Addition of HBr to Alkenes

by James


Description: Treatment of alkenes with hydrobromic acid will result in the formation of alkyl bromides.

Notes: This is an addition reaction. Note that the bromine always ends up at the more substituted carbon of the alkene (Markovnikoff-selectivity)



Notes: When a secondary carbocation is formed adjacent to a tertiary or quaternary carbon, rearrangements are possible. Note the third example – where Markovnikoff’s rule gives no clear preference, a mixture will be obtained. Note that the right hand molecule of the third example contains a stereocenter, which will be obtained as a racemic mixture (1:1 mixture of enantiomers).

Mechanism: Electrons from the C1-C2 π bond attack the hydrogen of HBr, expelling the bromide anion and leading to the formation of a carbocation (Step 1, arrows A and B). Note here that the carbocation preferentially forms on C2 (secondary) and not C1 (primary) since secondary carbocations are more stable. The bromide anion then attacks the carbocation, leading to formation of the alkyl bromide (Step 2, arrow C)

Notes: Since the carbocation is planar (flat) there is no preferred direction of attack of the bromide ion. If it’s possible to form a stereocenter, a mixture of stereoisomers will be obtained.


Additional example (advanced) : If the molecule contains adjacent stereocenters, the two directions of attack on the carbocation will no longer be of equal energy and a mixture of diastereomers will be obtained. In this example, attack of the bromide ion on the less hindered “top face” of the carbocation is favored, and a mixture of products (diastereomers) will be obtained.



{ 8 comments… read them below or add one }

William Wrench

This really is as good as anything out there- and I think the way it is neatly packaged makes it better than the rest. I am new as of yesterday to being a member- so if this is a frequently asked new kid question I apologize- but is there a clean way to print these? I filled out the mechanism blank sheets in Organic Chem as a Second Language and I wanted to print these out so that I can study from both my copy of notes and the reactions from this guide to make sure I have it all down- but printing it from a source like this takes a lot of pages since it prints out everything not just the text and pics about the mechanism (like this one short reaction printed is 9 pages). If not- that is okay- thanks for all of your hard work.



Why is there increase in energy in transition state at which Br- adds to carbocation ?



Good question! The transition state involves partial bonds and non-ideal bonding geometries, which is less stable than the carbocation itself.


Shrikant Saxena

Is there any variation in reaction when HBr in prescence of sunlight is used?



In the presence of ultraviolet light, the reaction undergoes radical chain addition. Covered in this post:



This is not 100% related to this video, but on the topic of alkenes in general. Why is 3-methyl-2-butene more reactive than 2-methyl-1-butene?



Think about the stability of the intermediate carbocation.



Will solvent change the product? Eg.- If water is used as a solvent will OH- attach instead of Br- as it is better nucleophile?


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