8-6 - Organic Chemistry (Mcmurry)

Question

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

Step-by-Step Solution

Answer

The opening of bromonium takes place at the secondary carbon and yields a Markovnikov product.

Markovnikov orientation

Markovnikov orientation is the electrophilic addition of hydrogen halides to the unsymmetrical alkenes; the addition occurs when hydrogen itself attaches to the least substituted carbon of the double bond. It means hydrogen is added to carbon with more hydrogens, and halogen is added to carbon with fewer hydrogen atoms.

To determine whether it is Markovnikov or non-Markovnikov orientation

The reaction of an alkene with  obtained from NBS forms a cyclic bromonium ion. This bromonium ion is opened by water, and a partial positive charge comes over carbon, whose bond with bromine gets cleaved.

The secondary carbon stabilizes the charge better as compared to a primary carbon. Thus, the opening of the bromonium ion takes place at the secondary carbon and yields a Markovnikov product.

            The comparison of primary and secondary carbon