Reactivity of halogenoalkanesEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student compares the rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane. A few drops of each halogenoalkane are added to separate test tubes containing aqueous silver nitrate in ethanol, held in a water bath at 60 °C, and the time for a precipitate to appear is measured.

   (a)

   Which halogenoalkane gives a precipitate first, and what colour is it?

   [1 mark]

   • A1-chlorobutane, white
   • B1-bromobutane, cream
   • C1-iodobutane, yellow
   • D1-iodobutane, white

   (b)

   Why is ethanol used as the solvent for the silver nitrate?

   [1 mark]

   • AIt dissolves both the halogenoalkane and the aqueous silver nitrate so they can mix
   • BIt acts as the nucleophile that hydrolyses the halogenoalkane
   • CIt is a catalyst for the precipitation of silver halide
   • DIt reacts with silver ions to form the precipitate

   (c)

   Explain the order of the rates of hydrolysis of the three compounds in terms of bond enthalpy.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician compares three isomers of C₄H₉Br: 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane. Equal volumes of each are added to separate tubes of aqueous silver nitrate in ethanol at the same temperature, and the time for the first precipitate is recorded.

   (a)

   Which isomer is expected to give a precipitate first?

   [1 mark]

   • A1-bromobutane
   • B2-bromo-2-methylpropane
   • C2-bromobutane
   • DAll three at the same time

   (b)

   Which statement gives the trend in the rate of hydrolysis of halogenoalkanes of the same halogen?

   [1 mark]

   • Aprimary > secondary > tertiary
   • Bsecondary > primary > tertiary
   • Ctertiary > primary > secondary
   • Dprimary < secondary < tertiary

   (c)

   The precipitate appeared after 15 s for 2-bromo-2-methylpropane and after 300 s for 1-bromobutane. Calculate how many times faster the tertiary isomer reacted, and state one variable that had to be kept constant.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student prepares 2-chloro-2-methylpropane by shaking 2-methylpropan-2-ol with concentrated hydrochloric acid in a separating funnel. The product is a colourless liquid that is immiscible with water. Its density is 0.84 g cm⁻³, the aqueous acid layer has a density of about 1.1 g cm⁻³, and the product boils at 51 °C.

   (a)

   After shaking and standing, two layers form. State which layer is the organic product, explain how you know, and describe what the student then does to the other layer.

   [3 marks]

   (b)

   The organic layer is washed with aqueous sodium hydrogencarbonate, dried and then distilled. Explain the purpose of each of these three stages and state the temperature range at which the product should be collected.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student plans to investigate how the structure of a halogenoalkane affects its rate of hydrolysis, using aqueous silver nitrate in ethanol warmed in a water bath at 60 °C. Mean bond enthalpies from the data booklet are C–F 467, C–Cl 346, C–Br 290 and C–I 228 kJ mol⁻¹.

   (a)

   Describe how the student could compare the rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane, including the measurements taken, the variables controlled, the expected results and one safety precaution.

   [6 marks]

   (b)

   Explain the order of reactivity of the three halogenoalkanes, evaluate whether bond polarity or bond enthalpy is the better explanation, and state the trend in rate for primary, secondary and tertiary isomers of the same halogenoalkane.

   [6 marks]

   Total for question 4: 12 marks

End of questions