Chemistry 9701 · AS & A Level · Organic synthesis

Organic synthesis — practice question

Ethanol is manufactured in industry by reacting ethene with steam, with $\text{H}_3\text{PO}_4$ present as the catalyst. Reaction 1: $\text{C}_2\text{H}_4(g) + \text{H}_2\text{O}(g) \rightleftharpoons \text{C}_2\text{H}_5\text{OH}(g)$
(a)[2]

Use the bond energy data in Table 4.1 to work out the enthalpy change, $\Delta H_r$, for reaction 1.

(b)[2]

Reaction 1 is at equilibrium when temperature and pressure are constant. Deduce the effect of increasing the pressure on the amount of ethanol in the new equilibrium mixture. Use Le Chatelier’s principle to explain your answer.

(c(i))[1]

Describe the behaviour of $\text{H}_3\text{PO}_4$ in step 1 in Fig. 4.1. Explain your answer.

(c(ii))[1]

Identify the species that behaves as an electrophile in step 2 in Fig. 4.1. Explain your answer.

(c(iii))[2]

Complete Fig. 4.2 to show the mechanism for step 3 of reaction 1. Include charges, dipoles, lone pairs of electrons and curly arrows, as appropriate.

(c(iv))[2]

Describe how a catalyst affects a reaction. Explain your answer.

(c(v))[1]

Use Fig. 4.1 and Fig. 4.2 to justify why $\text{H}_3\text{PO}_4$ is described as a catalyst in reaction 1.

(c(vi))[2]

Propene also reacts with steam. A mixture of organic products is produced. Explain why propan-2-ol is produced in the higher yield.

(d)[2]

Describe the covalent bonds present between the carbon atoms in an ethene molecule by completing Table 4.2.

Worked solution & mark scheme

This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: Correctly names the bonds broken and formed and gives a calculation that results in $\Delta H \approx -50\,\text{kJ mol}^{-1}$

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