CAIE A-Level Chemistry 30 Hydrocarbons

Practise arene reactions, electrophilic substitution, halogenation conditions and substituent effects.

[Maximum number: 5]

Cumene is an aromatic hydrocarbon used in the synthesis of other useful chemicals.

cumene

(a)

Cumene can be synthesised via a Friedel-Crafts alkylation reaction, as shown in Fig. 5.2.

Fig. 5.2

[ 5 ]

(i)

The first step of the reaction forms the $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}^{+}$ cation.

Identify a suitable reagent for the formation of this cation from 2-bromopropane, $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHBr}$ .

[ 1 ]

$\mathrm{AlBr}_{3}$

(ii)

Complete Fig. 5.3 to show the mechanism for the reaction of benzene with the $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}^{+}$ cation.

Include all relevant curly arrows and charges.

Fig. 5.3

[ 3 ]

Figure for Answer to Question (ii) — CAIE A-Level Chemistry A2

M1: curly arrow from inside the hexagon ring

AND towards positively charged carbon atom (in the electrophile $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}^{+}$ )

M2: structure of the intermediate

M3: curly arrow from C-H bond into the ring AND formation / loss of $\mathrm{H}^{+}$

(iii)

The Friedel-Crafts alkylation of benzene by 1-bromopropane, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}$ , also produces cumene as the major product.

The $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2}{ }^{+}$ cation formed in the first step quickly rearranges to form the $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}^{+}$ cation.

Suggest why this is the case. Explain your answer.

[ 1 ]

$2^{\circ}$ carbocation / $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}^{+}$

AND is more stable due to greater positive inductive effect (of alkyl groups)

[Maximum number: 6]

Benzene can be used to make benzoic acid in the two-step process shown in Fig. 7.1.

Fig. 7.1

(a)

Give the reagents and conditions for step 1 and step 2.
step 1
step 2

[ 2 ]

$\mathrm{CH}_{3} \mathrm{Cl}+\mathrm{AlCl}_{3}[1]$

hot alkaline) $\mathrm{MnO}_{4}^{-}$ [1]

(b)

When treated with $\mathrm{Cl}_{2}$ under suitable conditions, methylbenzene forms compound J .

When treated with $\mathrm{Cl}_{2}$ under different conditions with different reagents, methylbenzene forms compound K.

Suggest and draw structures of compounds J and K in the boxes. The molecular formula of each compound is given.

Figure for Question (b) — CAIE A-Level Chemistry A2

State the reagents and conditions required to form each product.
to form compound J
to form compound K

[ 4 ]

[Maximum number: 2]

Draw the major products from the nitration of benzoic acid and phenol in the boxes in Fig. 7.1. The molecular formula for each major product is given in the boxes.
major product from benzoic acid, $\mathrm{C}_{7} \mathrm{H}_{5} \mathrm{NO}_{4}$
```
major product from phenol, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{3}$
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Covered syllabus points

30. Hydrocarbons question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

30. Hydrocarbons question 2

Question (a)

Question (b)

30. Hydrocarbons question 3