Question 11

[jennifer.v7513]

still confused about the mechanism! can you clarify it further pls?

[jellywing_2058]

(P.S. Draw as you read!)
(P.P.S. 1,3 butadiene is a symmetrical molecule; therefore, in the following explanation, you can either use the carbon numbers written in green or written in blue since they are equivalent to each other (of course be consistent with the color choice for the entire explanation)

Basically, the double bond between C3 and C4 (C1 and C2) breaks and attacks a Br from dibromine, breaking dibromine apart (making 2Br instead of Br2).

The attacked Br attaches to C4 (C1) since it is less sterically hindered than C3 (C2), leaving a positive charge on C3 (C2).

The positive charge can be stabilized with a resonance structure using the other double bond (between C1 (C4) and C2 (C3)). This double bond breaks and reforms between C2 (C3) and C3 (C2); leaving a positive charge on C1 (C4).

Hope this helps!