A-LevelChemistryCarboxylic acids and derivatives9701/42 · Feb/Mar 2022 · Q614 Marks

Lidocaine is used as an anaesthetic. A synthesis of lidocaine is shown in Fig. 6.1. [Figure 6.1 showing synthesis of Lidocaine from W and X via Y]

(a)W can be formed by reacting HOCH2COOH with an excess of SOCl2. Write an equation for this reaction.[1]
(b)After W and X have reacted together, an excess of CH3COONa(aq) is added to the reaction mixture. Suggest why.[1]
(c)The reaction of W with X, reaction 1, follows an addition-elimination mechanism. Complete the mechanism for the reaction of W with X. Include all relevant curly arrows, lone pairs of electrons, charges and partial charges. Use Ar-NH2 to represent X. [Diagram showing W (chloroacetyl chloride) and Ar-NH2][4]
(d)(C2H5)2NH reacts with Y in reaction 2. Explain why (C2H5)2NH can act as a nucleophile.[1]
(e)(i)The purity of lidocaine can be checked using thin-layer chromatography. Ethyl ethanoate is used as the solvent. The Rf values of X and lidocaine are given in Table 6.1. Table 6.1 compound Rf X 0.49 lidocaine 0.71 Identify the substances used as the mobile and stationary phases in this thin-layer chromatography experiment.[1]
(e)(ii)Describe how an Rf value can be calculated.[1]
(e)(iii)Suggest why the Rf value for X is less than that for lidocaine.[1]
(f)(i)The proton (1H) NMR spectrum of lidocaine is shown in Fig. 6.2. [Figure 6.2] Name the splitting patterns at δ2.6 and δ1.1.[1]
(f)(ii)The relative peak area of the peaks at δ3.0 and δ2.3 is 1:3 respectively. Identify the protons in the 1H NMR spectrum of lidocaine that are responsible for the peaks at the following chemical shift values. δ7.1 δ3.0 δ2.3[2]
(f)(iii)Predict the number of peaks in the carbon-13 (¹³C) NMR spectrum of lidocaine.[1]

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 (Feb/Mar 2022). The mark or mark type is shown in brackets.

[1] HOCH2COOH + 2SOCl2 → CICH2COCl + 2SO2 + 2HCl
[1] to remove / neutralise excess H+/acid produced OR to react with any acidic by-products / HCl/ SO2 OR to react with any unreacted W
[4] M1: curly arrow from lone pair on :NH2 to carbonyl C(δ+)=O ; M2: correct dipole on δ+C=O AND curly arrow from bond C=O to O(δ–) ; M3: correct structure of the intermediate (inc. charges) ; M4: curly arrow from lone pair on :O¯ to C=O AND curly arrow from C-Cl to Cl
[1] N / nitrogen can donate its lone pair / LP / pair of electrons
[1] mobile = ethyl ethanoate ; stationary = SiO2 / silica or Al2O3 / alumina
[1] Rf = distance travelled by solute / substance / compound / component ÷ distance travelled by solvent (front)
[1] X is more attracted / more affinity / adsorbed more to the stationary phase OR lidocaine dissolves better in the solvent ORA
[1] quartet AND triplet
[2] • δ 7.1 = attached to aromatic ring / H—Ar ; • δ 3.0 = alkyl next to C=O / –CH(2)–C=O ; • δ 2.3 = alkyl next to aromatic ring / H(3)C–Ar (All three correct for two marks)
[1] 9 (nine)