HalogenoalkanesAQA A-Level Chemistry: Topic test

20 questions, 54 marks

AQA A-Level Chemistry

Halogenoalkanes topic test

Total 54 marks

Name

Class

Date

1
A chemist needs to convert a halogenoalkane, R–X, into the alcohol R–OH by warming it with aqueous sodium hydroxide. She can choose between the chloro, bromo, iodo and fluoro compounds with the same alkyl group R. Mean bond enthalpies: C–F 467 kJ mol⁻¹, C–Cl 346 kJ mol⁻¹, C–Br 290 kJ mol⁻¹, C–I 228 kJ mol⁻¹.
(a)
Under identical conditions, which compound would be hydrolysed fastest?
[1 mark]
AThe chloro compound
BThe bromo compound
CThe iodo compound
DThe fluoro compound
(b)
Why can a hydroxide ion attack the carbon atom of a halogenoalkane?
[1 mark]
AThe carbon atom has a lone pair of electrons that bonds to OH⁻
BThe carbon–halogen bond is polar, so the carbon atom is δ+
CThe halogen atom is positively charged
DThe carbon atom is part of a C=C double bond
(c)
The C–F bond is the most polar of the four carbon–halogen bonds, yet the fluoro compound is the least reactive. Explain this.
[2 marks]
Total for question 1: 4 marks
2
2-Bromobutane, CH₃CHBrCH₂CH₃, is heated under reflux with a solution of potassium hydroxide in ethanol. A mixture of alkenes is collected as a gas.
(a)
What is the role of the hydroxide ion in forming the alkenes?
[1 mark]
AA base, accepting a proton from the halogenoalkane
BA nucleophile, donating a lone pair to the carbon bonded to bromine
CAn electrophile, accepting a pair of electrons from the C–Br bond
DA catalyst, which is regenerated at the end of the reaction
(b)
How many different alkenes, including stereoisomers, can form in this reaction?
[1 mark]
A1
B2
C3
D4
(c)
Describe, in words, how the hydroxide ion brings about the formation of an alkene from 2-bromobutane.
[2 marks]
Total for question 2: 4 marks
3
Trichlorofluoromethane, CCl₃F, was widely used as a solvent and for making foam plastics. It is very unreactive near the ground, so it drifts upwards into the stratosphere where it is exposed to ultraviolet radiation. Mean bond enthalpies: C–F 467 kJ mol⁻¹, C–Cl 346 kJ mol⁻¹.
(a)
Write an equation for the formation of a chlorine atom from CCl₃F, name the type of bond fission involved and explain why it is a C–Cl bond that breaks rather than a C–F bond.
[3 marks]
(b)
Chlorine atoms catalyse the decomposition of ozone. Give the two equations for the steps in this catalytic cycle and the overall equation, and explain why a small amount of CCl₃F can destroy a large amount of ozone.
[4 marks]
Total for question 3: 7 marks
4
A chemist heats 2-bromobutane with sodium hydroxide under two sets of conditions. In experiment 1 the reagent is aqueous sodium hydroxide, warmed gently, and the main product is butan-2-ol. In experiment 2 the reagent is sodium hydroxide dissolved in ethanol, heated under reflux, and the main products are alkenes. Mean bond enthalpies: C–Br 290 kJ mol⁻¹, C–I 228 kJ mol⁻¹.
(a)
Describe the mechanism by which butan-2-ol forms in experiment 1, and explain why using 2-iodobutane instead of 2-bromobutane would give a faster reaction.
[6 marks]
(b)
Experiment 2 gives three alkenes. Name them, explain the role of the hydroxide ion in their formation and outline the mechanism for the formation of but-1-ene. Explain why the conditions used in experiment 2 favour elimination rather than substitution.
[6 marks]
Total for question 4: 12 marks
5
1,2-Dibromoethane, BrCH₂CH₂Br, is heated under reflux with an excess of potassium cyanide dissolved in ethanol. Both bromine atoms are replaced and butanedinitrile, NCCH₂CH₂CN, is formed. The reaction is carried out in ethanol rather than in water.
(a)
Why is ethanol used as the solvent rather than water?
[1 mark]
AWater would act as a competing nucleophile, forming alcohol by-products
BPotassium cyanide is insoluble in water
CEthanol acts as a catalyst for the substitution
DWater would oxidise the cyanide ion
(b)
Why is this reaction useful to a synthetic chemist?
[1 mark]
AIt removes carbon atoms from a chain
BIt forms a C=C double bond
CIt forms an alcohol directly
DIt lengthens the carbon chain, here from two to four carbon atoms
(c)
Describe, in words, how a cyanide ion reacts with a carbon atom bonded to bromine in the first substitution.
[2 marks]
Total for question 5: 4 marks
6
2-Bromo-2-methylbutane, (CH₃)₂CBrCH₂CH₃, is heated with a solution of potassium hydroxide in ethanol. A mixture of alkenes is formed.
(a)
How many different alkenes (structural isomers, ignoring any stereoisomers) can form?
[1 mark]
A1
B2
C3
D4
(b)
Which pair of inorganic products forms together with the alkenes?
[1 mark]
AKBr and H₂
BHBr and KOH
CKBr and O₂
DKBr and H₂O
(c)
State two conditions that favour elimination rather than substitution for a halogenoalkane and potassium hydroxide.
[2 marks]
Total for question 6: 4 marks
7
Bromoethane, CH₃CH₂Br (Mr = 109.0), is heated under reflux with an excess of aqueous sodium hydroxide to form ethanol, CH₃CH₂OH (Mr = 46.0). A student uses 5.45 g of bromoethane.
(a)
Calculate the maximum mass of ethanol that can be formed.
[3 marks]
(b)
After purification the student obtains 1.38 g of ethanol. Calculate the percentage yield. Name the organic by-product that forms in a competing reaction and state the role of hydroxide ions in forming it.
[4 marks]
Total for question 7: 7 marks
8
1,1,2-Trichloro-1,2,2-trifluoroethane, CF₂ClCFCl₂, was once used to clean electronic circuit boards. After independent research groups reported evidence linking chlorine-containing compounds to the loss of ozone, its use was banned and chlorine-free cleaning fluids were developed. Mean bond enthalpies: C–F 467 kJ mol⁻¹, C–Cl 346 kJ mol⁻¹.
(a)
Explain, using equations, how CF₂ClCFCl₂ released at ground level can lead to the destruction of ozone in the stratosphere. Explain why this destruction matters.
[6 marks]
(b)
A replacement liquid contains only C, H and F. Explain why it does not damage the ozone layer in the same way, and explain how the work of independent research groups led to legislation banning CFCs.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).