Name: bromohydrin addition in alkene reactions
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 2 min 41 s
Channel: JoVE

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&#8211;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&#8217;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.

formation of halohydrin from alkenes

Formation of Halohydrin from Alkenes

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.
<img alt="Alkene halohydrin formation equation, diagram of propene reacting with Br2 and H2O, yields Br and OH." src="/files/ftp_upload/11775/11775_PT_Figure_1.svg" style="text-align: center; height: 45px;" data-altupdatedat="1754904545000" data-alt="Allylic halogenation reaction, formula, organic chemistry equation, bromination of propene." />
Halohydrin formation commences when the &#960; 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 SN2 process. 
<img alt="Nucleophilic substitution reaction diagram, showing bromine leaving group and OH&#8322; attacking." src="/files/ftp_upload/11775/11775_PT_Figure_3.svg" style="text-align: center; height: 130px;" data-altupdatedat="1754904545000" data-alt="Organic reaction mechanism, showing nucleophilic substitution, with molecular structures and arrows." />
The deprotonation of the oxonium ion yields a hydronium ion and the neutral bromohydrin addition product.
<img alt="Organic reaction mechanism; hydroxyl displacement; molecular diagram; electrophilic addition." src="/files/ftp_upload/11775/11775_PT_Figure_4.svg" style="text-align: center; height: 110px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction mechanism; hydrobromination; organic compound transformation 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.
<img alt="Organic reaction mechanism, carbocation intermediate, molecular structure, electron cloud diagram." src="/files/ftp_upload/11775/11775_PT_Figure_5.svg" style="text-align: center; height: 150px;" data-altupdatedat="1754904545000" data-alt="Organic chemistry carbocation stability diagram; molecular structure and electron distribution." />
Additionally, the bond of the halogen with the more substituted carbon of the halonium ion is longer than that with the less substituted carbon.
<img alt="Static equilibrium in chemical bonding, C-Br bond length, molecular diagram, electron distribution." src="/files/ftp_upload/11775/11775_PT_Figure_6.svg" style="text-align: center; height: 150px;" data-altupdatedat="1754904545000" data-alt="C-Br bond length illustration; chemical structure and electron cloud model; bond polarity." />
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.
<img alt="Potential energy diagram, transition states in chemical reaction, molecules, energy change, reaction path." src="/files/ftp_upload/11775/11775_PT_Figure_7.svg" style="text-align: center; height: 400px;" data-altupdatedat="1754904545000" data-alt="Potential energy diagram of chemical reaction showing 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.
<img alt="Bromination reaction mechanism diagram; cyclohexene to enantiomeric bromonium ions." src="/files/ftp_upload/11775/11775_PT_Figure_8.svg" style="text-align: center; height: 175px;" data-altupdatedat="1754904545000" data-alt="Cyclohexene reaction mechanism, enantiomeric bromonium ions, diagram, organic chemistry process." />
The anti addition of water gives trans-2-bromo-1-methylcyclohexanol as a racemic mixture.
<img alt="Anti-addition reaction, chemical equation, mechanism diagram, organic synthesis process." src="/files/ftp_upload/11775/11775_PT_Figure_9.svg" style="text-align: center; height: 200px;" data-altupdatedat="1754904545000" data-alt="Anti-addition reaction mechanism, diagrams showing molecular structure and proton removal steps." />

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 ...

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Organic Chemistry

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Reactions of Alkenes

KEY TERMS AND CONCEPTS

bromohydrin addition in alkene reactions

Bromohydrin addition shows how an alkene can react with bromine in water to form a halohydrin. In this reaction, the product has a halogen and a hydroxyl group on neighboring carbons. When the halogen is bromine, the product is called a bromohydrin. When the halogen is chlorine, it is called a chlorohydrin.

The reaction begins when the pi electrons of the alkene react with electrophilic bromine. This step forms a bridged intermediate called a bromonium ion. Water then acts as a nucleophile. It uses a lone pair of electrons to open the three-membered bromonium ion ring in an SN2 process.

After water adds, the intermediate is an oxonium ion. Deprotonation of this ion gives hydronium ion and the neutral bromohydrin product. The attack of water is regioselective. It happens on the more substituted carbon of the alkene.

This preference can be explained in two ways. An electrostatic potential map of the bromonium ion shows greater carbocation character on the more substituted carbon. The bond between bromine and that carbon is also longer than the bond to the less substituted carbon. These features make ring opening easier at the more substituted site.

Because the mechanism goes through a halonium ion, the addition is anti. In 1-methylcyclohexene, bromine forms a pair of enantiomeric bromonium ions. Water then adds anti to give trans-2-bromo-1-methylcyclohexanol as a racemic mixture.

Bromohydrin Addition in Alkene Reactions

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Peroxide Effect in HBr Alkene Addition

Hydrogen bromide can add to an alkene in a way that goes against Markovnikov’s rule when organic peroxides are present. For example, 2-methylpropene reacts with HBr and peroxide to give 1-bromo-2-methylpropane. This change in product comes from a free radical mechanism, which reverses the usual regioselectivity. The reaction has three stages: initiation, propagation, and termination. In initiation, an oxygen-oxygen bond in the radical initiator breaks by homolytic cleavage, meaning each atom...

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Halogenation of alkenes adds chlorine or bromine across a carbon-carbon double bond to form a vicinal dihalide. This reaction is carried out in inert, non-nucleophilic solvents such as methylene chloride, chloroform, or carbon tetrachloride. The reaction starts when bromine is drawn toward the pi electrons of an alkene such as cyclopentene. One bromine atom acts as an electrophile and adds across the double bond. This forms a cyclic bromonium ion intermediate. The bromonium ion is more stable...

Bromohydrin addition shows how an alkene can react with bromine in water to form a halohydrin. In this reaction, the product has a halogen and a hydroxyl group on neighboring carbons. When the halogen is bromine, the product is called a bromohydrin. When the halogen is chlorine, it is called a chlorohydrin. The reaction begins when the pi electrons of the alkene react with electrophilic bromine. This step forms a bridged intermediate called a bromonium ion. Water then acts as a nucleophile. It...

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Carbocation Stability in Alkene Hydration

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Anti-Markovnikov Alcohols from Alkenes

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Hydroboration Reaction Paths and Product Control

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Alkene Dihydroxylation with Osmium Tetroxide

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Potassium Permanganate Tests for Alkene Bonds

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oxidation of alkenes: anti dihydroxylation with peroxy acids

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Epoxide Route to Trans Diols

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Alkene Bond Cutting by Ozonolysis

Ozonolysis is a reaction that cuts a carbon-carbon double bond in an alkene. Ozone attacks the double bond and breaks the molecule into smaller carbonyl products. Depending on the work-up, the products can be aldehydes, ketones, or carboxylic acids. Ozone is a symmetrical bent molecule with a resonance-stabilized structure. The reaction starts with electrophilic addition of ozone across the alkene double bond. This step forms an unstable molozonide, which then reacts further to give a carbonyl...

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Alkene hydrogenation turns an alkene into an alkane by adding molecular hydrogen. This reduction usually happens with a transition-metal catalyst, so the reaction is called catalytic hydrogenation. Palladium, platinum, and nickel are common catalysts in solid form as fine powder on an inert surface. These catalysts do not dissolve in the reaction mixture, so they are heterogeneous catalysts. The reaction takes place on the metal surface. First, hydrogen adsorbs onto the surface and the H-H...

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Chiral Alkene Hydrogenation for Drug Synthesis

Catalytic hydrogenation adds molecular hydrogen to an alkene double bond and turns it into an alkane. A transition-metal catalyst drives the reaction, and the hydrogen atoms add in syn stereochemistry, meaning they enter from the same side. The catalyst can be heterogeneous or homogeneous. When hydrogenation creates a chiral center, the products are usually a pair of enantiomers, which are mirror-image molecules. An enantiomeric excess of one product can be obtained with an enantioselective...

key terms and concepts

Learn how alkenes react with bromine and water to form bromohydrins through bromonium ion formation, anti addition, and regioselective ring opening.

Bromohydrin Addition in Alkene Reactions | JoVE K12

organic chemistry

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Bromohydrin Addition in Alkene Reactions

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