CIE AS Chemistry

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ASChemistryCIETopic Questions3. Organic Chemistry3.3 Halogen Compounds

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First teaching 2020

Last exams 2024

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3.3 Halogen Compounds

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1a2 marks

The structure of a halogenoalkane containing bromine is shown in Fig. 1.1.

3-3-1a-e-2-bromopropane

Fig. 1.1

i)
Name the compound shown in Fig. 1.1.

[1]

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

[1]

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1b1 mark

State the colour of the precipitate formed in the reaction between the halogenoalkane in Fig. 1.1 in part (a) and acidified silver nitrate, AgNO3.

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1c1 mark

The compound drawn in Fig. 1.1 in part (a) is reacted with alcoholic potassium cyanide, KCN. The reaction is heated under reflux. 

Draw the product of this nucleophilic substitution reaction. 

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1d2 marks

The halogenoalkane drawn in Fig. 1.2 contains a chlorine atom instead of a bromine atom. This halogenoalkane is reacted under the same conditions outlined in part (c). 

2-chloropropane

Fig. 1.2

The reaction for a halogenoalkane containing bromine would be faster than if the compound in Fig. 1.2 was used. Briefly explain why this reaction would be faster.

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2a3 marks

Fig. 2.1 shows the reaction profile for the production of propene. 

Propane   rightwards arrow with space step space 1 space on top   1-chloropropane   rightwards arrow with space step space 2 space on top Propene

Fig. 2.1

i)
State the conditions required for step 1.

[2]

ii)
Name the mechanism for step 1.

[1]

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2b2 marks
The mechanism for step 1 in part (a) 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.

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2c4 marks
i)
Name the mechanism for step 2.

[1]

ii)
State the reagent and necessary conditions for step 2.

[3]

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3a2 marks

Many of the reactions of halogenoalkanes involve a nucleophile attacking the carbon attached to the halogen atom. The nucleophile replaces the halogen atom in a nucleophilic substitution reaction.

The mechanism for the reaction is determined by the structure of the halogenoalkane. Primary and tertiary halogenoalkanes react via different reaction mechanisms.

i)
State what is meant by the term tertiary halogenoalkane.

[1]

ii)
Tertiary halogenoalkanes react via an SN1 mechanism, whereas primary halogenoalkanes react via an SN2 mechanism.

Explain what the numbers 1 and 2 refer to in SN1 and SN2.

[1]

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3b4 marks

2-bromo-2-methylpropane reacts with hydroxide ions via an SN1 mechanism.

i)
The incomplete reaction mechanism for the reaction is shown in Fig. 3.1
Complete the reaction mechanism shown in Fig. 3.1 clearly showing any partial charges, full charges and the transfer of electrons.
[3]

3-3-3b-e-incomplete-sn1-mechanism

Fig. 3.1

ii)
Identify which, step 1 or step 2, is the rate-determining step.

[1]

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3c3 marks

An ethanolic solution of excess ammonia (NH3 in ethanol) is heated under pressure with 2-bromo-2-methylpropane.

i)
Draw the structure of the resulting organic product.

[1]

ii)
State the name of the functional group of the organic product.

[2]

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1a4 marks

This question is about halogenoalkanes.

A student investigates the rate of reaction of six halogenoalkanes using the following method. 

  1. Mix ethanol with six drops of halogenoalkane.
  2. Warm the mixture in a water bath at 50 °C.
  3. Add silver nitrate solution.
  4. Record the time taken for the precipitate to form.

Table 1.1 shows the student's results.

 
Table 1.1 
 
Halogenoalkane Time taken for the precipitate to form (s) at 50 °C
CH3CH2CH2CH2CH2Cl 265
CH3CH2CH2CH2CH2Br 152
CH3CH2CH2CH2CH2I 65
CH3CH2CH2CHBrCH3 88
(CH3)2CHCHClCH3 190
CH3CH2C(CH3)2Cl 88
 
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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1b4 marks

Using Table 1.1, describe and explain two factors that influence the rate of this type of reaction in halogenoalkanes.

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1c3 marks

A fresh sample of aqueous 1-bromopentane reacts with an aqueous solution of sodium hydroxide.

 
i)
Fully identify the mechanism for this reaction and state the number of steps in the mechanism.
 
[1]
 
ii)
One of the products contains bromine. Name this product.
 
[1]
 
iii)
The reaction conditions are changed resulting in the formation of three products, including the product identified in part (c)(ii).
 
Suggest the change that is made to the conditions.
 
[1]

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1d4 marks

1-chloropentane was prepared in the laboratory by the reaction of an alcohol and thionyl chloride, SOCl2.

Two additional products are also produced in this reaction.

i)
Give the equation, including state symbols, for the preparation of 1-chloropentane.
 
[2]
 
ii)
Explain one safety precaution necessary for carrying out this reaction. Justify your answer.
 
[2]

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2a2 marks

This question is about the reactions of halogenoalkanes.

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. 

 

Complete Table 2.1 to name the organic products and identify the mechanisms involved.

 
Table 2.1
 
Organic product Name Mechanism involved
Alkene    
Alcohol    

 

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2b2 marks

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

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2c2 marks

An iodoalkane is prepared by the reaction of red phosphorus, P, with iodine, I2, to produce phosphorus(III) iodide, PI3. This then is reacted with an alcohol to produce the iodoalkane and phosphorous acid, H3PO3.

 

Give the equation for the reaction between butan-2-ol and phosphorus(III) iodide.

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2d1 mark

During a 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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3a2 marks

Compound G is a chloroalkane that can undergo two different reactions as outlined in Fig. 3.1.

 
reaction-scheme
 
Fig. 3.1
 

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

 

Draw the skeletal structures of compounds G and H.

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3b2 marks

State the reagents and conditions that will be required for reaction A in Fig. 3.1.

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3c4 marks

Name and draw the mechanism for reaction B in Fig 3.1. Include all charges, partial charges, lone pairs and curly arrows.

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4a1 mark

Bromobutanes react with hot ethanolic potassium hydroxide solution to produce gaseous butenes.q5-8cho-as-2-june-2019-qp-edexcel-a-level-chem

Procedure

  • 0.0080 mol of liquid 1-bromobutane was injected into a round bottom flask containing hot ethanolic potassium hydroxide.
  • After the reaction, the syringe was sealed using a clamp.
  • The syringe was then removed from the apparatus and allowed to cool to 25oC.

Result

  • The final volume of but-1-ene collected was 22.0 cm3.

State the purpose of the condenser.

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4b3 marks

Describe a chemical test and the expected observation to identify the functional group of the gas in the syringe.

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4c
Sme Calculator2 marks

Calculate the percentage of 1-bromobutane which was converted to but-1-ene.

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4d
Sme Calculator2 marks

Before cooling, the volume of but-1-ene in the gas syringe was 24.0 cm3.

 

Assuming a constant pressure and that no but-1-ene is lost from the gas syringe during cooling, calculate the temperature of the gas in the syringe. 





Temperature = ......................

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4e6 marks
i)
Another compound formed from 1-bromobutane under these conditions is butan-1-ol.
 
Fully identify the type of reaction taking place to form butan-1-ol.
 
[1]
 
ii)
Draw the mechanism for the reaction of 1-bromobutane with hydroxide ions to form butan-1-ol. Include all charges, partial charges, lone pairs and curly arrows.
 
[3]
 
iii)
Describe a chemical test and the expected observation(s) to confirm the presence of the functional group in butan-1-ol.
 
[2]

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4f3 marks

Isomeric alkene molecules are formed in the elimination reaction of 2-bromobutane.

 

Draw the displayed formulae of the isomers formed during this reaction.

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1a1 mark

The molecular formula C3H6 represents the compounds propene and cyclopropane are shown in Fig. 1.1.

propene cyclopropane
propene cyclopropane

Fig. 1.1

What is the H–C–H bond angle at the terminal =CH2 group in propene?

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1b4 marks

Under suitable conditions, propene and cyclopropane each react with chlorine.

i)
With propene, 1,2-dichloropropane, CH3CHClCH2Cl is formed.
State fully what type of reaction this is.
[1]
ii)
When cyclopropane reacts with chlorine, three different compounds with the molecular formula C3H4Cl2 can be formed.
Draw displayed structures of each of these three compounds.

[3]

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1c2 marks

Ethane reacts with chlorine according to the following equation.


C2H6 + Cl2 → C2H5Cl + HCl

i)
State the conditions needed for this reaction.

[1]

ii)
State the type of reaction occurring here.

[1]

How did you do?

1d
Sme Calculator3 marks
i)
One of the steps during the reaction in (c) is the following process.

Cl● + CH3CH3 → HCl + CH3CH2●

Table 1.1

Bond Bond enthalpy / kJ mol-1
C–H 410
H–Cl 431
H–I 299

Use the data in Table 1.1 to calculate the enthalpy change, ∆H, of this step.

Show your working.
[1]
∆H = ................................ kJ mol–1

ii)
Use the data in Table 1.1 to calculate the enthalpy change, ∆H, of the similar reaction:

I● + CH3CH3 → HI + CH3CH2●

Show your working.

[1]

∆H = ................................ kJ mol–1

iii)
Hence suggest why it is not possible to make iodoethane by reacting together iodine and ethane.
[1]

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1e3 marks

Complete the following equations of some possible steps in the formation of chloroethane.

Cl2  →  ..............

Cl● + CH3CH3  →  HCl  +  CH3CH2●

CH3CH2● + ..............  →  ........................  +  ..............

..............  +  ..............  →  CH3CH2Cl

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2a7 marks

This question concerns halogenoalkanes.

1-chloropropane can react to form organic products as shown in the reaction scheme in Fig. 2.1.

px-FxyDO_reactions-1-chloropropane
Fig. 2.1
 
i)
State the reagent and conditions used in Reaction 1.
[2]
 
ii)
Identify a suitable reagent for Reaction 2 and include a reason why this is a particularly useful type of reaction in organic chemistry.
[2]
 
Reagent ..................................................
 
Reason ..................................................
 
iii)
State the conditions which are used in Reaction 3.
[2]
 
iv)
Write the structural formula of the product that will be formed if 1-chloropropane is refluxed with aqueous sodium hydroxide solution.
[1]

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2b
Sme Calculator3 marks

2‐chloro‐2‐methylpropane can be reacted with aqueous alkali to form 2-methylpropan-2-ol.

The equation for this reaction is

(CH3)3CCl + OH− → (CH3)3COH + Cl−

The graph in Fig 2.2 shows how the concentration of 2‐chloro‐2‐methylpropane changes with time during an investigation of this reaction.

q6-8cho-as-2-oct-2021-edexcel-a-level-chem

Fig. 2.2

Calculate the rate of reaction at 50 s. Show your working on the graph.
Include units with your final answer.



Rate of reaction at 50 s = ............................

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2c1 mark

What is the classification of the mechanism for the reaction of 2‐chloro‐2‐methylpropane with aqueous alkali?

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2d3 marks

The letters X, Y and Z refer to three different halogenoalkanes:

X 1‐bromobutane
Y 1‐iodobutane
Z 1-chlorobutane


1 cm3 of each of these halogenoalkanes was added to separate test tubes containing 5 cm3 of ethanol and 5 cm3 of aqueous silver nitrate solution in a water bath at 50 °C.

i)
State the visible change in the reaction of an ethanol/silver nitrate solution with halogenoalkane X.
Include the formula of the compound responsible for this observation.
[2]
ii)
The three halogenoalkanes were placed in order of decreasing rate of reaction. 
State the order, starting with the halogenoalkane that reacted the fastest
[1]

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2e4 marks

2-bromo-2-methylpropane can be prepared from the addition of hydrogen bromide, HBr, to 2-methylpropene.


Draw the mechanism for this reaction.
Include curly arrows, and any relevant dipoles and lone pairs.

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3a2 marks

One method of making 1-bromobutane in the laboratory is described below.

Step 1

Place 35 g of powdered potassium bromide, 30 cm3 of water, and 25 cm3 of butan-1-ol, in a 250 cm3 two necked flask fitted with a tap funnel and reflux condenser.
Step 2 Concentrated sulfuric acid (25 cm3) is then placed in the tap funnel and added drop by drop to the reagents in the flask, keeping the contents well shaken and cooled occasionally in an ice-water bath.



The overall reaction may be considered to take place in two stages. In the first stage, the inorganic reagents react together to form HBr. In the second stage, the organic reagent reacts with the HBr that is formed in the first stage.

Write an equation for each of these stages.

Stage 1 ..............................................................................................................................

Stage 2 ..............................................................................................................................

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3b2 marks

1-bromobutane can be prepared using different reagents and conditions.


State two different ways that 1-bromobutane can be prepared.
In each case, include the reagents and conditions that are necessary

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3c5 marks
i)
Complete the following reaction scheme shown in Fig. 3.1 which starts with 1-bromobutane.

In each empty box, write the structural formula of the organic compound that would be formed.

1-bromobutane-reaction-with-naoh

Fig. 3.1

ii)
State the name of the type of reaction that occurs when X is produced from 1-bromobutane.

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3d2 marks

Compound W can also be formed from 1-bromobutane by heating it under reflux with aqueous silver nitrate. This causes the hydrolysis of 1-bromobutane.

Suggest why the rate of the formation of compound W is slower using this method than heating under reflux with aqueous sodium hydroxide.

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4a5 marks

This is a question about the hydrolysis of halogenoalkanes.

Devise an experiment, giving outline details only, that would enable the relative rates of hydrolysis of halogenoalkanes to be compared.

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4b2 marks

Explain the trend in the rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane.

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4c3 marks

The product of the hydrolysis of 2-bromobutane is butan-2-ol. Both molecules are chiral.

State what is meant by the term chiral, using three-dimensional diagrams of the enantiomers of butan-2-ol to illustrate your answer.

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4d6 marks

Compare and contrast the mechanism of hydrolysis, using aqueous sodium hydroxide, of the primary halogenoalkane, RCH2X, with that of the tertiary halogenoalkane, R3CX.
Include diagrams of any intermediate or transition state.

Curly arrows are not required.

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5a4 marks

Give the structures of the four structural isomers of C4H9Br and identify each as primary, secondary or tertiary.

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5b3 marks

Name the isomer of C4H9Br that contains a chiral centre and draw the three-dimensional structures of the two optical isomers.

name ...................................................................................................................................

structures

   

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5c2 marks

Aqueous silver nitrate solution was added to separate tubes containing chloroethane, bromoethane and iodoethane. The tubes were heated in a water bath.

A yellow precipitate appeared first in the tube containing iodoethane, followed by a cream precipitate in the tube containing bromoethane and finally a white precipitate appeared in the tube containing chloroethane.

Explain these observations.

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5d4 marks
i)
Give the full name of the mechanism for the reaction between aqueous sodium hydroxide and bromoethane.

[2]

ii)
Complete the diagram below to represent this mechanism. Include all necessary curly arrows, partial charges and lone pairs.

3-3-5d-m-3-3m-q5d-bromoethane-mechanism-a
[2]

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5e5 marks

Primary and teriary halgenoalkanes have different reaction mechanisms when they react with aqueous sodium hydroxide.

i)
State the name that is given to the reaction mechanism when aqueous sodium hydroxide is reacted with the following types of halogenoalkane.

primary halogenoalkane .......................................................................................

tertiary haologenolkane ......................................................................................

[2]

ii)
Tertiary halogenoalkanes form a tertiary carbocation as an intermediate during this reaction. Explain why tertiary carbocations are more stable than primary carbocations.

[3]

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