Chemistry 9701 · AS & A Level · Carboxylic acids and derivatives

Carboxylic acids and derivatives — practice question

Lidocaine is used as an anaesthetic. Figure 6.1 shows how lidocaine is synthesised.
(a)[1]

$W$ is produced when $\text{HOCH}_2\text{COOH}$ is treated with excess $\text{SOCl}_2$. Write the equation for this reaction.

(b)[1]

Once $W$ and $X$ have reacted, excess $\text{CH}_3\text{COONa(aq)}$ is then added to the reaction mixture. Suggest why.

(c)[4]

Reaction 1, the reaction of $W$ with $X$, proceeds by an addition-elimination mechanism. Complete the mechanism for the reaction of $W$ with $X$. Include every relevant curly arrow, lone pair of electrons, charge and partial charge. Use Ar-$\text{NH}_2$ to stand for $X$.

(d)[1]

$(\text{C}_2\text{H}_5)_2\text{NH}$ reacts with $Y$ in reaction 2. Explain why $(\text{C}_2\text{H}_5)_2\text{NH}$ can act as a nucleophile.

(e)

Thin-layer chromatography can be used to check the purity of lidocaine. Ethyl ethanoate is the solvent. The $R_f$ values for X and lidocaine are listed in Table 6.1.

(e(i))[1]

Identify the materials used as the mobile and stationary phases in this thin-layer chromatography experiment.

(e(ii))[1]

Describe how an $R_f$ value can be calculated.

(e(iii))[1]

Suggest why the $R_f$ value for X is less than that for lidocaine.

(f)

The proton ($^1\text{H}$) NMR spectrum of lidocaine is shown in Fig. 6.2.

(f(i))[1]

Name the splitting patterns at $\delta 2.6$ and $\delta 1.1$.

(f(ii))[2]

The relative peak area of the peaks at $\delta 3.0$ and $\delta 2.3$ is $1:3$ respectively. Identify the protons in the $^1\text{H}$ NMR spectrum of lidocaine that are responsible for the peaks at the following chemical shift values: $\delta 7.1$, $\delta 3.0$, $\delta 2.3$.

(f(iii))[1]

Predict the number of peaks in the carbon-13 ($^{13}\text{C}$) NMR spectrum of lidocaine.

(i)[1]

Name the splitting patterns at $\delta 2.6$ and $\delta 1.1$.

(ii)[2]

The relative peak area of the peaks at $\delta 3.0$ and $\delta 2.3$ is $1 : 3$ respectively. Identify the protons in the $^1\text{H}$ NMR spectrum of lidocaine that are responsible for the peaks at the following chemical shift values: $\delta 7.1$, $\delta 3.0$, $\delta 2.3$.

(iii)[1]

Predict the number of peaks in the carbon-13 ($^{13}\text{C}$) NMR spectrum of lidocaine.

Worked solution & mark scheme

This 18-mark question has a full step-by-step worked solution and mark scheme. One marking point: Equation: $\mathrm{HOCH_2COOH+2SOCl_2\rightarrow ClCH_2COCl+2SO_2+2HCl}$

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