Formation of bromohydrins from alkenes using water and NBS

by James

Description: Alkenes treated with N-bromosuccinimde (NBS) and water will form bromohydrins.
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Notes: The reaction is Markovnikoff selective and provides the trans-product. NBS stands for N-bromosuccinimide, it looks like this:

 

 

 

Examples:

Notes: Note how in examples 1 and 2 the oxygen is attached to the most substituted carbon (Markovnikoff). Note the trans stereochemistry in example 1. Finally in the third example, this is an example as an intramolecular reaction, which often gives students trouble.

Mechanism: The alkene attacks NBS to form a bromonium ion (Step 1, arrows A and B) which is attacked at the more substituted carbon by water (Step 2, arrows C and D). The resulting positively charged oxygen is then deprotonated to give the neutral bromohydrin (Step 3, arrows E and F).

Notes: Although not important for this molecule, the bromine and oxygen should be trans, since attack of water on the bromonium ion is a backside attack.

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