2500–3000 due to RCO2-H; M2 range within 3200–3650 due to RO-H OR Spectrum Y would NOT have: M1 2500–3000 due to RCO2-H; M2 range within 3200–3650 due to RO-H
[2] M1 ANY ester group AND valid C4H4O4 molecule; M2 correct cyclic structure