13 – Reaction Mechanisms Leaving Certificate Chemistry Organic Chemistry

The conversion of an alkane to a chloroalkane A free radical mechanism

chemical reactions of the alkanes Reaction with Chlorine Formation of the Chloroalkanes A substitution reaction involving ultraviolet light 2003 Q. 6 (b) (3) 2005 Q. 7(b) (3)

Mechanism of the reaction How does this reaction happen and How do we know?

The initiation (starting) step Fact: The reaction needs heat of over 300 o C or the presence of UV light to happen This input of energy allows the reaction: Cl 2 2Cl. Chlorine free radicals This is called homolytic fission ( equal splitting) Two chlorine free radicals (chlorine atoms with an unpaired electron each) are made for each photon of light.

Cl Initiation Ultraviolet light splits chlorine molecules into extremely reactive chlorine free atoms

The propagation (repeating middle steps) Fact: Hydrogen chloride is a major product of this reaction Reaction 1: Cl. + CH 4 HCl + CH 3. Hydrogen chloride is formed A methyl free radical is formed

Cl C H HH H Hydrogen Chloride formed Methyl Radical

The propagation ( repeating middle steps) Fact: Chloromethane is a major product of this reaction. Reaction 2: CH 3. + Cl 2 CH 3 Cl + Cl. Chloromethane is formed A new chlorine free radical is formed – which can go on to cause more propagation steps to happen. These middle steps are called a chain reaction

Cl Initiation Ultraviolet light splits chlorine molecules into extremely reactive chlorine free atoms C H HH H Hydrogen Chloride formed Methyl Radical Cl Chloromethane formed 2007 Q. 11(b) (19)

The termination (ending) steps Fact: The reaction can’t go on forever as the reactants run out Ethane is a minor product of the reaction How the reaction eventually stops… 1.Cl. + CH 3. CH 3 Cl 2.CH 3. + CH 3. C 2 H 6 3.Cl. + Cl. Cl 2 More chloromethane is formed A small amount of ethane is also formed

Cl Chlorine formed H3CH3CCH 3 Ethane (C 2 H 6 ) formed H3CH3CCl Chloromethane formed Termination Steps

Evidence for this mechanism 1. No reaction will occur in the absence of ultraviolet light. If irradiation is stopped the speed of the reaction slows down 2. The formation of ethane (C 2 H 6 ) in the reaction of methane with chlorine provides evidence of the formation of methyl radicals (CH 3 ) 3. Addition of a radical promoter (like tetraethyl lead) speeds up the reaction.

Recap - The reaction of methane with chlorine C H H H H Cl Methane Chlorine Chloromethane Hydrogen chloride In the presence of ultraviolet light

Cl Initiation Ultraviolet light splits chlorine molecules into extremely reactive chlorine free atoms C H HH H Hydrogen Chloride formed Methyl Radical Cl Chloromethane formed 2007 Q. 11(b) (19)

Cl Chlorine formed H3CH3CCH 3 Ethane (C 2 H 6 ) formed H3CH3CCl Chloromethane formed Termination Steps

Class activity Try to work out the mechanism for the reaction between ethane and chlorine. Include the iniation, propagation and termination steps. Facts: It is a substitution reaction UV light is needed for the reaction to happen at room temperature The main products are Chlorine and chloroethane. A minor product is butane

The reaction of ethane with chlorine C H H H C H H H Cl Ethane Chlorine Chloroethane Hydrogen chloride In the presence of ultraviolet light 2003 Q. 6 (d) (9)

The initiation (starting) step Fact: The reaction needs heat of over 300 o C or the presence of UV light to happen This input of energy allows the reaction: Cl 2 2Cl. Chlorine free radicals This is called homolytic fission ( equal splitting) Two chlorine free radicals (chlorine atoms with an unpaired electron each) are made for each photon of light.

The propagation ( repeating middle steps) Fact: Hydrogen chloride is a major product of this reaction Reaction 1: Cl. + C 2 H 6 HCl + C 2 H 5. Hydrogen chloride is formed An ethyl free radical is formed

The propagation ( repeating middle steps) Fact: Chloroethane is a major product of this reaction. Reaction 2: C 2 H 5. + Cl 2 C 2 H 5 Cl + Cl. Chloroethane is formed A new chlorine free radical is formed – which can go on to cause more propagation steps to happen. These middle steps are called a chain reaction

The termination (ending) steps Fact: The reaction can’t go on forever as the reactants run out Ethane is a minor product of the reaction How the reaction eventually stops… 1.Cl. + C2H 5. C2H 5 Cl 2.C 2 H 5. + C 2 H 5. C 4 H 10 3.Cl. + Cl. Cl 2 More chloroethane is formed A small amount of ethane is also formed

Cl C H H H C H H H Hydrogen Chloride formed Cl Ethyl free radical formed Chloroethane formed

Cl Chlorine formed H5C2H5C2 C2H5C2H5 Butane (C 4 H 10 ) formed H5C2H5C2 Cl Chloroethane formed Termination Steps

Evidence for this mechanism 1. No reaction will occur in the absence of ultraviolet light. If irradiation is stopped the speed of the reaction slows down 2. The formation of butane (C 4 H 10 ) in the reaction of ethane with chlorine provides evidence of the formation of ethyl radicals (C 2 H 5 ) 3. Addition of a radical promoter (like tetraethyl lead) speeds up the reaction.

The conversion of an alkene to a chloroalkane Ionic Addition

Addition of Hydrogen Chloride CC H H H H Ethene HCl Hydrogen Chloride C H H H C H Cl H Chloroethane

C H H C HH H Cl ∂+ ∂― Region of high electron density (negative charge) The polar Hydrogen Chloride molecule becomes further polarised on approach to the double bond A pair of electrons moves out from the double bond to form a covalent bond with hydrogen Ethenechloroethane formed

How do we represent this on paper?

C H H C HH Ethene H Cl ∂+ H Cl ∂- Chloroethane formed

Addition of Chlorine CC H H H H Ethene Cl 2 Chlorine C H H Cl C H H 1, 2 - dichloroethane Industrially Important As 1, 2 – dichloroethane is used to make chloroethene which is the raw material for the important plastic PVC (polyvinylchloride)

C H H C HH Cl Region of high electron density (negative charge) The nonpolar chlorine molecule becomes polarised on approach to the double bond A pair of electrons moves out from the double bond to form a covalent bond with one of the chlorines Ethene1, 2 - dichloroethane formed

How do we represent this on paper?

C H H C HH Ethene Cl ∂+ Cl ∂- 1, 2 - dichloroethane formed

Addition of Bromine CC H H H H Ethene Br 2 Bromine C H H Br C H H 1, 2 - dibromoethane Industrially Important As 1, 2 – dibromoethane is used to prevent lead accumulating in car engines

C H H C HH Br Region of high electron density (negative charge) The nonpolar bromine molecule becomes polarised on approach to the double bond A pair of electrons moves out from the double bond to form a covalent bond with one of the bromine atoms Ethene1, 2 - dibromoethane formed 2006 Q. 9(c) (9)

How do we represent this on paper?

C H H C HH Ethene Br ∂+ Br ∂- 1, 2 - dibromoethane formed

2003 Q. 6(e) (6)2006 Q. 9(c) (6)

C H H C HH Ethene Br ∂+ Br v ∂- 1, 2 - dibromoethane formed Cl Bromine salt water is a source of Cl - ions Cl 1-bromo-2-chloroethane formed OH Bromine salt water is a source of OH - ions OH 2-bromoethanol formed