Electron Sharing Reactions (DP IB Chemistry: HL): Exam Questions

Methane reacts with chlorine in the presence of ultraviolet (UV) light to form chloromethane.

i) Write one equation for the initiation step.

[1]

ii) Write two equations for the propagation steps.

[2]

Write one equation for a termination step.

[1]

Define the term free radical and explain, with reference to bond enthalpy values from section 12 of the data booklet, why the Cl–Cl bond is broken during initiation rather than a C–H bond in methane.

Chlorine free radicals also catalyse the destruction of ozone (O₃) in the stratosphere. This process can be represented by a two-step catalytic cycle, which involves the intermediate chlorine monoxide radical (ClO•).

Write two balanced equations for this catalytic cycle.

The chlorofluorocarbon CCl₃F is a source of the chlorine radicals in part (c).

Draw the three-dimensional structure of the CCl₃F molecule, name its molecular geometry, and state the value of the Cl–C–Cl bond angle.

The successive free-radical substitution of methane with excess chlorine produces a mixture of chloromethanes.

Write the overall balanced chemical equation for the formation of dichloromethane (CH₂Cl₂) from methane.

The formation of trichloromethane (CHCl₃) from dichloromethane involves an initiation and two propagation steps.

i) Write the equation for the initiation step.

[1]

ii) Write the two equations for the propagation steps.

[2]

A different halogenated alkane is analysed and found to contain 20.3% carbon, 30.0% chlorine, and 48.1% fluorine by mass, with the remainder being hydrogen.

i) Determine the empirical formula of this compound.

[2]

ii) State the name of the mechanism by which this compound would be formed from a suitable alkane and halogen mixture.

[1]

Suggest the structural formula of one organic product formed by a termination step during the reaction of dichloromethane and chlorine.

Consider the following reaction

RCH₃ RCH₂Cl

i) The alkane contains 17.4% of hydrogen. Calculate the empirical formula of the alkane.

[3]

ii) Equal volumes of methyl vinyl ether, CH₃OCH=CH₂ and the unknown alkane are found to have the same mass, measured to an accuracy of two significant figures, at the same temperature and pressure. Deduce the molecular formula of the alkane.

[1]

The reaction in part a) involves a free-radical mechanism to form the chloroalkane.

Describe the mechanism, by giving equations to represent the initiation, propagation and termination steps.

Draw the full displayed formula of the haloalkane formed if excess chlorine is used in the reaction in part a).

Dichloromethyl benzene reacts with chlorine to produce trichloromethyl benzene. State the name of this type of mechanism and the required condition. 

Outline the mechanism for the reaction occurring in part a).

A reaction pathway is shown below. Compound J reacts with bromine water to form a colourless solution.

State the IUPAC name for Compound J.

Identify the reagents and conditions for the formation of Compound Y from Compound J. 

For the reaction profile outlined in the reaction profile below, state the mechanism or type of reaction for steps 1 and 2. 

Propane  1-bromopropane  Compound X  Propanal 

Step 1 ...............................................................

Step 2 ...............................................................

Outline the mechanism for step 2.

Compound X can be oxidised by the reaction with acidified potassium dichromate to give propanal. Compound X will oxidise to propanoic acid if allowed to fully oxidise. Explain how full oxidation can be prevented. 

State the following for step 1.

Reagents and conditions ..........................................

Mechanism ..........................................