OCR A Level Chemistry

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A LevelChemistryOCRTopic Questions4. Core Organic Chemistry4.5 Haloalkanes

4.5 Haloalkanes

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1a2 marks

The list shows the structural formulas of some haloalkanes.

W CH3CH2Cl
X (CH3)3CBr
Y CH3CH2I
Z CH3CHBrCH2CH(Cl)CH3

i)
State the IUPAC name of haloalkane W.
[1] 

ii)
State the IUPAC name of haloalkane X.
[1] 

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1b1 mark

Haloalkanes reaction with aqueous hot potassium hydroxide to form alcohols.

Select a haloalkane from the list in part a) which would form a diol (a molecule with two OH groups).

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1c3 marks

Using 'curly arrows' complete the mechanism of the reaction between haloalkane W and hot aqueous potassium hydroxide to form an alcohol. Include relevant dipoles in the mechanism.


4-5_q1c-ocr-a-as--a-level-easy-sq
 

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1d1 mark

The same reaction in part c) was repeated using haloalkane Y. State whether the reaction with haloalkane Y would be faster or slower than the reaction with haloalkane W. 

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2a1 mark

A common chlorofluorocarbon is CCl3F. State the IUPAC name of this molecule.

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2b2 marks

Name two chemical properties of CFC molecules that make them suitable for use as refrigerants and propellants for aerosols. 

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2c1 mark

CFCs are very beneficial, however, they have had a destructive effect on the ozone layer. The CFC molecule will break up in the presence of UV light. The Cl● radical produced in this reaction will react with an ozone molecule as outlined in
the reaction below.

Cl● + O3 → ClO● + O2

Give another equation showing how the ClO● breaks up an ozone, O3 molecule to produce another Cl● radical. 

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2d2 marks

Explain why more recent alternative molecules to CFCs such as HFCs do not have the same destructive effect on the ozone layer.

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2e1 mark

Nitrogen monoxide radicals can be formed in the atmosphere and can also react with ozone to form a nitrogen dioxide radical. Write the equation for this reaction.

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3a1 mark

The rates of hydrolysis of chloroethane, bromoethane and iodoethane are different. From slowest to fastest, state the order for rate of hydrolysis.

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3b1 mark

The rate of reaction for the hydrolysis of the haloalkanes listed in part a) can be measured by timing how long a precipitate takes to form. 

State the reagent required for precipitation reaction to occur. 

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3c3 marks

State the reagents and conditions required for the hydrolysis reaction to occur when measuring the rate of reaction. 

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3d1 mark

Identify the haloalkane that could form a yellow precipitate during the hydrolysis reaction.

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4a1 mark

Explain the term nucelophile. 

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4b2 marks

A student drew the mechanism using curly arrows to describe the reaction between 1-bromopropane and aqueous potassium hydroxide. Their mechanism is shown in Figure 1.

Figure 1

4-5_q4b-ocr-a-as--a-level-easy-sq

State two changes that should be made to this reaction mechanism. 

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4c1 mark

Which of the following changes would cause the reaction to be quicker. Tick the appropriate box.

Using 1-chloropropane  
Using 1-iodopropane  
Using water as a nucleophile  

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4d1 mark

The reaction conditions from part a) were changed to using potassium cyanide in ethanol which was heated under reflux. Name the organic compound that would be formed.

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1a4 marks

This question is about haloalkanes.

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

Step 1

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

   Step 2

   They then added silver nitrate solution. 

   Step 3

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

Haloalkane compound Time taken for precipitate to form (s) at 50 °C Time taken for precipitate to form (s) at 60 °C
CH3CH2CH2CH2CH2Cl 265 160
CH3CH2CH2CH2CH2Br 152 98
CH3CH2CH2CH2CH2I 65 33
CH3CH2CHBrCH3 88 41
(CH3)2CHCHClCH3 190 80
CH3CH2C(CH3)2Cl 88 42

i)
Suggest how the student could improve the method.
[3]
ii)
Other than precipitation, state what type of reaction is occurring in this method.
[1]

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1b6 marks

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

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1c1 mark

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.

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1d4 marks

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.

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

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2a3 marks

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 nucleophillic substitution. 

Suggest a mechanism for this reaction 

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2b5 marks

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

  • CH3CH2CH2CH2Cl
  • (CH3)3CBr

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2c
Sme Calculator3 marks

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.

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2d
Sme Calculator2 marks

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

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3a2 marks

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.

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3b3 marks

The formula of a CFC is CF2Cl2.

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

[1]

ii)
Give the shape of the molecule.

[1]

iii)
Explain why CF2Cl2 does not hydrolyse in water. 

[1]

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3c4 marks

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.

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3d3 marks

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.  

[2]

ii)
Write an overall equation for the process

[1]

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4a6 marks

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.

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4b6 marks

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. 

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4c3 marks

The following diagram shows a reaction sequence for propane. 

4-5_q4c-ocr-a-as--a-level-hard-sq

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

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4d4 marks

Draw the mechanism that takes place in reaction 2. 
   

Show any relevant dipoles. 

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1a9 marks

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

1-4

i)

Give the IUPAC names for halogenoalkane A and B.

ii)

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

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

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1b7 marks

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

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i)

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

ii)

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

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1c4 marks

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. 

ii)

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

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1d2 marks

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

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

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2a4 marks

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.

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. 

How did you do?

2b3 marks

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.

4-2

i)

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

ii)

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

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2c6 marks

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.

ii)

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

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2d3 marks

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

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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

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3a3 marks

Butan-1-ol can be prepared by the alkaline hydrolysis of 2-bromobutane as follows.

CH3CH2CH2CH2Br + OH- → CH3CH2CH2CHOH + Br- 

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

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3b2 marks

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

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3c3 marks

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

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3d1 mark

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

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4a
Sme Calculator5 marks

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

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4b4 marks

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

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4c4 marks

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.

ii)

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

iii)

Explain why this molecule does not hydrolyse with water.

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4d2 marks

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. 

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