Chemistry 9701 · AS & A Level · Hydrocarbons

Hydrocarbons — practice question

Cumene is an aromatic hydrocarbon that is used to make other useful chemicals.
(a)[1]

Complete Table 5.1 to show how many $sp^2$ and $sp^3$ hybridised carbon atoms are present in one molecule of cumene.

(b(i))[1]

Name the mechanism involved in the Friedel-Crafts alkylation shown in Fig. 5.2.

(b(ii))[1]

The initial step of the reaction produces the $(\text{CH}_3)_2\text{CH}^+$ cation. Identify an appropriate reagent for forming this cation from 2-bromopropane, $(\text{CH}_3)_2\text{CHBr$.

(b(iii))[3]

Complete Fig. 5.3 to show the mechanism for the reaction of benzene with the $(\text{CH}_3)_2\text{CH}^+$ cation. Include every relevant curly arrow and charge.

(b(iv))[1]

The Friedel-Crafts alkylation of benzene using 1-bromopropane, $\text{CH}_3\text{CH}_2\text{CH}_2\text{Br}$, also gives cumene as the main product. In the first step, the $\text{CH}_3\text{CH}_2\text{CH}_2^+$ cation rapidly rearranges to the $(\text{CH}_3)_2\text{CH}^+$ cation. Suggest why this happens. Explain your answer.

(c(i))[1]

Cumene is oxidised by air to give phenol, $\text{C}_6\text{H}_5\text{OH}$, and propanone, $\text{CH}_3\text{COCH}_3$. Reaction 1: $\text{C}_6\text{H}_5\text{CH}((\text{CH}_3)_2) + \text{O}_2 \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{CH}_3\text{COCH}_3$ , $\Delta H^\circ = -371\ \text{kJ mol}^{-1}$. Calculate the standard entropy change, $\Delta S^\circ$, of reaction 1.

(c(ii))[2]

Show that reaction 1 is feasible at $25^\circ\text{C}$.

(d(i))[4]

Fig. 5.4 shows two reactions of phenol. State the conditions for the bromination of phenol in reaction 2. Explain why these conditions are different from those for the bromination of benzene.

(d(ii))[1]

Draw the structure of organic compound Q in Fig. 5.4.

(d(iii))[1]

Identify organic reagent R.

(d(iv))[1]

Name the functional group that is formed in reaction 3.

(d(v))[2]

The reaction of phenol with $\text{HNO}_3$ gives a mixture of isomers with molecular formula $\text{C}_6\text{H}_5\text{NO}_3$. Identify the two isomers produced in the greatest amounts. Explain your answer.

Worked solution & mark scheme

This 19-mark question has a full step-by-step worked solution and mark scheme. One marking point: Number of $sp^2$ carbons = 6 and $sp^3$ carbons = 3

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