Halogenoalkanes (AQA A Level Chemistry)

Exam Questions

4 hours45 questions
1a2 marks

The structure of a halogenoalkane containing bromine is shown in Figure 1.

i)
Name the compound shown in Figure 1.

ii)
State the class of halogenoalkane to which this compound belongs. 

Figure 1

Kj2nhi3P_1

1b1 mark

State the colour of the precipitate formed in the reaction between the halogenoalkane in Figure 1 in part (a) and acidified silver nitrate, AgNO3

1c1 mark

The compound drawn in Figure 1 in part (a) is reacted with alcoholic potassium cyanide, KCN. The reaction is heated under reflux.

Draw the product of this nucleophilic substitution reaction.

1d2 marks

The halogenoalkane drawn in Figure 2 contains a chlorine atom instead of a bromine atom. This halogenoalkane is reacted under the same conditions outlined in part (c).

Figure 2

cacNWsIi_2

The reaction for a halogenoalkane containing bromine would be faster than if the compound in Figure 2 was used. Briefly explain why this reaction would be faster.

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2a2 marks

Figure 1 shows part of a nucleophilic substitution mechanism. The reactants are 2-iodopropane, CH3CH(I)CH3 and warm aqueous potassium hydroxide, KOH.

Complete the mechanism by drawing in the remaining arrow and partial charges to the diagram in Figure 1.

Figure 1

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2b2 marks

2-iodopropane, CH3CH(I)CH3, was also reacted with hot potassium hydroxide, KOH, in ethanol. Complete the reaction mechanism for this process which is started is Figure 2.

Figure 2

nOOdZxTX_4

2c1 mark

Name the reaction mechanism that is outlined in part (b).

2d2 marks

Two different halogenoalkanes are shown in Table 1. 1-iodopropane, CH3CH3CH2I, and 2-iodo-2-methylpropane, CH3C(CH3)ICH3.

Table 1

Halogenoalkane

Class of halogenoalkane

Mechanism favoured

CH3CH3CH2I

Primary

 

CH3C(CH3)ICH3.

Tertiary

 


Complete Table 1 by indicating which type of reaction mechanism these halogenoalkanes will tend to favour, when they undergo a reaction with potassium hydroxide, KOH.

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3a3 marks

Halogenoalkanes are useful compounds in reaction pathways as they can undergo nucleophilic substitution. One example is shown the equation below where chloroethane, CH3CH2Cl is reacted with ammonia, NH3, to produce an amine and a gaseous product.

State the reagents and conditions required for the following reaction:

CH3CH2Cl (aq) + NH3 (aq) → CH3CH2NH2 (aq) + HCl (g)

3b2 marks

Give the name of both products in the reaction outlined in part (a).

3c1 mark

The mechanism for this reaction is nucleophilic substitution where the ammonia molecule acts as a nucleophile. Explain why a molecule of ammonia can behave as a nucleophile. 

3d2 marks

Figure 1 below shows part of the final step of the nucleophilic substitution reaction outlined in part (a). Draw in the final arrow and include the correct charge on the nitrogen atom.

Figure 1

VDtYR-nM_5

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4a1 mark

A common chlorofluorocarbon is CCl3F.

Give the IUPAC name of this molecule.

4b2 marks

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

4c1 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.

4d2 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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5a3 marks

Figure 1 shows the reaction profile for the production of propene.

Figure 1

gqkEV1fH_13

i)
State the conditions required for Step 1.

ii)
Name the mechanism for Step 1.
5b2 marks

The mechanism for Step 1 involves three different steps, initiation, propagation and termination.

Initiation:
Cl2 → 2Cl•

Propagation:
Equation 1
Equation 2

Termination:
Cl• + CH3CH2CH2

The two propagation steps are missing.
Write both of the equations that are required for this step.

5c4 marks

Name and outline the mechanism for Step 2.

5d3 marks

State the conditions required for Step 2

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1a9 marks

Figure 1 below shows the skeletal formulae of two mono-substituted halogenoalkanes.

Figure 1

bngs2yfv_15

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 Table 1 below by placing an X in the correct box. 
Justify your choice for ‘higher boiling point’.

Table 1

Property

A

B

Greater solubility in water

 

 

Most reactive

 

 

Higher boiling point

 

 

1b7 marks

Halogenoalkanes react with nucleophiles.

The rate of reaction between a halogenoalkane, X, and different nucleophiles are shown in Table 2 below.

Table 2

Nucleophile

Rate of reaction (x 10-3 mol dm-3 s-1)

H2O

0.34

OH-

1.7

Cl-

2.9

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. 
1c4 marks

Halogenoalkanes, like those in Figure 1 of part (a), will react readily with dilute aqueous hydroxide solutions.

i)
Outline the mechanism for the reaction of halogenoalkane B with potassium hydroxide, including any relevant dipoles on the halogenoalkane. 

ii)
Give the IUPAC name of the major product of this reaction. 
1d2 marks

The rates of reaction when halogenoalkanes A and B are reacted with aqueous potassium hydroxide are summarised in Table 3 below.

Table 3

Halogenoalkane

Rate of reaction (x 10-3 mol dm-3 s-1)

A

1.34

B

1.78


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

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2a5 marks

A chemist has a sample of 1-chloropropane, which he wants to convert to a molecule with a carbon chain length of four.

Name and draw the reaction mechanism for the reaction which the chemist would use, and suggest suitable reagents and conditions for this conversion to occur.

2b5 marks

Hydroxide ions will react with halogenoalkanes in different ways, depending on the reaction conditions. The hydroxide ions either act as a base or as a nucleophile, and different products are formed by two different reaction mechanisms.

i)
Define the term base.

ii)
Complete Table 1 below.

Table 1

 

Dilute, aqueous OH- ions at room temperature

Hot, concentrated ethanolic OH- ions

Name of reaction mechanism

 

 

Type of product formed

 

 

2c4 marks

The reaction of 1-chloropropane with hot, ethanolic potassium hydroxide results in the formation of a product with a molecular weight of 42.

Draw the mechanism for this reaction and state the IUPAC name of the product formed. 

2d6 marks

Whether the hydroxide ion acts as a nucleophile or base when reacted with halogenoalkanes does not only depend on the reaction conditions.

i)
State one other factor which determines the type of reaction that occurs when hydroxide ions react with halogenoalkanes.

ii)
Predict all of the possible major products which can form when 2-bromobutane reacts with hydroxide ions.
Explain your answer.

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3a3 marks

Butylamine is an organic compound used as an ingredient in the manufacture of pesticides.

Figure 1 below shows the overall reaction equation for the formation of butylamine from 1-chlorobutane.

Figure 1

Q_FaNofq_16

i)

State the type of reaction that occurs in Step 1.

ii)
State the reagents and reaction conditions of the reaction in Step 1.
3b4 marks

Draw the reaction mechanism for the reaction that occurs in Step 1 and label compound X in your mechanism.

Include all necessary partial and full charges.

3c3 marks

Compound X undergoes another reaction in Step 2 before butylamine is formed.

i)
Describe what occurs in Step 2 of the reaction shown in Figure 1

ii)
Give the molecular formula of compound Y.

iii)

The by-products Cl- and compound Y can combine together to form compound Z.
State the name of compound Z.

3d5 marks

A chemist can form but-1-ene either by thermally cracking octane or by reacting 1-chlorobutane with hydroxide ions. The incorrect reaction mechanism for this elimination reaction is shown in Figure 2.

Figure 2

bxLdX6OQ_17

Identify the errors shown in the reaction mechanism in Figure 2.

Suggest which of the methods stated above for synthesising but-1-ene would give a higher atom economy of the product. 

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4a4 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.

4b3 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 in Figure 1.

Figure 1

c1~sZBQi_18

i)
State the IUPAC name for the compound shown in Figure 1 and draw its skeletal Formula.

ii)

Explain why the compound shown in Figure 1 does not lead to the depletion of the ozone in the upper atmosphere.

4c6 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.
4d3 marks

The bond enthalpies of some carbon-halogen bonds are shown in Table 1.

Table 1 

Bond

Bond enthalpy (kJ mol-1)

C-F

485

C-Cl

327

C-Br

285


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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5a3 marks

The halogenoalkanes have a number of properties and undergo a number of different reactions.

Draw the structure of 1-bromo-2-methylpropane and explain why it is not soluble in water.

5b3 marks

A chemist reacts 2-chlorobutane with potassium hydroxide to form product A and product B.            

Table 1 summarises some properties of the two products formed in the two different reactions.

Table 1

Product

Decolourises bromine water?

Difference in Mr between product & 2-chlorobutane

Boiling point (degreeC)

Forms stereoisomers?

A

Yes

-18.5

-6.3

No

B

No

-36.5

99

No


State the type of reaction that has occurred to give each product and draw the skeletal formulae of both products.

5c4 marks

When 2-chlorobutane reacts with hot ethanolic potassium hydroxide, two structural Isomers are formed. One of these structural isomers also displays stereoisomerism.

i)
Draw the displayed formula of the two structural isomers formed in this reaction and state their IUPAC names.

ii)
State the type of structural isomers drawn in part (i). 

iii)
Draw the structures of the two stereoisomers formed in this reaction.
5d2 marks

Predict whether isomers are formed in the reaction of 2-chlorobutane with cold, aqueous potassium hydroxide.

State the IUPAC name of any products formed during this reaction.

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1a2 marks

If stored for a long time, compound A, drawn in Figure 1, will turn from a colourless to a straw colour liquid. It can react with excess ammonia to form compound B.

Figure 1

30

i)
Give the name of compound A.

ii)
Draw the structure of compound B.
1b1 mark

When a large excess of chloroethane is reacted with ammonia, a solid product C is formed. Name compound C.

1c5 marks

Chloroethane can react with methylamine to give compound D. Name and outline the full mechanism of this reaction.

1d3 marks

Compound G is a bromoalkane was analysed and found have the following % composition by mass:

51.92% C;       4.87% H;         43.21% Br

The Mr of this compound was found to be 184.9 g mol-1. This compound will undergo a substitution reaction to produce compound J.

When compound J is reacted with acidified potassium dichromate, an orange to green colour change is observed. The product of this reaction will not form a red precipitate when reacted with Fehling's solution.

i)
Determine the molecular formula of the halogenoalkane.

ii)
Suggest a structure for the bromoalkane.

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2a4 marks

A student investigates the rate of reaction of five halogenoalkanes in the method outlined below:

Step 1
The student mixes ethanol with six drops of halogenoalkane 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 Table 1.

Table 1

Halogenoalkane compound Time taken for precipitate to form (s) at 50 °C
CH3CH2CH2CH2CH2Cl 265
CH3CH2CH2CH2CH2Br 152
CH3CH2CH2CH2CH2I 43
CH3CH2CH2CHICH3 35
(CH3)2CHCHClCH3 190
CH3CH2C(CH3)2Cl 88

i)
Suggest how the student could improve the method.

ii)
Other than precipitation, state what type of reaction is occurring in this method.
2b6 marks

Use the information in Table 1 given in part (a) to describe and explain the factors that influence the rate of this type of reaction in halogenoalkanes.

2c1 mark

When a fresh sample of aqueous 1-bromopentane reacts with ethanol a compound that contains bromine will be produced. Give the name of the compound containing bromine.

2d4 marks

1-chloropentane, which has a boiling point of 108 degreeC, was prepared in the laboratory through the reaction of an alcohol and thionyl chloride, SOCl2, which has a boiling point of 74.6 degreeC. 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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3a5 marks

2-bromo-2-methylpropane is refluxed with ethanolic potassium hydroxide, KOH. The result is a mixture of products containing roughly a 4 : 1 ratio of alkene to alcohol.

i)
Name the alkene and alcohol produced.

ii)
Name and outline the mechanism for the production of the alkene in this reaction.
3b2 marks

Suggest why there is a mixture of an alkene and an alcohol produced in the reaction outlined in part (a).

3c2 marks

An iodoalkane can be prepared by the reaction between red phosphorus, P, and iodine, I2, to produce phosphorus (III) iodide, PI3.

This then is reacted with an alcohol to produce the iodoalkane and phosphonic acid, H3PO3.

Give the equation for the reaction between butan-2-ol and phosphorus (III) iodide. You do not have to include state symbols.

3d1 mark

During the series of reactions to produce a bromoalkane a 50 : 50 mix of sulfuric acid and water is reacted with potassium bromide to produce hydrogen bromide. The mixture is kept very cool.

Explain why sulfuric acid can not be used in the preparation of an iodoalkane

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4a4 marks

Compound G is a chloroalkane that can undergo two different reactions as outlined in Figure 1.

Figure 1

31

Compound H is an alkene which does not exhibit geometric isomerism.

i)
Draw the skeletal structures of compounds G and H.

ii)
State the reagents and conditions that will be required for reaction A.
4b4 marks

Name and outline the mechanism for reaction B, shown in part (a).

4c6 marks

Air conditioners manufactured before 1995 commonly used CFCs as a refrigerant. Equipment manufactured before 2010 used hydrochlorofluorocarbons (HCFC) as a refrigerant. New equipment will now use hydrofluorocarbons (HFC) which contain no chlorine.

i)
Explain the environmental impact of CFCs. Your answer should include relevant symbol equations.

ii)

Give two environmental advantages and one environmental disadvantage of using HFCs instead of CFCs.

4d5 marks

2-chloropropane can be formed from propane through the mechanism of free radical substitution. The mechanism requires three steps; initiation, propagation and termination.

Figure 2

32

i)
The curly arrows for the initiation step are shown in Figure 2. Using your knowledge of free radical substitution, outline a mechanism for the first propagation step. 

ii)
Give the formulas for the two possible organic products from the termination step.

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5a4 marks

The reaction pathway outlined in Figure 1 shows two different reactions of a cyclic iodoalkane to produce compound A and compound B.

Compound A reacts with bromine water to give a colourless solution.

Use your knowledge of halogenoalkane reactions to name and outline a possible mechanism of reaction 1 and draw the structure of the compound produced.

Compound B can be oxidised to produce a ketone.

Figure 1

triukBpU_33

5b4 marks

Compound B is also produced in the reaction scheme given in part (a).

i)
Give suitable reagents and conditions for reaction 2.

ii)
Draw the structure for compound B and state in which class of compound it belongs.
5c2 marks

1,5-dibromooctane is reacted with ammonia to produce a cyclic compound known as coniine which is a poisonous compound. 

i)
Draw the skeletal structure of 1,5-dibromooctane.

ii)
Draw the structure of the cyclic compound coniine. 
5d3 marks

A reaction pathway to produce a carboxylate salt, compound M, which contains three carbon atoms is shown in Figure 2.

Figure 2

34 

Give the correct reagents to form compound L from 1-bromoethane.

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