18A.2 Electrophilic substitution of benzene Quiz: Pearson Edexcel Chemistry, Unit 18

The 20 questions

1. What type of reaction does benzene undergo with bromine in the presence of an iron or aluminium halide catalyst?

   • Nucleophilic substitution, with bromide acting as the nucleophile that replaces a hydrogen atom
   • Electrophilic addition across a delocalised double bond, in which the bromine adds to two carbon atoms
   • Free-radical addition that adds two bromine atoms to the ring in the presence of ultraviolet light
   • Electrophilic substitution, in which a hydrogen atom is replaced by bromine

2. Which electrophile is generated when benzene is nitrated?

   • NO+, the nitrosonium ion
   • NO3-, the nitrate ion
   • NO2-, the nitrite ion
   • NO2+, the nitronium ion

3. Which reagents are used to nitrate benzene?

   • Concentrated nitric acid alone, heated under reflux for a long time
   • A mixture of concentrated nitric acid and concentrated sulfuric acid
   • Dilute nitric acid and sodium hydroxide, heated gently in a water bath
   • Nitrogen dioxide gas dissolved in water, with no catalyst added

4. What is the role of aluminium chloride in Friedel-Crafts alkylation of benzene?

   • It acts as a catalyst by polarising the halogenoalkane to generate an electrophilic carbocation
   • It reduces the halogenoalkane to an alkane, which then reacts with the benzene ring in a second separate step
   • It acts as a nucleophile that attacks the benzene ring first, forming a bond to carbon before the electrophile is formed
   • It is a solvent that dissolves the benzene ring so the reagents can mix more easily during the reaction stage

5. Which catalyst is needed to generate the electrophile Br+ from bromine?

   • Sodium hydroxide, which removes a proton from bromine
   • Concentrated sulfuric acid, which oxidises bromine to bromate
   • Iron(III) bromide or aluminium bromide, which polarises the Br-Br bond
   • Platinum, which adsorbs bromine molecules onto its surface

6. Which halogenoalkane and catalyst would give methylbenzene by Friedel-Crafts alkylation?

   • Iodomethane with concentrated sulfuric acid
   • Chloroethane with sodium hydroxide
   • Bromoethene with iron(III) chloride
   • Chloromethane with aluminium chloride

7. In electrophilic substitution of benzene, what intermediate forms after the electrophile attacks the ring?

   • A carbanion with a negative charge on one carbon, stabilised by the electron-withdrawing nitro group
   • A free radical delocalised over the whole ring, formed by homolytic fission of the C-H bond
   • An arenium ion, a carbocation stabilised by delocalisation over five carbon atoms
   • A nitrene formed by loss of two hydrogen atoms, which then inserts into a neighbouring C-H bond

8. What is the last step in the electrophilic substitution mechanism of benzene?

   • Loss of a proton from the arenium ion, regenerating the delocalised ring
   • Loss of a bromide ion, leaving a positively charged ring that is then neutralised
   • Reaction of the intermediate ion with water to form phenol and hydrogen ions
   • Addition of a second electrophile to the ring, giving a disubstituted product in one step

9. Benzene reacts with ethanoyl chloride in the presence of aluminium chloride. What is the organic product?

   • Phenylethanol, formed by reduction of the acyl group
   • Phenylethanone (acetophenone)
   • Benzoic acid, formed by oxidation of the side chain
   • Ethylbenzene, formed by reduction of the ketone group

10. Calculate the relative molecular mass of nitrobenzene, C6H5NO2, using Ar values C = 12, H = 1, N = 14, O = 16.

    • 138
    • 123
    • 77
    • 122

11. What is the molecular formula of bromobenzene?

    • C6H4Br2
    • C6H5Br
    • C6H5Br2
    • C6H6Br

12. Why does electrophilic substitution occur with benzene rather than addition?

    • Addition is impossible because the bromine molecule is too large to approach the carbon atoms of the ring closely
    • Substitution restores the delocalised pi system after the electrophile attacks, keeping its stabilisation energy
    • Substitution is faster because benzene has more hydrogen atoms available on the ring to be removed in each step
    • Addition would form a stable carbocation that is too unstable to be formed from the delocalised ring system

13. Which step is usually the slow, rate-determining step in electrophilic substitution of benzene?

    • The step where the bromine molecule is polarised by the catalyst, which requires a large amount of energy
    • The first step, where the electrophile attacks the ring and disrupts its delocalisation
    • The final step, where the proton is removed from the ring and the catalyst is released to react again
    • The step where the catalyst is regenerated at the end of the reaction, which removes the last trace of product

14. Which species is the electrophile in the bromination of benzene with a catalyst?

    • Br2 molecule with no polarisation, which attacks the ring directly as a neutral species
    • Br+, or a polarised Br-Br-FeBr3 complex that acts as Br+
    • Br-, the bromide ion, which attacks the ring as a nucleophile in each step
    • BrO-, the hypobromite ion, formed by reaction with water in the mixture

15. What is the by-product of the bromination of benzene in the presence of a catalyst?

    • Bromine chloride
    • Carbon dioxide
    • Water
    • Hydrogen bromide

16. Why is the acylation product of benzene not easily acylated a second time?

    • The acyl group is electron-donating, which makes the ring too electron-rich to react
    • The acyl group blocks all positions on the ring, so no site is left
    • The product is a salt that cannot be attacked by any electrophile
    • The acyl group is electron-withdrawing, which deactivates the ring towards a second electrophilic attack

17. Describe the generation of the electrophile in Friedel-Crafts alkylation with CH3Cl and AlCl3.

    • AlCl3 donates a chloride ion to the ring, forming an anion that attacks the methyl group
    • AlCl3 reacts with the ring to give an aluminium complex, which is then methylated
    • CH3Cl adds across the delocalised ring to form a cyclohexyl chloride
    • AlCl3 polarises the C-Cl bond, forming a CH3+ electrophile that attacks the ring, then H+ is lost

18. What is the role of concentrated sulfuric acid in mixed-acid nitration of benzene?

    • It is a solvent that dilutes the nitric acid so the reaction is slower and easier to control in the lab
    • It protonates nitric acid, which loses water to generate the nitronium ion electrophile
    • It removes the hydrogen from benzene to form a benzyl anion, which then reacts with nitric acid in the next step
    • It dehydrates benzene to form an alkyne intermediate, which is then nitrated by the nitric acid present

19. Why does benzene not react with bromine without a catalyst?

    • Benzene is too polar to accept bromine molecules into the ring, so the bromine stays in the solvent layer
    • Bromine is not polarised enough to form a strong enough electrophile to disrupt the ring's stability
    • Benzene reacts with bromine only at temperatures above 500 C, which cannot be reached in a normal laboratory
    • Bromine reacts only with alkenes, which have localised pi electrons that are more exposed than those in benzene

20. Benzene reacts with a mixture of concentrated nitric and sulfuric acids at about 50 C. What is the main organic product?

    • Nitrobenzene, C6H5NO2
    • Phenylamine, C6H5NH2
    • Benzoic acid, C6H5COOH
    • Phenol, C6H5OH

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