Adding Hydrogen Bromide (HBr) to an Alkene

An alkene is an organic (carbon-containing) compound that has one or more double bonds between adjacent carbon atoms. The simplest example of an alkene is ethylene. It is derived from ethane by removal of a hydrogen molecule.

H₃C-CH₃ → H₂C=CH₂ + H₂

Adding hydrogen bromide to this simplest alkene can result in only one product, since ethylene is symmetrical about the double bond. Whether one adds the bromine to the first or to the second carbon atom, the reactions produce the same product, bromoethane..

H₂C=CH₂ + HBr → BrH₂C-CH₃
(which is the same thing as H₃C-CH₂Br).

When there is Lack of Symmetry

The next simplest alkene is propene,

H₂C=CH-CH₃.

The situation with propene is entirely different. This molecule is not symmetrical about the double bond. To the left of the double bond is CH₂, whereas to the right of the double bond is CH-CH₃. If the bromine atom goes to the leftmost carbon, an entirely different product results from a reaction in which it goes to the second carbon atom.

The two different reaction sequences¹ are called Markovnikov and Anti-Markovnikov additions, and each results in a different product. Thus,

H₂C=CH-CH₃ + HBr → H₃C-CHBr-CH₃ (Markovnikov addition)

H₂C=CH-CH₃ + HBr → BrH₂C-CH₂-CH₃ (Anti-Markovnikov addition).

The rule is that for Markovnikov addition, the bromine atom goes to the most substituted carbon atom-the acid hydrogen goes to the other double-bonded carbon atom.

What Determines Product Outcome?

Chemical reaction paths (and hence, resultant products) often vary due to the application of different reaction "mechanisms." In the case of carbon chemistry, carbocations may be involved, or perhaps carbanions or free radicals. The addition of hydrogen bromide ordinarily proceeds via a carbocation reaction mechanism², however specific adjustments may be made to alter this, if it is the Anti-Markovnikov product that is desired.

What Specific Adjustments Produce Anti-Markovnikov Products?

One example of a mechanistic adjustment to yield Anti-Markovnikov products is the use of free radicals, employing a peroxide or similar radical initiator. Another is the introduction of negative charges by employing a carbanion mechanism, perhaps via a strong Lewis base. Such mechanisms depend upon the relative ease certain atoms have in bearing charges beyond that atoms normally carry. Various factors are involved in this, none more important than the principle of atom electronegativity.

¹ Markovnikov additions are not restricted to it, but hydrogen bromide affords the simplest example or demonstration of Markovnikov's Rule. Markovnikov reactions employ so-called protic or Bronsted acids, and includes sulfuric acid and even water.

² A carbocation is a positively charged structure containing an even amount of electrons with a significant amount of the charge located on one or more carbon atoms.

Resources and References:

Michigan State University, Department of Chemistry - Addition Reactions of Alkenes

McGraw Hill Higher Education - Reaction of Alkenes with Hydrogen Halides

Khanacademy Video - Markovnikov's Rule and Carbocations