Question

Why is this CH2 in the center marked as having a positive charge?

I've been studying too much today, and my head refuses to work. Please give just a hint. I want to refresh my knowledge about this reaction.

Answer 1

Consider what the electrons are doing in the pi bond, they are attacking the `delta+` bromide (in that mechanism, although it's a tad weird: I would think they go to the methylene group at the end, but I digress).

Now the carbon atom only has three bonds: one to carbon and two to hydrogen, hence the positive charge, that is a very great substrate for the nucleophilic bromide hanging out in the solution.

This is what's (roughly) happening in the transition state you're confused about,