Free Radical Chlorination Experimental Evidence Helps to Determine

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 Cl 2 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

H’s are not abstracted at the same rate. For secondary: 70%/4 H = 17. 5 For primary: 30%/6 H = 5. 0 Therefore… Relative rate of H abstraction for 1 o : 2 o is 1 : 3. 5

Chlorination of Methylpropane

3 o Radicals are Easiest to Form

Tertiary H’s removed five times more readily than primary H’s in chlorination reactions Relative reactivity of 1 o H abstraction: 65% / 9 H = 7. 2 Relative reactivity of 3 o H abstraction: 35% / 1 H = 35 Therefore… Relative rate of H abstraction for 1 o : 3 o 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: 1 o: 2 o: 3 o = 1: 3. 5: 5

2. (12) Consider the free radical monochlorination of 1, 4 -dimethylcyclohexane. The reaction affords three different products (all C 8 H 15 Cl). 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. )