Free Radical Chlorination

Experimental Evidence Helps to Determine Mechanism

• Chlorination does not occur at room temperature in the dark.

• The most effective wavelength of light is blue that is strongly absorbed by Cl2 gas.

• The light-initiated reaction has a high quantum yield (many molecules of product are formed from each photon of light).

Free Radical Species are Constantly Generated Throughout the Reaction

Propagation

Termination: Reaction of any 2 Radicals

Chlorination of Propane

30%

70%

H’s are not abstracted at the same rate.

For secondary:70%/4H = 17.5For primary:

30%/6H = 5.0 Therefore…Relative rate of H abstraction for 1o : 2o is

1 : 3.5

Chlorination of Methylpropane

C

CH3

CH3

CH3 H HCH3

CH3

CH2

C CH3

CH3

CH3

C.++ Cl

.

.

.

C

CH2

CH3

CH3 H

.C

CH3

CH3

CH3

+ Cl2

+ Cl2 C

CH3

CH3

CH3 Cl

C

CH2Cl

CH3

CH3 H 65%

35%

+ Cl

+ Cl

.

.

3o Radicals are Easiest to Form

Tertiary H’s removed five times more readily than primary H’s in chlorination reactions

Relative reactivity of 1o H abstraction:65% / 9H = 7.2

Relative reactivity of 3o H abstraction:35% / 1H = 35

Therefore…Relative rate of H abstraction for 1o : 3o

1:5

Stability of Free Radicals

Bromination is Very Selective

RDS in Bromination is highly endothermic

Consider the free radical monochlorination of 2,2,5-trimethylhexane. Draw all of the unique products (ignore stereoisomers; use zig-zag structures please) and predict the ratio or percent composition of the products.The relative reactivity of H abstraction in a chlorination reaction: 1o: 2o: 3o = 1: 3.5: 5

2.(12) Consider the free radical monochlorination of 1,4-dimethylcyclohexane. The reaction affords three different products (all C8H15Cl). In this reaction, the rates of hydrogen abstraction of primary : secondary : tertiary are 1 : 3.5 : 5. Draw the three products and predict the percent composition (or ratio) of the product mixture. (Disregard cis / trans isomers for this problem.)