Phenol and bromine waterEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student adds bromine water to aqueous phenol, C₆H₅OH, at room temperature. The orange colour of the bromine water disappears and a white precipitate forms.

   (a)

   What is the white precipitate?

   [1 mark]

   • A2-bromophenol
   • B4-bromophenol
   • Cbromobenzene
   • D2,4,6-tribromophenol

   (b)

   What amount of bromine, in mol, reacts with 1.00 mol of phenol to form the white precipitate?

   [1 mark]

   • A3.00
   • B2.00
   • C1.00
   • D6.00

   (c)

   Write an equation for the reaction and name its type.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Benzene does not react with bromine water. It reacts with bromine only when a catalyst of anhydrous iron(III) bromide is present. Phenol, however, reacts with aqueous bromine at room temperature with no catalyst.

   (a)

   Why is phenol more reactive than benzene towards bromine?

   [1 mark]

   • AThe O–H bond is polar, so bromine is attracted to it
   • BOxygen is electronegative and withdraws electrons from the ring
   • CA lone pair on oxygen is delocalised into the ring, increasing its electron density
   • DThe OH group is a good leaving group

   (b)

   Which statement about the reaction of phenol with bromine water is correct?

   [1 mark]

   • AIt is an addition reaction across a C=C bond
   • BIt is a substitution reaction, in which hydrogen atoms on the ring are replaced
   • CIt is a nucleophilic substitution
   • DIt requires anhydrous FeBr₃ as a catalyst

   (c)

   Explain why phenol reacts without a catalyst but benzene needs one.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student treats 0.470 g of phenol, C₆H₅OH, with excess bromine water at room temperature and obtains 1.24 g of dry 2,4,6-tribromophenol, C₆H₂Br₃OH. The OH group in phenol is attached directly to the benzene ring.

   (a)

   Explain why phenol reacts with bromine water more readily than benzene.

   [3 marks]

   (b)

   Calculate the percentage yield of 2,4,6-tribromophenol. Use relative atomic masses: H = 1.0, C = 12.0, O = 16.0, Br = 79.9.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist compares the reactions of cyclohexene, benzene and phenol with bromine, and then uses bromine water to analyse a waste-water sample for phenol.

   (a)

   Compare the reactions of cyclohexene, benzene and phenol with bromine water. Explain the differences in terms of electron density.

   [6 marks]

   (b)

   A 50.0 cm³ sample of the waste water is treated with excess bromine water and gives 0.0662 g of dry 2,4,6-tribromophenol, M = 330.7 g mol⁻¹. Calculate the concentration of phenol (M = 94.0 g mol⁻¹) in the sample in mol dm⁻³ and in g dm⁻³. Suggest one source of error in the method and state its effect on the calculated concentration.

   [6 marks]

   Total for question 4: 12 marks

End of questions