Halogenoalkanes (CIE A Level Chemistry)

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

propene	cyclopropane

Fig. 1.1

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

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

[3]

Ethane reacts with chlorine according to the following equation.

C₂H₆ + Cl₂ → C₂H₅Cl + HCl

[1]

[1]

Cl● + CH₃CH₃ → HCl + CH₃CH₂●

Table 1.1

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

I● + CH₃CH₃ → HI + CH₃CH₂●

[1]

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

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

This question concerns halogenoalkanes.

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

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.

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 = ............................

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

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

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.

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.

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

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 ..............................................................................................................................

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

Fig. 3.1

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.

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.

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

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.

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.

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

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

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

structures

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.

[2]

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

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

[2]

[3]

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.

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

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

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.

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.

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

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.

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

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

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

Draw the skeletal structures of compounds G and H.

State the reagents and conditions that will be required 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.

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 25^oC.

Result

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

State the purpose of the condenser.

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

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

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. 

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

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