Why is there a color change when ethene reacts with bromine?

1 Answer
Nov 2, 2016

Because neither ethylene nor ethyl bromide have a characteristic colour.


On the other hand, bromine has a characterstic red-orange colour. When bromine reacts with an olefin (which are typically colourless), the orange colour dissipates as it reacts with the bromine.

Typically, we use orange-coloured bromine water, and the reaction gives an halohydrin:

#RCH=CH_2 + Br_2(aq) rarr RCH(OH)CH_2Br +HBr(aq)#

Bromine is the electrophile, i.e. #""^(delta+)Br-Br^(delta-)#, and it reacts to give a #2^@# carbocation. This carbocation then reacts with the most abundant nucleophile, which of course is water in aqueous solution.

Bromine is the one of the most corrosive substances you can handle in a laboratory. You treat this as you would treat your girlfriend/boyfriend, i.e. with care and respect.