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AS & A Level Chemistry 33.2 Esters: Exam Style Questions Paper 4

A Level Chemistry Paper 4 - All Chapters
Question

Benzene reacts with chlorine gas to form chlorobenzene. This reaction can be described as the reaction between benzene molecules and Cl⁺ ions. The Cl⁺ ions are formed by adding a suitable catalyst to the chlorine gas.
(a) Give the name or formula of a catalyst that can be used for this reaction.

(b) The mechanism for this reaction is shown.

(i) The movement of a pair of electrons is represented by x in diagram 1.
• State where this pair of electrons is before step 1 takes place.
• State where this pair of electrons is after step 1 has taken place.
(ii) The movement of another pair of electrons is represented by y in diagram 2.
• State where this pair of electrons is before step 2 takes place.
• State where this pair of electrons is after step 2 has taken place.

There are six carbon atoms in diagram 2.
State how many of these carbon atoms are sp hybridised, \(sp^2\) hybridised, and \(sp^3\) hybridised.

(d) Complete the equation for this reaction between benzene and chlorine.

(e) The mechanism for this reaction is electrophilic substitution. Complete the following sentence. Write formulae in the gaps provided. During this reaction, the electrophile is …………………………… and a …………………………… atom in benzene is substituted by a …………………………… atom.
(f) Chloroethane reacts with NaOH(aq). Chlorobenzene does not.
(i) Name the mechanism of the reaction that chloroethane undergoes with NaOH(aq), and identify the major organic product that is formed.
(ii) Explain the difference in reactivity of chloroethane and chlorobenzene when treated with NaOH(aq).

▶️ Answer/Explanation
Solution

(a) Aluminium chloride (AlCl₃) or iron(III) chloride (FeCl₃).

Explanation: These Lewis acids catalyze the formation of Cl⁺ by polarizing Cl₂, making it more electrophilic for benzene substitution.

(b)(i) • Before step 1: The electron pair (x) is in the delocalized π-system of benzene.
• After step 1: The electron pair forms a new C–Cl bond with the electrophile Cl⁺.

Explanation: The π-electrons of benzene attack Cl⁺, breaking the aromaticity temporarily to form a σ-complex.

(b)(ii) • Before step 2: The electron pair (y) is in the C–H bond of the σ-complex.
• After step 2: The electron pair rejoins the delocalized π-system, restoring aromaticity.

Explanation: Deprotonation regenerates the aromatic ring, completing the electrophilic substitution.

(c) sp hybridised: 0, \(sp^2\) hybridised: 5, \(sp^3\) hybridised: 1.

Explanation: Five carbons remain \(sp^2\) hybridized (part of the aromatic ring), while one carbon bonded to Cl becomes \(sp^3\) hybridized.

(d) \(C_6H_6 + Cl_2 \rightarrow C_6H_5Cl + HCl\).

Explanation: Benzene reacts with chlorine in the presence of a catalyst to form chlorobenzene and hydrogen chloride.

(e) The electrophile is \(Cl^+\), a hydrogen (H) atom in benzene is substituted by a chlorine (Cl) atom.

Explanation: Electrophilic substitution involves \(Cl^+\) replacing H on the benzene ring.

(f)(i) Mechanism: Nucleophilic substitution. Major product: Ethanol (CH₃CH₂OH).

Explanation: NaOH attacks chloroethane via \(S_N2\), replacing Cl with OH⁻ to form ethanol.

(f)(ii) Chlorobenzene’s C–Cl bond is strengthened by resonance with the ring’s π-electrons, making it inert to nucleophilic substitution. Chloroethane lacks this stabilization.

Explanation: The lone pair on Cl in chlorobenzene delocalizes into the ring, giving the C–Cl bond partial double-bond character and higher bond strength.

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