[zeoblade]

in Harris chapter 17, he describes coulometry divided into potentiometric or amperometric. the amperometric is constant current like in a coulometric titration. the example is Br2/cyclohexene where adding electrons causes the reaction to proceed until completion where the voltage changes indicating the endpoint.

2Br^- --> Br₂ + e^-
Br₂ + cyclohexene --> 1,2-dibromocyclohexane

but why wouldnt the Br just add to the cyclohexene without electron input? in galvanic cells the solutions have to be separated so that the redox reaction doesnt directly happen and will go through the circuit. but electrolytic cells there seems to be no separation so the redox reaction will proceed.

[Borek]

but why wouldnt the Br just add to the cyclohexene

You don't have Br (more precisely: Br₂) in the solution.

[tamim83]

but why wouldnt the Br just add to the cyclohexene without electron input?

Br- is electron rich and so is the pi bond in cyclohexene.  They would not be very reactive toward one another.  However, once Br- is oxidized to Br₂, bromonation can proceed through a bromonium ion mechanism.  First add "Br⁺" to the pi system forming a bromonium ion.  Then, the remaining Br- adds to the cation in an S_N2  type reaction to form 1,2-dibromocyclohexane. 

[zeoblade]

thanks, it makes everything so clear