Name: alkyne halogenation and anti addition
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 2 min 38 s
Channel: JoVE

Electrophilic addition reactions involve the conversion of multiple bonds, such as carbon-carbon double and triple bonds, into other functional groups.
In these reactions, the high electron density around the &#960; bond allows them to function as nucleophiles and attack electrophilic centers. The net result is the addition of a simple molecule across a &#960; bond.
If the simple molecule is a halogen, such as bromine or chlorine, the reaction is called a halogenation reaction. For every mole of the added halogen, one &#960; bond is broken, and two new &#963; bonds are formed.
Recall that the halogenation of alkenes is a stereospecific reaction that proceeds via an anti addition forming vicinal dihalides.
Halogenation of alkynes follows a similar pattern. However, since alkynes have two &#960; bonds, halogens can add twice across the multiple bonds.
The addition of one equivalent of the halogen forms the trans-dihalide as the major product; another equivalent gives the tetrahaloalkane.
Analogous to alkenes, one of the &#960; bonds in alkynes acts as a nucleophile and attacks the electrophilic center on the polarized halogen molecule. 
As this happens, the halogen atom with the partial negative charge leaves as a halide ion, resulting in the formation of a cyclic halonium ion intermediate.
Next, the halide ion attacks either carbon of the halonium intermediate from the backside of the ring, causing the ring to open and form the trans-dihaloalkene.
Further addition of another equivalent of the halogen follows a similar mechanism to yield a tetrahaloalkane.
For example, the addition of one mole of bromine to 2-butyne in the presence of acetic acid and lithium bromide selectively forms E-2,3-dibromo-2-butene. Addition of a second mole of bromine yields 2,2,3,3-tetrabromobutane.
Lastly, alkynes are less reactive to electrophilic additions than alkenes. This is because the &#960; electrons are held more tightly in C&#8801;C bonds than in C=C bonds.
Additionally, the halonium ion formed from alkynes is highly strained and more unstable than the corresponding alkene intermediate.

electrophilic addition to alkynes: halogenation

Electrophilic Addition to Alkynes: Halogenation

<h4>Introduction</h4>
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a &#960; bond. In alkynes, the presence of two &#960; bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
<img alt="Organic reaction diagram: alkyne halogenation process of alkenes to alkanes with X&#8322;." src="/files/ftp_upload/11836/11836_PT_Figure_1.svg" style="text-align: center; height: 200px;" data-altupdatedat="1754904545000" data-alt="Alkyne halogenation process, chemical reaction diagram, showing major and minor product formation." />
<h4>Reaction Mechanism</h4>
In the first step, a &#960; bond from the alkyne acts as a nucleophile and attacks the electrophilic center on the polarized halogen molecule, displacing the halide ion and forming a cyclic halonium ion intermediate. In the next step, a nucleophilic attack by the halide ion opens the ring and forms the trans-dihaloalkene. Since the nucleophile attacks the halonium ion from the backside, the net result is an anti addition where the two halogen atoms are trans to each other.
<img alt="Alkyne halogenation mechanism, showing electron pair movement, chemical reaction diagram." src="/files/ftp_upload/11836/11836_PT_Figure_2.svg" style="text-align: center; height: 130px;" data-altupdatedat="1754904545000" data-alt="Alkyne halogenation mechanism, chemical reaction diagram, illustrating electron pair movements." />
The addition of a second equivalent of halogen across the alkene &#960; bond also proceeds via the formation of a bridged halonium ion to give the tetrahalide as the final product.
<img alt="Radical polymerization mechanism diagram with electron movement and bond formation equations." src="/files/ftp_upload/11836/11836_PT_Figure_3.svg" style="text-align: center; height: 130px;" data-altupdatedat="1754904545000" data-alt="Organic reaction mechanism diagram, showing steps of electrophilic addition with intermediate formation." />
For example, the addition of bromine to 2-butyne in the presence of acetic acid and lithium bromide favors anti addition and preferentially forms the trans or (E)-2,3-dibromo-2-butene as the major product. The corresponding cis isomer, (Z)-2,3-dibromo-2-butene, is formed in lower yields. A second addition gives 2,2,3,3-tetrabromobutane.
<img alt="2-butyne bromination reaction mechanism with (E/Z)-2,3-dibromo-2-butynes, chemical diagram." src="/files/ftp_upload/11836/11836_PT_Figure_4.svg" style="text-align: center; height: 270px;" data-altupdatedat="1754904545000" data-alt="2-Butyne halogenation reaction, chemical equations diagram, product formation steps shown" />
<h4>Reactivity of alkynes and alkenes towards electrophilic addition</h4>
Alkynes are less reactive than alkenes towards electrophilic addition reactions. The reasons are twofold. First, the carbon atoms of a triple bond are sp hybridized in contrast to the double bonds that are sp2 hybridized. Since the sp hybrid orbitals have a higher s-character and are more electronegative, the &#960; electrons in C&#8801;C are held more tightly than in C=C. As a result, in alkynes, the &#960; electrons are not readily available for the nucleophilic attack, making them less reactive towards electrophilic addition than alkenes. 
Secondly, the cyclic halonium ion formed from alkynes is a three-membered ring with a double bond where the 120&#176; bond angle of an sp2 carbon is constrained into a triangle.
<table style="margin-left: auto; margin-right: auto;">
	<tbody style="border: 1px solid black;">
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;"><img alt="Carbocation intermediate diagram, chemical structure, electrophilic addition process." src="/files/ftp_upload/11836/11836_PT_Figure_5_Left.svg" style="text-align: center; height: 55px;" data-altupdatedat="1754904545000" data-alt="Carbocation intermediate, chemical structure diagram, organic chemistry reaction mechanism." /></td>
			<td style="color: black; border: 1px solid black; text-align: center;"><img alt="Organic chemistry free radical reaction; diagram; carbon-centered radical structure." src="/files/ftp_upload/11836/11836_PT_Figure_5_Right.svg" style="text-align: center; height: 55px;" data-altupdatedat="1754904545000" data-alt="Carbocation structure diagram with positive charge, chemical bonding, organic chemistry concept." /></td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Alkyne halonium ion</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Alkene halonium ion</td>
		</tr>
	</tbody>
</table>
In contrast, the cyclic intermediate in alkenes is a three-membered ring with an sp3 hybridized carbon where a bond angle of 109&#176; is constrained into a triangle. Therefore, the larger ring strain associated with the alkyne halonium ions makes them more unstable and hinders their formation.

Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a &#960; bond. In alkynes, the ...

electrophilic-addition-to-alkynes-halogenation

Education

JoVE Core

Organic Chemistry

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

alkyne halogenation and anti addition

Alkyne halogenation is an electrophilic addition reaction where a halogen molecule adds across a triple bond. Because alkynes have two π bonds, they can add two equivalents of bromine or chlorine. The first addition usually gives a trans-dihaloalkene as the major product and the cis isomer as the minor product. A second equivalent of halogen then forms the tetrahalide.

The reaction begins when a π bond in the alkyne acts as a nucleophile and attacks the electrophilic end of the polarized halogen molecule. This step pushes out a halide ion and forms a cyclic halonium ion intermediate. The halide ion then attacks from the backside and opens the ring. That backside attack makes the overall addition anti, so the two halogen atoms end up trans to each other.

The second halogen addition follows the same bridged halonium ion pathway. It adds across the alkene π bond in the intermediate and gives the final tetrahalide product. In the example of bromine addition to 2-butyne with acetic acid and lithium bromide, the trans, or E, product 2,3-dibromo-2-butene forms in the highest amount. The cis, or Z, isomer forms in lower yield, and a second addition produces 2,2,3,3-tetrabromobutane.

Alkynes are less reactive than alkenes in electrophilic addition reactions. One reason is that alkyne carbons are sp hybridized, while alkene carbons are sp2 hybridized. The sp orbitals have more s-character, so the π electrons in a C≡C bond are held more tightly. That makes them less ready to react with an electrophile.

Another reason is the structure of the halonium ion intermediate. The ring formed from an alkyne is a three-membered, bridged structure with a double bond, and it is highly strained. The alkyne intermediate is more strained than the similar intermediate from an alkene, where the carbon atoms are sp3 hybridized. This extra ring strain makes the alkyne halonium ion less stable and harder to form.

Alkyne Halogenation and Anti Addition

structure and physical properties of alkynes

structure-and-physical-properties-of-alkynes

Alkyne Bonds, Shape, and Physical Traits

Alkynes are hydrocarbons with carbon-carbon triple bonds. They are unsaturated compounds and are written with the general formula CnH2n-2. This group includes simple molecules like ethyne, also called acetylene, which is a colorless gas used as a fuel for welding because it burns at high temperatures. Alkynes also appear in natural and synthetic compounds. Some natural alkynes are found in the poison from South American tree frogs. Synthetic alkyne-containing compounds are found in medicines...

nomenclature of alkynes

Naming Alkynes with IUPAC Rules

Alkynes are unsaturated hydrocarbons with carbon-carbon triple bonds. Their general formula is CnH2n-2, and their names use the suffix "-yne" instead of "-ane" or "-ene." Alkynes can be named with IUPAC names or common names. The IUPAC system uses a step-by-step method based on the molecule’s structure. First, find the longest carbon chain that contains the triple bond. Add the suffix "-yne" to the parent chain name. In the example shown, the longest chain has seven carbon atoms, so the parent...

acidity of 1-alkynes

Why Terminal Alkynes Form Acetylide Ions

Terminal alkynes are more acidic than alkenes and alkanes. Acid strength is usually measured with pKa, and a lower pKa means a stronger acid. Ethane has a pKa of 51, ethene has a pKa of 44, and acetylene has a pKa of 25. That means acetylene is far more acidic than the other two hydrocarbons. This difference comes from the stability of the carbanion formed after deprotonation. A carbanion is a negatively charged carbon species. In ethane, ethene, and acetylene, the lone pair sits in an sp3,...

preparation of alkynes: alkylation reaction

preparation-of-alkynes-alkylation-reaction

Alkyne Chain Extension by Alkylation

Terminal alkynes can be made into longer carbon-chain alkynes by alkylation with primary alkyl halides. This reaction uses a strong base such as sodium amide. One example is 1-propyne, which reacts with sodium amide and then ethyl bromide to give 2-pentyne. The reaction happens in two steps. First, the strong base removes the acidic hydrogen from the terminal alkyne and forms an acetylide ion. An acetylide ion is the carbon-based species that then takes part in the next step. Next, the...

preparation of alkynes: dehydrohalogenation

preparation-of-alkynes-dehydrohalogenation

Making Alkynes from Dihalides

Alkynes can be made from vicinal or geminal dihalides by dehydrohalogenation. This reaction uses a strong base, such as sodium amide, in liquid ammonia. Two equivalents of hydrogen halide are lost through two successive E2 elimination reactions. In a vicinal dihalide, the two halogen atoms are on neighboring carbon atoms. The first E2 step removes a proton that is anti to the leaving group. Because E2 reactions are concerted, the proton is removed and the halide leaves at the same time. This...

Alkyne halogenation is an electrophilic addition reaction where a halogen molecule adds across a triple bond. Because alkynes have two π bonds, they can add two equivalents of bromine or chlorine. The first addition usually gives a trans-dihaloalkene as the major product and the cis isomer as the minor product. A second equivalent of halogen then forms the tetrahalide. The reaction begins when a π bond in the alkyne acts as a nucleophile and attacks the electrophilic end of the polarized...

electrophilic addition to alkynes: hydrohalogenation

electrophilic-addition-to-alkynes-hydrohalogenation

Alkyne Reactions with HX and HBr Peroxides

Hydrohalogenation of alkynes adds hydrogen halides, HX, across a carbon-carbon triple bond. In this reaction, X can be chlorine, bromine, or iodine. The first addition gives a vinyl halide, and the second addition gives a geminal dihalide. When HCl reacts with an alkyne, one pathway begins with proton transfer to the triple bond. The pi electrons attack the hydrogen in hydrogen chloride and push out chloride ion. This forms a secondary vinylic carbocation, and chloride then attacks to make a...

alkynes to aldehydes and ketones: acid-catalyzed hydration

alkynes-to-aldehydes-and-ketones-acid-catalyzed-hydration

Keto-Enol Tautomerism in Alkyne Hydration

Alkyne hydration uses acid, water, and a mercuric salt to convert alkynes into aldehydes and ketones. It is similar to alkene hydration, but the product type is different. A mercuric salt such as mercuric sulfate, HgSO4, is usually added because the reaction is much slower than alkene hydration. Terminal alkynes follow Markovnikov’s rule during hydration. In contrast, internal alkynes do not add water in a regioselective way. That means the product outcome depends on the alkynes structure. <br...

alkynes to aldehydes and ketones: hydroboration-oxidation

alkynes-to-aldehydes-and-ketones-hydroboration-oxidation

Alkyne Hydration via Borane and Peroxide

Alkyne hydration can make aldehydes and ketones through hydroboration-oxidation. In this indirect hydration reaction, an alkyne first reacts with borane, then with alkaline peroxide. The alkyne forms an enol, and that enol quickly changes into a carbonyl product. Terminal alkynes give aldehydes, while internal alkynes give ketones. The reaction happens in two steps. The hydroboration step is a concerted syn addition, which means the boron and hydrogen add to the same side of the carbon-carbon...

alkynes to carboxylic acids: oxidative cleavage

alkynes-to-carboxylic-acids-oxidative-cleavage

Triple-Bond Cleavage in Alkynes

Alkynes can be broken apart by oxidative cleavage. This reaction uses strong oxidizing reagents such as potassium permanganate and ozone. It cuts the triple bond, which has one sigma bond and two pi bonds, and turns the alkyne into carboxylic acids. With warm, basic aqueous potassium permanganate, the alkyne first forms an unstable alpha-diketone intermediate. The reaction then gives carboxylate salts. A mild acid treatment protonates these carboxylate anions and produces the free carboxylic...

reduction of alkynes to cis-alkenes: catalytic hydrogenation

reduction-of-alkynes-to-cis-alkenes-catalytic-hydrogenation

Stopping Alkyne Hydrogenation at cis-Alkenes

Alkyne hydrogenation can stop at a cis-alkene instead of going all the way to an alkane. Transition metal catalysts such as Pt, Pd, or Ni add hydrogen in two syn steps, which means both hydrogens add to the same side. The cis-alkene is the key intermediate in this process. Hydrogenation also helps compare the thermodynamic stability of hydrocarbons. Acetylene has a heat of hydrogenation of −176 kJ/mol, while ethylene has a value of −137 kJ/mol. The larger heat released for acetylene shows that...

reduction of alkynes to trans-alkenes: sodium in liquid ammonia

reduction-of-alkynes-to-trans-alkenes-sodium-in-liquid-ammonia

Dissolving Metal Reduction of Alkynes

Dissolving metal reduction turns alkynes into trans-alkenes with sodium or lithium in liquid ammonia. This reaction gives the alkene with anti addition across the carbon-carbon triple bond, which leads to the trans product. The reaction must be kept very cold because ammonia is a gas at room temperature. It is run with a dry ice and acetone bath, which holds the mixture at low temperature. When sodium dissolves in liquid ammonia, it forms sodium ions and free electrons. Ammonia molecules...

key terms and concepts

Explore how bromine or chlorine adds to alkynes, why anti addition forms trans products, and why a second equivalent gives tetrahalides.

Alkyne Halogenation and Anti Addition | 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.

Alkyne Halogenation and Anti Addition

Description

Transcript

Tags