8.4 : Formation of Halohydrin from Alkenes

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.

An alkene, such as propene, reacts with bromine in the presence of water to yield a halohydrin. Halohydrins contain a halogen and a hydroxyl group attached to adjacent carbons. When the halogen is bromine, it is called a bromohydrin, while a chlorohydrin has chlorine as the halogen.

Alkene reaction addition of Br2 and H2O, chemical equation, organic chemistry process.

Halohydrin formation commences when the π electrons of the alkene react with electrophilic bromine to form a bridged intermediate called a bromonium ion. Water, acting as a nucleophile, uses a lone pair of electrons to open the three-membered bromonium ion ring and forms a bond with the carbon in an S_N2 process.

Organic reaction mechanism, Br⁻ attack, hydration process; chemical equation diagram.

The deprotonation of the oxonium ion yields a hydronium ion and the neutral bromohydrin addition product.

Organic reaction mechanism, acid-catalyzed hydration, structural formulas, diagram.

The preferred opening of the cyclic bromonium ion intermediate by the regioselective attack of water on the more substituted carbon of the alkene can be explained based on two factors. Primarily, the electrostatic potential map of a bromonium ion shows that the more substituted carbon exhibits a greater carbocation character.

Organic chemistry reaction mechanism, carbocation formation diagram, electron density visualization.

Additionally, the bond of the halogen with the more substituted carbon of the halonium ion is longer than that with the less substituted carbon.

Organic chemistry diagram; C-Br bond length comparison, molecular structure, electron distribution.

This difference in bond lengths in the cyclic intermediate indicates that the ring-opening transition state can be attained more efficiently by the attack of the nucleophile at the more substituted carbon.

Potential energy diagram for chemical reaction; charts transition states and reaction coordinate.

Since the mechanism involves a halonium ion, the stereochemistry of addition is anti. When 1-methylcyclohexene is treated with bromine, a pair of enantiomeric bromonium ions is obtained.

Bromination reaction mechanism showing enantiomeric bromonium ions; organic chemistry diagram.

The anti addition of water gives trans-2-bromo-1-methylcyclohexanol as a racemic mixture.

Anti-addition reaction mechanism diagram; illustrates bromine and hydroxyl group addition.

Tags

Alkene Halohydrin Bromine Water Bromonium Ion Chlorohydrin Nucleophile SN2 Process Oxonium Ion Regioselective Attack Carbocation Character Bond Length Ring opening Transition State Stereochemistry

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