When an unsymmetrical alkene such as propene is treated with...

When an unsymmetrical alkene such as propene is treated with $N$ -bromosuccinimide in aqueous dimethyl sulfoxide, the major product has the bromine atom bonded to the less highly substituted carbon atom. Is this Markovnikov or non-Markovnikov orientation? Explain.

Key Concepts

Markovnikov's Rule

This rule predicts the regioselectivity of electrophilic additions to alkenes, stating that the electrophile adds to the carbon with more hydrogen atoms, leading to the more stable (often more substituted) carbocation intermediate, and ultimately placing the other substituent on the more substituted carbon.

Non-Markovnikov Addition

In non-Markovnikov addition, the regioselectivity of the reaction is reversed compared to what is predicted by Markovnikov's rule. Instead of the electrophile adding to the site that leads to the most stable carbocation, alternative reaction mechanisms, such as those involving radical intermediates or cyclic intermediates, lead to the electrophile (or other group) adding to the less substituted carbon.

Bromonium Ion Mechanism

The bromonium ion mechanism involves the formation of a three-membered cyclic intermediate when an alkene reacts with a brominating agent. In this intermediate, the positive charge is delocalized between the two carbons of the original double bond, and a subsequent nucleophilic attack typically occurs at the more substituted carbon, which has a greater degree of positive character, ultimately dictating the regiochemistry of the product.

Halohydrin Formation

Halohydrin formation is a specific type of reaction where an alkene reacts with a halogen in the presence of water. The process involves the initial formation of a cyclic halonium ion (such as a bromonium ion), followed by nucleophilic attack by water on the more substituted carbon. This attack opens the ring and leads to the formation of a halohydrin, where the halogen and hydroxyl group end up on different carbons with regioselectivity that is opposite to the typical Markovnikov addition for the halogen.

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