Substitution Reactions of Alkanes

Alkanes

What is a substitution reaction?

A reaction in which an atom or group is replaced by another atom or group.

Why are alkanes generally unreactive?

They contain only carbon and hydrogen atoms joined by strong, relatively non-polar single covalent bonds.

What happens when methane reacts with chlorine?

A hydrogen atom in methane is replaced by a chlorine atom.

What is the overall equation for the reaction between methane and chlorine?

CH₄ + Cl₂ → CH₃Cl + HCl

What is chlorination?

A substitution reaction in which a chlorine atom replaces another atom, such as hydrogen, in a molecule.

What is halogenation?

A reaction in which a halogen atom is introduced into a molecule.

What is a reaction mechanism?

A sequence of two or more steps showing how a reaction takes place, including changes in bonding and electrons.

Why is a mechanism more useful than an overall equation?

It shows how bonds break and form and how electrons move during each stage of the reaction.

What conditions are needed for methane to react readily with chlorine?

Ultraviolet radiation, such as sunlight.

What effect does ultraviolet radiation have in the chlorination of methane?

It breaks the Cl–Cl bond to form chlorine free radicals.

What type of bond breaking occurs when Cl₂ is exposed to ultraviolet radiation?

Homolytic fission.

What is homolytic fission?

The breaking of a covalent bond so that each atom takes one electron from the bonding pair, forming two free radicals.

What is the initiation step in the chlorination of methane?

Cl₂ → 2Cl•

What causes the initiation step?

Ultraviolet radiation causes homolytic fission of the Cl–Cl bond.

What is a free radical?

A species containing an unpaired electron.

What does the dot in Cl• represent?

An unpaired electron.

Why are free radicals very reactive?

They contain an unpaired electron.

What is initiation in a free-radical mechanism?

The first step that starts the reaction by forming free radicals, usually through bond breaking caused by ultraviolet radiation.

What happens in the first propagation step of methane chlorination?

A chlorine radical reacts with methane and removes a hydrogen atom, producing HCl and a methyl radical.

What is the equation for the first propagation step of methane chlorination?

Cl• + CH₄ → HCl + CH₃•

What is a methyl free radical?

CH₃•, a methyl species containing an unpaired electron.

What happens in the second propagation step of methane chlorination?

A methyl radical reacts with chlorine to form chloromethane and another chlorine radical.

What is the equation for the second propagation step of methane chlorination?

CH₃• + Cl₂ → CH₃Cl + Cl•

What is propagation in a free-radical mechanism?

Steps in which a molecule and a free radical react to produce a different molecule and a different free radical.

Why can the propagation steps repeat many times?

They regenerate a chlorine free radical, allowing the chain reaction to continue.

What is the overall result of the two propagation steps?

CH₄ and Cl₂ are converted into CH₃Cl and HCl.

What is termination in a free-radical mechanism?

A step in which two free radicals combine to form one molecule, removing radicals and stopping the chain reaction.

What are the three possible termination reactions in methane chlorination?

Cl• + Cl• → Cl₂; Cl• + CH₃• → CH₃Cl; CH₃• + CH₃• → C₂H₆.

What is formed when two chlorine radicals combine?

Cl₂.

What is the termination equation for two chlorine radicals?

Cl• + Cl• → Cl₂

What is formed when a chlorine radical and methyl radical combine?

CH₃Cl.

What is the termination equation between a chlorine radical and a methyl radical?

Cl• + CH₃• → CH₃Cl

What is formed when two methyl radicals combine?

Ethane, C₂H₆.

What is the termination equation between two methyl radicals?

CH₃• + CH₃• → C₂H₆

Why do termination reactions stop the chain reaction?

Two reactive free radicals are converted into a stable molecule, reducing the number of radicals available to continue propagation.

What are the three stages of a free-radical substitution mechanism?

Initiation, propagation and termination.

How can you recognise an initiation step?

One molecule forms two free radicals.

How can you recognise a propagation step?

A free radical and a molecule react to form a different free radical and molecule.

How can you recognise a termination step?

Two free radicals combine to form one molecule.

Can chloromethane undergo further substitution reactions?

Yes, its remaining hydrogen atoms can also be replaced by chlorine atoms.

What happens when chloromethane undergoes further chlorination?

Another hydrogen atom is replaced by chlorine, producing dichloromethane.

What is the equation for the formation of dichloromethane?

CH₃Cl + Cl₂ → CH₂Cl₂ + HCl

What is the equation for the formation of trichloromethane?

CH₂Cl₂ + Cl₂ → CHCl₃ + HCl

What is the equation for the formation of tetrachloromethane?

CHCl₃ + Cl₂ → CCl₄ + HCl

What products can therefore form when methane reacts with excess chlorine?

Chloromethane, dichloromethane, trichloromethane and tetrachloromethane, as well as HCl.

Why can chlorination of methane produce a mixture of organic products?

Substitution can continue after chloromethane forms because its remaining hydrogen atoms can also be replaced by chlorine.

Why can the yield of chloromethane be low?

Further substitution reactions occur, producing several different chlorinated products.