NCERT Class XI Chemistry Hydrocarbons Solutions
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Question : 16
Total: 25
Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1- bromopropane. Explain and give mechanism.
Solution:
Addition of HBr to propene takes place as per the Markownikoff’s rule via the formation of most stable carbocation as follows :
Step 1 :
Step 2 :
Thus, it is an example of electrophilic addition reaction.
But, when the same reaction is carried out in the presence of peroxide, which is a free-radical generator, the mechanism follows free radical mechanism where the intermediate is a carbon free radical.
Step 1 :
2 C 6 H 5 −
−
Step 2 : Generation of
+
B r C 6 H 6 +
Step 3 :
C H 3 −
− C H 2 − B r
Here, the free radical is the 2° free radical and therefore it is the one which finally forms 1-bromopropane.
Step 4 :C H 3 −
− C H 2 B r
B r C H 3 C H 2 C H 2 B r +
Step 5 :
+
B r 2
This explains why different products are formed under different reaction conditions.
Step 1 :
Step 2 :
Thus, it is an example of electrophilic addition reaction.
But, when the same reaction is carried out in the presence of peroxide, which is a free-radical generator, the mechanism follows free radical mechanism where the intermediate is a carbon free radical.
Step 1 :
Step 2 : Generation of
Step 3 :
Here, the free radical is the 2° free radical and therefore it is the one which finally forms 1-bromopropane.
Step 4 :
Step 5 :
This explains why different products are formed under different reaction conditions.
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