A-LevelChemistryHydrocarbons9701/43 · May/June 2019 · Q714 Marks

Benzene can be converted into cyclohexane.

(a)(i)Benzene can be converted into cyclohexane. [Figure 7(a)(i)] (Diagram shows benzene ring converting to cyclohexane ring.) For this reaction name the type of reaction and identify the reagent and conditions needed. type of reaction reagent and conditions[2]
(a)(ii)State the bond angles in benzene and cyclohexane. bond angle in benzene bond angle in cyclohexane Explain your answers.[2]
(b)(i)When benzene reacts with SO3, benzenesulfonic acid is produced. benzenesulfonic acid [Figure 7(b)] (Diagram shows benzene + SO3 forming benzenesulfonic acid.) The mechanism of this reaction is similar to that of the nitration of benzene. Concentrated H2SO4 is used in an initial step to generate the SO3H+ electrophile as shown. SO3 + H2SO4 → SO3H+ + HSO4¯ Draw a mechanism for the reaction of benzene with SO3H+ ions. Include all necessary curly arrows and charges. benzenesulfonic acid [Figure 7(b)(i)] (Diagram shows incomplete mechanism for reaction of benzene with SO3H+ ions.)[3]
(b)(ii)Write an equation to show how the H2SO4 catalyst is reformed.[1]
(c)3-dodecylbenzenesulfonic acid can be prepared from benzenesulfonic acid. benzenesulfonic acid 3-dodecylbenzenesulfonic acid [Figure 7(c)] (Diagram shows benzenesulfonic acid converting to 3-dodecylbenzenesulfonic acid.) Suggest the reagents and conditions and name the mechanism for this reaction. reagents and conditions mechanism[2]
(d)(i)When concentrated sulfuric acid is added to water, dissociation takes place in two stages. stage 1 H2SO4 ⇌ H+ + HSO4¯ stage 2 HSO4¯ ⇌ H+ + SO42- Ka2 = 1.0 × 10-2 mol dm-3 Ka2 is the acid dissociation constant for stage 2. Write the expression for the acid dissociation constant Ka2. Ka2 =[1]
(d)(ii)H2SO4 is considered a strong acid whereas HSO4¯ is considered a weak acid. Suggest how the magnitude of the acid dissociation constant for stage 1 compares to Ka2.[1]
(e)Benzoic acid, C6H5CO2H, is a weak acid. A solution of 0.0250 mol dm-3 benzoic acid has a pH of 2.90. Calculate the Ka of benzoic acid. Ka =[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] (a)(i) M1: reduction / hydrogenation
[1] (a)(i) M2: H2 + Ni / Pt catalyst
[1] (a)(ii) M1: benzene (120°) and cyclohexane (109.5°)
[1] (a)(ii) M2: as π-bonds are transformed into σ-bonds
[1] (b)(i) M1: first curly arrow to the sulfur atom
[1] (b)(i) M2: intermediate shown
[1] (b)(i) M3: 2nd curly arrow and H+ formed / lost
[1] (b)(ii) HSO4 + H+ → H2SO4
[1] (c) M1: C12H25Br and halogen carrier e.g. AlBr3 (+ heat)
[1] (c) M2: electrophilic substitution
[1] (d)(i) Ka2 = [H+][SO42-] / [HSO4]
[1] (d)(ii) Ka of H2SO4 is larger than Ka2
[1] (e) M1: [H+] = 10-2.90 = 1.26 × 10-3
[1] (e) M2: Ka = [1.26 × 10−3]2 / 0.025 = 6.3 × 10−5 (mol dm-³)