1,2 and 1,4 Addition reactions

1,3 - butadiene reacts with halogen acids (HBr or HCl) to yield a mixture of two compounds. they are 3-bromo 1 butene and 1 bromo 2 butene. The first product results from 1,2 Addition to one of the double bonds. The second product results from addition to terminal (1,4) positions with the formation of a new double bond between C - 2 and C -3 This letter process is known as 1, 4 addition at low temperatures the 1,2 addition is preferred where as  high temperatures 1,4 addition predominates.

Mechanism :-

Step1 : HBr ionises to give a proton (electrophile).

Step2 : Proton attacks a double bond according to Markovnikov rule to give a resonance stabilized carbonium ion.

Step3 : Bromide ion combines with (A) to give 3 bromo 1 butene (1,2 addition). It combines with (B) to give 1 bromo 2 butene (1,4 addition). Thus a mixture of two compounds is obtained.

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