8.4 : Formation d'halohydrine à partir d'alcènes

Alkenes, such as propene, react with a halogen in the presence of water to yield a halohydrin, a compound having a halogen and a hydroxyl group on adjacent carbons.

The first step in the mechanism involves the electrophilic addition of bromine across the double bond giving a bromonium ion.

Water, present in excess, acts as a nucleophile and attacks the more substituted carbon of the bromonium ion to open the three-membered ring.

The steric crowding and non-availability of the bonding orbitals direct the attack of the nucleophile to an antibonding orbital on the opposite side of the carbon–bromine bond. 

Since the protonated bromohydrin is a strong acid, it loses a proton to water giving the product 1-bromo-2-propanol.

The regioselective addition of a hydroxyl group at a more substituted carbon is explained based on the combination of two factors. The electrostatic potential map of the bromonium ion shows that the more substituted carbon possesses more carbocation character, facilitating the nucleophile’s attack.

Furthermore, the bond between bromine and the more substituted carbon is longer than the bond of the less substituted carbon, meaning that the ring-opening transition state requires lower energy when the nucleophile attacks the more substituted carbon.

The next step is to identify the stereochemical outcome when a chiral center is generated. The addition of a halogen to a propene produces enantiomerically bridged intermediates and the addition of a nucleophile from the opposite face leads to a pair of halohydrin enantiomers.

In another instance, 1-methylcyclohexene reacts with bromine to give a pair of enantiomeric bromonium ions. The anti addition of water delivers trans-2-bromo-1-methylcyclohexanol as a racemic mixture.

Because alkenes are insoluble in water, the reaction is usually carried out in solvents like aqueous dimethyl sulfoxide. N-Bromosuccinimide serves as a stable and less harmful source of bromine.

Interestingly, in biological systems, bromoperoxidase oxidizes the bromine ion to the corresponding hypobromous acid, which upon electrophilic addition to the substrate and subsequent reaction with water, yields 2-bromo-1-phenyl-1,3-propanediol, a bromohydrin.