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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).
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
.
.
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
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.)