Haloalkanes Science for Safety and Sustainability

OCR Chemistry A

Haloalkanes – science for safety and sustainability

Specification reference

· 4.2.2

· HSW9

· HSW10

· HSW12

Introduction

Haloalkanes, also known as halogenoalkanes or alkyl halides, comprise an alkane and one or more halogens. They were first synthesised in the 15th century, but it wasn’t until the 19th century that our understanding of organic chemistry was sufficient to allow the development of systematic methods to reliably produce specific haloalkanes.

Haloalkanes are now used for a wide range of applications, such as fire extinguishers, refrigerants and in the production of medicines.

In this task, you will explore one of the uses of haloalkanes – as refrigerants – arguably their most famous use.

Learning outcomes

After completing this worksheet you should be able to:

· appreciate the role of scientists in providing evidence for national and international legislation

· understand the role of haloalkanes in environmental issues.

Background

Questions

1 Summarise the evidence that led to the replacement of CFCs as refrigerants. (3 marks)

2 Chlorotrifluoromethane (CF3Cl), or CFC-13, was widely used as a refrigerant as well as a propellant in aerosols.

a Suggest a property of CF3Cl that makes it suitable for its use in aerosols. (1 mark)

b Suggest a property of CF3Cl that means it has a lifespan exceeding 100 years. (1 mark)

c Draw the three-dimensional structure of CF3Cl. (2 marks)

d Identify the shape of this molecule. (1 mark)

e Predict the angles of bonds in a molecule of CF3Cl. (1 mark)

f When exposed to ultraviolet radiation, CF3Cl homolytic fission occurs causing free radicals to be produced. Suggest which bond is most likely to be broken when CF3Cl is exposed to ultraviolet radiation. Explain your answer. (2 marks)

g Identify the two radicals most likely to be formed by this process. (3 marks)

3 CFCs have, for most applications, been replaced by HCFCs, HFCs and hydrocarbons.

a Evaluate the suitability of these alternatives in terms of their sustainability. (4 marks)

b C4H9F is an example of an HFC. Draw the possible isomers with this formula. (4 marks)

c Explain, using an appropriate diagram with curly arrows, what is meant by the term ‘nucleophilic substitution’ and why this is unlikely to be the mechanism by which HFCs break down in the environment. (3 marks)

This resource sheet may have been changed from the original 2