Haloalkanes (OCR AS Chemistry)

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

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

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

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.

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.

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

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 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.

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

The reaction in part (a) was repeated using 1-chlorobutane. State an 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.

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. 

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.

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.

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 

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

• CH₃CH₂CH₂CH₂Cl
• (CH₃)₃CBr

A student prepared a sample of 1-bromobutane. 

The equation for the reaction that occurs is:

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 CF₂Cl_2.

[1]

[1]

[1]

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 CCl₂F_2.

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. 

[2]

[1]

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.