whats the mech for this: (E)-2,5-dimethyl-hex-3-ene and bromine

Answer 1

CH3-CH(CH3)-CH=CH-CH(CH3)-CH3 + Br2 ->

CH3-CH(CH3)-CH(Br)-CH(Br)-CH(CH3)-CH3

Note that C-2 and C-5 are chiral centres so you may want to restrict your answer by starting with one set of configurations about the chiral centres.

I can't show the stereochemistry, in any case I suggest that you make a model, add the bromine and transcribe the structure.

The bromine will add across the double bond in the usual fashion. This page shows the addition with an example where the Br2 is added across Z and E isomers.

http://en.wikipedia.org/wiki/Halogen_addition_reaction