Haloalkanes (OCR A Level Chemistry A): Structured Questions

The skeletal formulae of two mono-substituted haloalkanes are shown below

i) Give the IUPAC names for halogenoalkane A and B.

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ii) Draw the dipole on halogenoalkane A and explain how this dipole arises. 

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iii) Complete the table below by placing an X in the correct box. Justify your choice for ‘higher boiling point’.

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Haloalkanes react with nucleophiles. The rate of reaction between a haloalkane, X, and different nucleophiles are shown in below. 

i) Define the term nucleophile and name the type of reaction mechanism which would take place in these reactions.

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ii) Using the data in the table, evaluate the strength of these nucleophiles.

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Haloalkanes, like those in part (a), will react readily with dilute aqueous hydroxide solutions. 

i) Using the curly arrow method, outline the mechanism for the reaction of haloalkane B with potassium hydroxide, including any relevant dipoles on the haloalkane. 

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ii) Give the IUPAC name of the major product of this reaction. 

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The rates of reaction when haloalkanes A and B are reacted with aqueous potassium hydroxide are summarised below.

Explain the difference in the rates of reaction between the two haloalkanes with aqueous potassium hydroxide using the table above.

CFCs were initially introduced in the 1930s by the American engineer Midgley for use in refrigerators. They were especially popular due to their non-toxicity and non-flammability. However, later research showed that CFCs were involved in the depletion of the ozone layer as they release radicals in the upper atmosphere.

An example of a CFC is chlorotrifluoromethane.

i) Explain the importance of ozone.

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ii) Write two equations to show how the radicals decompose the ozone layer and use these to explain why the radicals act as a catalyst in the decomposition of ozone.

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Due to their damaging effect on the ozone layer, chemists have now developed alternative compounds to CFCs which can be used as solvents and refrigerants. 

An example of such a compound is shown below.

i) State the IUPAC name for the compound shown in and draw its skeletal formula. 

[2]

ii) Explain why the compound shown does not lead to the depletion of the ozone in the upper atmosphere. 

Bromotrifluoromethane is another halogenated organic compound which is used in fire extinguishers in aircrafts.

The overall equation for the formation of bromotrifluoromethane from trifluoromethane is as follows:

CHF₃ + Br₂ → CBrF₃ + HBr

i) State the type of reaction that occurs when bromine is reacted with trifluoromethane and state one condition required for the first step of this reaction to occur.

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ii) Write an equation for each of the following steps in the mechanism for this reaction.

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The bond enthalpies of some carbon-halogen bonds are shown in the table below:

Explain, using the data in the table above, whether the environmental impact of CBrF₃ on the ozone layer is more or less severe than that of CClF₃.

Butan-1-ol can be prepared by the alkaline hydrolysis of 1-bromobutane as follows.

CH₃CH₂CH₂CH₂Br + OH^- → CH₃CH₂CH₂CHOH + Br^- 

Use the curly arrow model to outline the mechanism for the alkaline hydrolysis of 1-bromobutane. In your answer, include the name of the mechanism, the type of bond fission and relevant dipoles.

State the reagents for the conversion of a haloalkane into an alcohol.

The reaction in part (a) was repeated using 1-chlorobutane. State and explain one difference that should be made to the reaction conditions.

State, giving your reason, the role of the hydroxide ion in the reaction outlined in part (a).

The rates of hydrolysis of chloroethane and iodoethane are different. Describe how you would monitor the reaction rates.

Explain why chloroethane and iodoethane react at different rates. Use suitable equations in your answer.

Chlorofluorocarbons, CFCs, have been used as propellants in aerosols. A CFC compound has the formula CF₃Cl.

i) State the bond angle of the Cl-C-F bond and name the shape of this molecule.

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ii) Explain why this CFC is suitable for being used as a propellant in aerosols.

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iii) Explain why this molecule does not hydrolyse with water.

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Chlorofluorocarbons form free radicals in the upper atmosphere which catalyse the breakdown of the ozone layer.

•NO free radicals can also catalyse this reaction as well.

Write two equations for the breakdown of ozone by the •NO radical. 

A student compares the rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane.

For each haloalkane the student:

• places 5 drops of the haloalkane in a test tube containing 2 cm³ of ethanol

• places the test tube in a water bath at 50 °C

• adds 2 cm³ of aqueous silver nitrate and starts a stopwatch

• records the time taken for a precipitate to appear

The results are shown in Table 1.

Table 1

Explain why ethanol is added to each test tube.

Give the colour of the precipitate formed with 1-bromobutane.

Give the formula of this precipitate.

Other than the volume of aqueous silver nitrate, state two variables that the student must keep the same for each haloalkane.

Explain the order of the times shown in Table 1.

This question is about haloalkanes.

A student investigates the rate of reaction of six haloalkanes in the method outlined below:

1. The student mixes ethanol with six drops of haloalkane and warms the mixture in a water bath at 50 °C. 

2. They then added silver nitrate solution. 

3. The time taken for the precipitate to form is given in the table below.

i) Suggest how the student could improve the method.

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ii) Other than precipitation, state what type of reaction is occurring in this method.

[1]

Use the information in part (a) to describe and explain the factors that influence the rate of this type of reaction in haloalkanes.

When a fresh sample of 1-bromopentane reacts with an aqueous solution of sodium hydroxide with ethanol, a compound that contains bromine will be produced.

Give the name of the compound containing bromine.

1-chloropentane, which has a boiling point of 108°C, was prepared in the laboratory through the reaction of an alcohol and thionyl chloride, SOCl₂, which has a boiling point of 74.6°C. Two additional products are also produced in this reaction.

i) Give the equation, including state symbols, for the preparation of 1-chloropentane.

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ii) Give and explain one safety precaution necessary for carrying out this reaction and give a reason for your answer.

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This question is about the reactions of haloalkanes.

1-bromobutane is used as an intermediate in organic synthesis and as a solvent for cleaning and degreasing. 

1-bromobutane can react with methoxide ions by nucleophilic substitution. 

Suggest a mechanism for this reaction 

Compare the rate of reaction of methoxide ions and 1-bromobutane with the reaction between methoxide ions and:

• CH₃CH₂CH₂CH₂Cl

• CH₃CH₂CH₂CH₂I

A student prepared a sample of 1-bromobutane. 

The equation for the reaction that occurs is:

C₄H₉OH + KBr + H₂SO₄ → C₄H₉Br + KHSO₄ + H₂O

To do this they reacted 2.96 g of butan-1-ol with an excess of sulfuric acid and potassium bromide.

After purification, 4.62 g of 1-bromobutane are collected. 

Calculate the percentage yield. 

Give your answer to three significant figures.

Calculate the atom economy for the formation of 1-bromobutane in the reaction in part (c).

Chlorofluoroalkanes, CFCs, have been used in aerosols and refrigerants. They are now largely banned because of their effect on the ozone layer and have been replaced in many applications.

International agreements have reduced the use of CFCs but there are still concerns about ozone depletion.  Suggest two reasons why.

The formula of a CFC is CF₂Cl₂.

i) Give the bond angle for F-C-Cl.

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ii) Give the shape of the molecule.

[1]

iii) Explain why CF₂Cl₂ does not hydrolyse in water. 

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The American engineer, Thomas Midgley demonstrated a new refrigerant in 1930. 

He inhaled a lung full of CCl₂F₂ and blew out a candle with it. 

Suggest two properties of CCl₂F₂ using his demonstration to help you.

Other than a refrigerant, explain two other uses of chemicals such as CCl₂F₂.

CFCs are not solely responsible for ozone depletion. Nitrogen oxide radicals can be formed naturally during lightning strikes and also cause the breakdown of ozone, by a mechanism similar to that involving chlorine radicals. 

i) Write equations for the two step propagation process involving nitrogen oxide radicals.  

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ii) Write an overall equation for the process

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A student was investigating the rates of hydrolysis of chloroethane, bromoethane and iodoethane. 

Describe how you would monitor the reaction rates and explain why they react at different rates.

Another student completed the same investigation on three unlabelled solutions of chlorobutane, bromobutane and iodobutane. 

Outline a method the student could use to determine the identity of the unlabelled solutions. 

You should include an equation in your answer. 

The following diagram shows a reaction sequence for propane. 

Identify the reactions taking place in steps 1, 3 and 4. 

Draw the mechanism that takes place in reaction 2. Show any relevant dipoles. 

Halothane, CF₃CHBrCl, was widely used as an anaesthetic until the 1990s.

It has been replaced by anaesthetics such as desflurane, CF₃CHFOCHF₂, in which fluorine is the only halogen present.

Halothane that escapes into the upper atmosphere is broken down by ultraviolet radiation.

Identify the bond in halothane that breaks, and explain your answer.

Write two equations to show how the bromine radical, formed when this bond breaks, catalyses the breakdown of ozone.

Explain how a single radical of this type can break down many thousands of ozone molecules.

Explain why desflurane does not deplete the ozone layer.

Halothane is not hydrolysed by water and is unreactive in the lower atmosphere.

Explain why this makes halothane more damaging to the ozone layer than a compound that is broken down close to ground level.