L5 - Free Radical Substitution

objectives

Understandings:

Alkanes:

• Alkanes have low reactivity and undergo free-radical substitution reactions.

Applications and Skills:

• Explanation of the reaction of methane and ethane with halogens in terms of a free-radical substitution mechanism involving photochemical homolytic fission.

Guidance:

• Reference should be made to initiation, propagation and termination steps in free-radical substitution reactions. Free radicals should be represented by a single dot.

BREAKING COVALENT BONDS

There are 3 ways to split the shared electron pair in an unsymmetrical covalent bond:

UNEQUAL SPLITTING

• produces IONS known as HETEROLYSIS or HETEROLYTIC FISSION

EQUAL SPLITTING

• produces RADICALS known as HOMOLYSIS or

HOMOLYTIC FISSION

• If several bonds are present the weakest bond is usually broken first

• Energy to break bonds can come from a variety of energy sources - heat / light

• In the reaction between methane and chlorine either can be used, however...

• In the laboratory a source of UV light (or sunlight) is favoured.

There are 3 ways to split the shared electron pair in an unsymmetrical covalent bond:

• If several bonds are present the weakest bond is usually broken first
• Energy to break bonds can come from a variety of energy sources - heat / light
• In the reaction between methane and chlorine either can be used, however...
• In the laboratory a source of UV light (or sunlight) is favoured.

Free Radicals

TYPICAL PROPERTIES

• reactive species (atoms or groups) which possess an unpaired electron
• their reactivity is due to them wanting to pair up the single electron
• formed by homolytic fission (homolysis) of covalent bonds
• formed during the reaction between chlorine and methane
• formed during thermal cracking
• involved in the reactions taking place in the ozone layer

Reaction of Chlorine with Methane

Reagents chlorine and methane

Conditions UV light or sunlight - heat is an alternative energy source

Equation(s)

CH₄(g) + Cl₂(g) ——> HCl(g) + CH₃Cl(g) chloromethane

Further Substitution:

CH₃Cl(g) + Cl₂(g) ——> HCl(g) + CH₂Cl₂(l) dichloromethane

CH₂Cl₂(l) + Cl₂(g) ——> HCl(g) + CHCl₃(l) trichloromethane

CHCl₃(l) + Cl₂(g) ——> HCl(g) + CCl₄(l) tetrachloromethane

• free radicals are very reactive - they are trying to pair their electron
• with sufficient chlorine, every hydrogen will eventually be replaced.

• Mechanisms portray what chemists think is going on in the reaction, whereas an equation tells you the ratio of products and reactants.
• Chlorination of methane proceeds via FREE RADICAL SUBSTITUTION because the methane is attacked by free radicals resulting in hydrogen atoms being substituted by chlorine atoms.
• The process is a chain reaction.
• In the propagation step, one radical is produced for each one used

Initiation

Cl₂ ——> 2Cl• RADICALS CREATED

The single dots represent UNPAIRED ELECTRONS

During initiation, the WEAKEST BOND IS BROKEN by HOMOLYTIC FISSION as it requires less energy.

There are three possible bonds in a mixture of alkanes and chlorine.

The Cl-Cl bond is broken in preference to the others as it is the weakest and requires requires less energy to separate the atoms.

Propagation

Cl• + CH₄ ——> CH₃• + HCl RADICALS USED and

Cl₂ + CH₃• ——> CH₃Cl + Cl• then RE-GENERATED

• Free radicals are very reactive because they want to pair up their single electron.
• They do this by abstracting a hydrogen atom from methane; a methyl radical is formed
• The methyl radical is also very reactive and attacks a chlorine molecule
• A chlorine radical is produced and the whole process can start over again

Termination

Cl• + Cl• ——> Cl₂ RADICALS REMOVED

Cl• + CH₃• ——> CH₃Cl

CH₃• + CH₃• ——> C₂H₆

Removing the reactive free radicals brings an end to the reaction.

This is not very likely at the start of the reaction because of their low concentration.

Summary

• Due to lack of reactivity, alkanes need a very reactive species to persuade them to react
• Free radicals need to be formed by homolytic fission of covalent bonds
• This is done by shining UV light on the mixture (heat could be used)
• Chlorine radicals are produced because the Cl-Cl bond is the weakest
• You only need one chlorine radical to start things off
• With excess chlorine you get further substitution and a mixture of chlorinated products

Further Propagation

• If excess chlorine is present, further substitution takes place
• The equations show the propagation steps for the formation of...