A-LevelChemistryHydrocarbons9701/42 · May/June 2022 · Q616 Marks

The reagent and conditions required for the nitration of benzene, benzoic acid and phenol are shown in Table 6.1. [Figure Table 6.1] Concentrated HNO3 reacts with concentrated H2SO4 to generate the electrophile NO2+.

(a)(i)Complete Fig. 6.1 to show the mechanism of the reaction between benzene and NO2+. Include all relevant curly arrows and charges. [Figure Fig. 6.1][3]
(a)(ii)Write an equation to show how H₂SO₄ is regenerated.[1]
(b)Draw the major products from the mononitration of benzoic acid and of phenol.[2]
(c)Compare the relative ease of nitration of benzene, benzoic acid and phenol. Explain your reasoning; include reference to the structures of the three compounds in your answer.[4]
(d)(i)The azo compound Congo Red is used as an acid-base indicator and can be made by the route shown in Fig. 6.2. [Figure Fig. 6.2] In step 3 of this synthesis, compound Y reacts with compound Z. Compound Z is made from compound X. Assume that the –SO3-Na+ groups do not react. Suggest structures for compounds X, Y and Z and draw them in the boxes in Fig. 6.2.[3]
(d)(ii)Give the reagents and conditions for step 1 and step 2.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[3] M1 first curly arrow from within hexagon to N of the NO2+
[3] M2 correct intermediate
[3] M3 second curly arrow from C-H bond into the ring AND H+ formed / lost
[1] HSO4 + H+ → H2SO4
[2] M1 benzoic acid (meta-nitration product)
[2] M2 phenol (ortho/para-nitration product)
[4] M1 phenol > benzene > benzoic acid
[4] M2/M3 for phenol: lone pair/p-orbital on oxygen delocalises into the ring / overlaps with π-delocalised ring accepts / attracts / polarises NO2+ / electrophiles better
[4] M2/M3 for benzoic acid: COOH / C=O is an electron-withdrawing / positive inductive effect
[4] M4 (phenol-oxygen) increases electron density in the ring (as compared to benzene as a result of the OH group) OR (benzoic acid–COOH) decreases electron density in the ring (as compared to benzene as a result of the COOH group)
[3] M1 Structure of X (4,4'-diaminobiphenyl / benzidine)
[3] M1 Structure of Z (4,4'-bis(diazonium)biphenyl)
[3] M1 Structure of Y (Sodium 4-aminonaphthalene-1-sulfonate)
[3] M1 (step 1) Sn and hydrochloric acid / HCl
[3] M2 (step 1) concentrated (HCl) + heat / reflux (followed by NaOH)
[3] M3 (step 2) HNO2 and (HCl) <10°C OR NaNO2, HCl and ≤10°C

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