Formation of Bromohydrins from alkenes using water and Br2
Description: Alkenes treated with bromine (Br2) in the presence of water will form bromohydrins.
Notes: Note that this reaction is Markovnikoff selective and delivers the trans-product. This is essentially the same reaction as with NBS.
Notes: Note how in examples 1 and 2 the oxygen is attached to the most substituted carbon (Markovnikoff). The trans-stereochemistry is evident in example 1. Finally in the third example this is an example of an intramolecular reaction – often gives students trouble!
Mechanism: The alkene attacks Br2 to form a bromonium ion (Step 1, arrows A and B) which is then attacked on the more substituted carbon by water (Step 2, arrows C and D). The positively charged oxygen is then deprotonated to give the neutral alcohol.
Notes: It’s probably more reasonable to show water as the base in step 3. Although not important for this molecule, the bromine and oxygen are in a trans orientation since water attacks the bromonium ion from the back.