Halogenoalkanes (Cambridge (CIE) AS Chemistry): Exam Questions

Propene and cyclopropane are structural isomers with the molecular formula C₃H₆. Their structures are shown in Fig. 1.1.

Fig. 1.1

State the H–C–H bond angle at the terminal =CH₂ group in propene.

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

i) Propene reacts with chlorine to form 1,2-dichloropropane, CH₃CHClCH₂Cl.

Name the mechanism for this reaction.

[1]

ii) When cyclopropane reacts with chlorine, three different compounds with the molecular formula C₃H₄Cl₂ can be formed.

Draw the displayed formulae of these three compounds.

[3]

Ethane reacts with chlorine according to the following equation.

C₂H₆ + Cl₂ → C₂H₅Cl + HCl

i) State the conditions needed for this reaction.

[1]

ii) Name the mechanism for this reaction.

[1]

i) One of the steps during the reaction in (c) is the following process.

Cl● + CH₃CH₃ → HCl + CH₃CH₂●

Table 1.1

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

[1]

ΔH = ................................ kJ mol^-1

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

I● + CH₃CH₃ → HI + CH₃CH₂●

[1]

ΔH = ................................ kJ mol^-1

iii) Use your answer to (d)(ii) to suggest why it is not possible to make iodoethane by reacting together iodine and ethane.

[1]

Complete the equations to show the steps in the mechanism for the formation of chloroethane.

Cl₂ → ..............

Cl● + CH₃CH₃ → HCl + CH₃CH₂●

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

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

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

Fig. 2.1

i) State the reagent and conditions for Reaction 1.

[2]

ii) Identify a suitable reagent for Reaction 2. Explain why this reaction is useful in organic synthesis.

[2]

Reagent ..................................................

Explanation ..................................................

iii) State the conditions for Reaction 3.

[2]

iv) Draw the structural formula of the organic product formed when 1-chloropropane is heated under reflux with aqueous sodium hydroxide.

[1]

2‑chloro‑2‑methylpropane reacts with aqueous alkali to form 2-methylpropan-2-ol.

(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.

Fig. 2.2

Use the graph in Fig. 2.2 to calculate the rate of reaction at t = 50 s. Show your working on the graph.

Rate of reaction = ............................ units ............................

Name the mechanism for the reaction of 2‑chloro‑2‑methylpropane with aqueous sodium hydroxide.

X is 1‑bromobutane, Y is 1‑iodobutane and Z is 1-chlorobutane.

Equal volumes of X, Y and Z are added to separate test-tubes containing aqueous silver nitrate in ethanol at 50 °C.

i) State what is observed in the test-tube containing X. Give the formula of the compound responsible for this observation.

[2]

ii) Deduce the order of the rate of reaction of X, Y and Z, starting with the fastest.

fastest ........................ > ........................ > ........................ slowest

[1]

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

Complete the diagram to show the mechanism for this reaction.

Include charges, dipoles, lone pairs of electrons and curly arrows, as appropriate.

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

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

The overall reaction takes place in two stages. In the first stage, the inorganic reagents react to form HBr. In the second stage, the organic reagent reacts with the HBr formed in the first stage.

Construct an equation for each of these stages.

stage 1 ......................................................................

stage 2 ......................................................................

Identify two different reagents and conditions that can be used to prepare 1-bromobutane. Include the reagents and conditions for each method.

i) Complete the reaction scheme in Fig. 3.1, which starts with 1-bromobutane.

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

[4]

Fig. 3.1

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

[1]

Compound W can also be formed by heating 1-bromobutane under reflux with aqueous silver nitrate.

Suggest why the rate of formation of compound W is slower in this reaction than when 1-bromobutane is heated under reflux with aqueous sodium hydroxide.

Describe an experiment that would allow the relative rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane to be compared.

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

The hydrolysis of 2-bromobutane produces butan-2-ol.

i) Explain what is meant by the term chiral centre.

[1]

ii) Draw the three-dimensional structures of the two optical isomers of butan-2-ol.

[2]

Primary halogenoalkanes, such as RCH₂X, and tertiary halogenoalkanes, such as R₃CX, undergo hydrolysis with aqueous sodium hydroxide via different mechanisms.

i) Name the mechanism for the hydrolysis of a primary halogenoalkane.

Draw the structure of the transition state formed in this reaction.

[2]

ii) Name the mechanism for the hydrolysis of a tertiary halogenoalkane.

Draw the structure of the intermediate formed in this reaction.

[2]

iii) State one difference between these two mechanisms in terms of the species involved in the rate-determining step.

[2]

There are four structural isomers with the molecular formula C₄H₉Br.

Complete Table 1.1 to draw the skeletal formula of each isomer and state whether each is a primary, secondary or tertiary halogenoalkane.

Table 1.1

One of the isomers of C₄H₉Br contains a chiral centre.

i) State the systematic name of this isomer.

[1]

ii) Draw the three-dimensional structures of the two optical isomers of this compound.

[2]

Aqueous silver nitrate in ethanol was added to separate test-tubes containing chloroethane, bromoethane and iodoethane. The test-tubes were placed in a hot 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.

Bromoethane reacts with aqueous sodium hydroxide.

i) Name the mechanism for this reaction.

[1]

ii) Complete the diagram below to show the mechanism for this reaction.

Include charges, dipoles, lone pairs of electrons and curly arrows, as appropriate.

[2]

Primary and tertiary halogenoalkanes have different reaction mechanisms when they react with aqueous sodium hydroxide.

i) State the name of the mechanism when aqueous sodium hydroxide reacts with each of the following types of halogenoalkane.

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

tertiary halogenoalkane ....................................................................................

[2]

ii) Explain why a tertiary carbocation is more stable than a primary carbocation.

[2]

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

i) Suggest three modifications to the experimental procedure to improve the reliability of the results.

[3]

ii) Name the type of reaction occurring in this method, other than precipitation.

[1]

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

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

i) Name the mechanism for this reaction.

[1]

ii) State the systematic name of the inorganic product formed.

[1]

iii) State the reagents and conditions required to favour an elimination reaction rather than nucleophilic substitution.

[2]

1-chloropentane and two other products can be prepared in the laboratory by the reaction of pentan-1-ol with thionyl chloride, SOCl₂.

i) Construct an equation for this reaction. Include state symbols.

[2]

ii) Identify one safety precaution necessary for carrying out this reaction. Explain why it is required.

[2]

2-bromo-2-methylpropane is refluxed with ethanolic potassium hydroxide, KOH.

Complete Table 2.1 to state the systematic name of each organic product and identify the mechanism involved.

Table 2.1

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

Butan-2-ol reacts with phosphorus(III) iodide, PI₃, to produce an iodoalkane and phosphorous acid, H₃PO₃.

Construct an equation for this reaction.

Explain why concentrated sulfuric acid cannot be used to prepare iodoalkanes from potassium iodide.

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

Fig. 3.1

Compound H is an alkene which does not show geometrical (cis/trans) isomerism.

Draw the skeletal structures of compounds G and H.

State the reagents and conditions for reaction A in Fig. 3.1.

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

Bromobutanes react with hot ethanolic potassium hydroxide solution to produce gaseous butenes.

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 and removed from the apparatus. It was then allowed to cool to 25 °C.

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

State the purpose of the condenser.

Describe a chemical test and expected observation to confirm the presence of the carbon-carbon double bond in the gas in the syringe.

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

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

Assuming 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 before cooling. Show your working.

Temperature = ...................... K

i) Another compound formed from 1-bromobutane under these conditions is butan-1-ol.

Name 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 charges, dipoles, lone pairs of electrons and curly arrows, as appropriate.

[3]

iii) Describe a chemical test and expected observation to confirm the presence of the hydroxyl group in butan-1-ol.

[2]

The elimination reaction of 2-bromobutane produces a mixture of three isomeric alkenes.

Draw the displayed formulae of the three alkene isomers formed.