Name: anti addition in alkene halogenation
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 2 min 46 s
Channel: JoVE

The addition of bromine or chlorine to an alkene produces a 1,2-dihalide product. Halogenation of alkenes is typically performed in non-nucleophilic solvents, like methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene to obtain trans-1,2-dibromocyclopentane. The reaction is initiated by the polarization of molecular bromine in the proximity of the electron-rich &#960; bond of an alkene. The electrophilic bromine adds across the alkene double bond generating a bromonium ion. 
This cyclic intermediate is more stable than a carbocation, as it contains more covalent bonds, and all the atoms have a filled octet.
However, the ring strain and a positive charge on the electronegative bromine make the bromonium ion susceptible to the nucleophilic attack by the bromide ion. The leaving group is the bromine in the intermediate and it remains tethered by a bond to the other carbon.
Due to the steric overcrowding by the cyclic bromonium ion and non-availability of bonding orbitals, the nucleophile approaches the antibonding orbital, pointing opposite to the carbon&#8211;bromine bond, leading to the anti addition. Thus, only trans-1,2-dibromocyclopentane is obtained as a product and not cis-1,2-dibromocyclopentane.
Furthermore, as both of the carbons of a bromonium ion are accessible to the nucleophile, the product is a racemic mixture.
The different stereoisomers of the starting alkene give different stereoisomers of the product. The bromination of cis-2-butene produces a pair of enantiomers, while another stereoisomer, trans-2-butene, forms a meso compound. Therefore, the halogenation of alkenes is a stereospecific reaction.
In a simple qualitative test for the presence of olefinic-double bonds, when bromine is added to an alkene, the deep red-colored bromine solution turns colorless, as the bromine adds across the double bond. Alternatively, a solution of bromine in an alkane retains a deep red color. 
Although the bromination and chlorination of alkenes are common, the addition of fluorine and iodine are not synthetically useful. Fluorine reacts too vigorously, and the iodination products decompose back to the alkene and iodine quickly.

halogenation of alkenes

Halogenation of Alkenes

Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the &#960; electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
<img alt="Cyclopentene bromination reaction mechanism; chemical equations diagram showing bromonium ion formation." src="/files/ftp_upload/11774/11774_PT_Figure_1.svg" style="text-align: center; height: 175px;" data-altupdatedat="1754904545000" data-alt="Cyclopentene to bromonium ion formation; chemical reaction diagram with Br2 intermediate." />
A bromonium ion is more stable than the analogous carbocation, as it has more covalent bonds and all the atoms have filled octets.
<img alt="Bromine ion stability comparison; cyclohexane diagrams; molecular stability analysis." src="/files/ftp_upload/11774/11774_PT_Figure_2.svg" style="text-align: center; height: 110px;" data-altupdatedat="1754904545000" data-alt="Bromine stability comparison in organic molecules; diagram shows less stable vs more stable structures." />
In the second step, the nucleophile, a bromide ion, attacks one of the carbon atoms in the bridged bromonium ion. Due to the non-availability of bonding orbitals and steric crowding, the nucleophile approaches the antibonding orbitals, pointing opposite to the carbon&#8211;bromine bond. This accounts for the anti addition.
<img alt="Bromination of cyclohexene mechanism diagram; electrophilic addition; reaction pathway illustration." src="/files/ftp_upload/11774/11774_PT_Figure_3.svg" style="text-align: center; height: 150px;" data-altupdatedat="1754904545000" data-alt="Bromination reaction mechanism; cyclohexene diagram; electrophilic addition of bromine; organic chemistry." />
Thus, the addition of two bromine atoms takes place from the opposite faces of the double bond in cyclopentene to yield trans-1,2-dibromocyclopentane.
The configuration of the starting alkene decides the stereochemical outcome for halogenation reactions. For example, the addition across cis-2-butene generates a pair of enantiomers, while addition across trans-2-butene produces a meso compound. Therefore, the halogenation of alkenes is a diastereospecific reaction.

Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence ...

Education

JoVE Core

Organic Chemistry

Unit 1

Reactions of Alkenes

KEY TERMS AND CONCEPTS

anti addition in alkene halogenation

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 than the similar carbocation intermediate. It has more covalent bonds, and the atoms keep filled octets. In the next step, a bromide ion attacks one carbon of the bridged ion.

Because bonding orbitals are not easily available and the bridge causes steric crowding, the nucleophile attacks from the opposite side. This gives anti addition. In cyclopentene, the two bromine atoms add from opposite faces of the double bond to form trans-1,2-dibromocyclopentane.

The starting alkene controls the stereochemical outcome. Addition to cis-2-butene gives a pair of enantiomers, while addition to trans-2-butene gives a meso compound. For this reason, halogenation of alkenes is a diastereospecific reaction.

Anti Addition in Alkene Halogenation

regioselectivity of electrophilic additions-peroxide effect

regioselectivity-of-electrophilic-additions-peroxide-effect

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...

free-radical chain reaction and polymerization of alkenes

free-radical-chain-reaction-and-polymerization-of-alkenes

How Alkene Monomers Form Polymers

Alkene monomers can join together to form polymers, which are large macromolecules used in many commercial products. In a polymer, the repeating unit defines the structure, while the terminal groups are considered insignificant. The average degree of polymerization shows how many repeating units are in a polymer molecule and is written as the subscript n. Alkenes form polymers through a free-radical chain reaction with three main stages: initiation, propagation, and termination. In the...

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...

formation of halohydrin from alkenes

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...

acid-catalyzed hydration of alkenes

Hydrating Alkenes to Form Alcohols

Alkenes can be converted into alcohols by adding water in the presence of an acid. Without acid, this hydration happens too slowly to matter in practice. The acid is not used up in the reaction, so this is an acid-catalyzed process. Strong acids such as sulfuric acid dissociate completely in water. In that case, the reacting acid species is the hydronium ion. The first step is a slow protonation of the alkene at the less-substituted end. This forms the more-substituted carbocation, which is...

regioselectivity and stereochemistry of acid-catalyzed hydration

regioselectivity-and-stereochemistry-of-acid-catalyzed-hydration

Carbocation Stability in Alkene Hydration

Acid-catalyzed hydration of alkenes forms alcohols through a carbocation intermediate. The reaction starts when hydronium ion protonates the alkene. This step is slow and controls the overall rate. The alkene structure matters because alkyl substituents at the double bond can change how fast the reaction occurs. The kind of carbocation formed also affects the rate. A reaction that passes through a tertiary carbocation is faster than one that forms a secondary or primary carbocation. This is...

oxymercuration-reduction of alkenes

Markovnikov Hydration with Mercury

Oxymercuration-reduction is a way to turn an alkene into an alcohol. It hydrates the alkene using mercuric acetate in a mix of tetrahydrofuran and water. The reaction first makes an organomercury adduct and then reduces it to an alcohol with sodium borohydride. The solvent mix has two jobs. Tetrahydrofuran dissolves the alkene and the aqueous mercuric acetate solution. Water acts as both a reactant and a solvent for mercuric acetate. The mechanism can be seen in the conversion of...

hydroboration-oxidation of alkenes

Anti-Markovnikov Alcohols from Alkenes

Hydroboration-oxidation turns alkenes into alcohols with anti-Markovnikov orientation. It is a complementary route to oxymercuration-demercuration, which gives Markovnikov products. In this reaction, borane adds across the alkene first, and basic hydrogen peroxide finishes the conversion to an alcohol. Borane is very reactive because the boron atom has only six electrons in its valence shell. It also has an empty 2p orbital that sits perpendicular to the plane formed by boron and its three...

regioselectivity and stereochemistry of hydroboration

regioselectivity-and-stereochemistry-of-hydroboration

Hydroboration Reaction Paths and Product Control

Hydroboration shows how alkene reactions control both where atoms add and how the product is shaped. In hydroboration–oxidation, borane attacks the alkene π bond in a concerted way, which means the bond changes happen together. This forms a cyclic, four-centered transition state. In this transition state, the BH2 group attaches to the less substituted carbon, while H attaches to the more substituted carbon. The addition happens across the same face of the alkene, so the reaction gives syn...

oxidation of alkenes: syn dihydroxylation with osmium tetraoxide

oxidation-of-alkenes-syn-dihydroxylation-with-osmium-tetraoxide

Alkene Dihydroxylation with Osmium Tetroxide

Alkene dihydroxylation with osmium tetroxide adds two hydroxyl groups across a double bond to make 1,2-diols, also called glycols. The reaction can give either anti or syn products, and this method follows syn stereochemistry. That means both new O atoms add to the same face of the alkene. The reaction occurs in two steps. First, osmium tetroxide adds to the alkene double bond in a concerted step, where both bonds form at the same time. This creates a five-membered cyclic osmate ester...

oxidation of alkenes: syn dihydroxylation with potassium permanganate

oxidation-of-alkenes-syn-dihydroxylation-with-potassium-permanganate

Potassium Permanganate Tests for Alkene Bonds

Potassium permanganate can be used to test and change alkene double bonds. When an alkene reacts with cold, dilute potassium permanganate under basic conditions, it forms a cis-diol, which is a molecule with two hydroxyl groups on the same side. A brown precipitate of manganese dioxide also appears during the reaction. The reaction starts when the permanganate ion adds to the same side of the alkene pi bond. This syn addition forms a cyclic manganate ester intermediate. Water then hydrolyzes...

oxidation of alkenes: anti dihydroxylation with peroxy acids

oxidation-of-alkenes-anti-dihydroxylation-with-peroxy-acids

Epoxide Route to Trans Diols

Epoxides provide a route to trans diols from alkenes. In anti dihydroxylation, an alkene is treated with a peroxycarboxylic acid to first make an epoxide. A diol is a compound with two hydroxyl groups, and in this pathway the two OH groups end up on opposite sides. An epoxide is a highly strained three-membered ring. Oxygen and two carbon atoms sit at the corners of an equilateral triangle. The first step uses a peracid, which is a strong oxidizing agent related to a carboxylic acid but with...

oxidative cleavage of alkenes: ozonolysis

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...

reduction of alkenes: catalytic hydrogenation

reduction-of-alkenes-catalytic-hydrogenation

Alkene Hydrogenation and Alkane Stability

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...

reduction of alkenes: asymmetric catalytic hydrogenation

reduction-of-alkenes-asymmetric-catalytic-hydrogenation

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

See how bromine or chlorine adds across an alkene double bond through a bromonium ion intermediate and why halogenation gives anti addition products.

Anti Addition in Alkene Halogenation | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Anti Addition in Alkene Halogenation

Description

Transcript

Tags