Description: Treatment of alkenes with hydrobromic acid will result in the formation of alkyl bromides.
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Notes: This is an addition reaction. Note that the bromine always ends up at the more substituted carbon of the alkene (Markovnikoff-selectivity)
Notes: The first two examples show a simple addition of HBr to give the Markovnikov product. In the third example Markovnikov’s rule gives no clear preference, so a mixture is obtained. The fifth example gives a mixture of diastereomers, since addition of HBr across the alkene will not affect the initial (R) stereochemistry. The final (sixth) example will give a dominant product, despite identical substitution, because formation of the resonance-stabilized carbocation will be favored!
When a secondary carbocation is formed adjacent to a tertiary or quaternary carbon, rearrangements are possible. See these pages for more examples:
- Addition with 1,2- hydride shift
- Addition with 1,2- alkyl shift
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.
Rearrangements: When secondary (or primary) carbocations are formed adjacent to a more substituted carbon, adjacent hydrogen atoms or alkyl groups can shift, resulting in formation of a more stable carbocation.
The first step of the reaction below is formation of a carbocation. Here, the formation of a carbocation via attack of the alkene upon H-Br is shown. (Step 1, arrows A and B). Since we have a secondary carbocation adjacent to a tertiary carbon, shift of a hydrogen to the secondary carbocation will result in a (more stable) tertiary carbocation (Step 2, arrow C). The tertiary carbocation is then trapped, in this case, by Br(-) (Step 3, arrows D). The rearrangement step goes through a transition state such as the one pictured.
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.
Rearrangements can also occur. See Additions To Alkenes Accompanied By 1,2-Hydride Shifts
(Advanced) References And Further Reading:
- Early example The Stereochemistry of the Addition of Hydrogen Bromide to 1,2-Dimethylcyclohexene George S. Hammond and Thomas D. Nevitt Journal of the American Chemical Society 1954 76 (16), 4121-4123 DOI: 10.1021/ja01645a020 Early paper from the 50’s by Prof. George Hammond (of Hammond’s Postulate) on the mechanism of HBr addition to 1,2-dimethylcyclohexane. He prefers a concerted pathway, although that might due to the conditions he employs – in pentane, a very nonpolar solvent, polar intermediates are disfavored.
- Mechanistic studies Hydrochlorination of cyclohexene in acetic acid. Kinetic and product studies Robert C. Fahey, Michael W. Monahan, and C. Allen McPherson Journal of the American Chemical Society 1970 92 (9), 2810-2815 DOI: 10.1021/ja00712a034 Detailed kinetic studies of the addition of HCl to cyclohexene in acetic acid, discussing a possible third-order mechanism (rate = k[cyclohexene][HX]2).
- Experimental Procedure
Lee, T. V.; Porter, J. R. Org. Synth. 1995, 72, 189 DOI: 10.15227/orgsyn.072.0189 The first reaction in the above procedure involves two steps – addition of HBr across the double bond and converting the aldehyde to a dimethyl acetal.
Org. Synth. 1938, 18, 47
DOI Link: 10.15227/orgsyn.018.0047
Org. Synth. 1940, 20, 64
DOI Link: 10.15227/orgsyn.020.0064