Name: alkyne reactions with hx and hbr peroxides
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 2 min 35 s
Channel: JoVE

Hydrohalogenation is another class of electrophilic addition reactions. It proceeds with the addition of a hydrogen halide, such as HCl or HBr, across a &#960; bond.
The presence of two &#960; bonds allows for the addition of two equivalents of hydrogen halide. Here, the first addition of HCl produces vinyl chloride, and the second yields a geminal dichloride.
Both additions follow Markovnikov&#39;s regioselectivity, with the chlorine getting added to the more substituted carbon.
One possible mechanism begins with a proton transfer, where the &#960; electrons attack the hydrogen atom of HCl and displace the chloride ion to form a stable secondary vinylic cation.
Next, the chloride ion attacks the vinyl cation to form a vinyl chloride.
The addition of a second equivalent of HCl proceeds with the protonation of vinyl chloride, resulting in two possible carbocations.
Here, the secondary carbocation is favored over the primary, despite the positive charge being centered on the carbon attached to an electron-withdrawing halogen group. This is because the positive charge on the secondary carbocation is delocalized through resonance, thereby stabilizing the intermediate and supporting it&#39;s formation.
The final step involves the attack of the chloride ion to form a geminal dichloride.
Another competing mechanism is a termolecular process. It involves simultaneous interaction between three molecules, the alkyne and two equivalents of hydrogen halide. 
However, instead of a vinylic carbocation, the reaction proceeds via a low-energy transition state that bears a partial positive charge at the more substituted carbon. 
The trans addition of a hydrogen from one hydrogen halide and the addition of a halogen from the other hydrogen halide gives a haloalkene that reacts with the displaced hydrogen halide to form a geminal dihalide.
Lastly, like with alkenes, when a hydrohalogenation of terminal alkynes is carried out in the presence of peroxides and HBr, it proceeds in an anti-Markovnikov manner. Here, the bromine gets added to the less substituted carbon, producing a mixture of E and Z alkenes.

electrophilic addition to alkynes: hydrohalogenation

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.
<img alt="Alkyne to geminal dichloride via vinyl chloride; chemical reaction pathway diagram." src="/files/ftp_upload/11837/11837_PT_Figure%201.svg" style="text-align: center; height: 150px;" data-altupdatedat="1754904545000" data-alt="Alkyne halogenation mechanism, showing reaction with HX forming vinyl chloride and geminal dichloride." />
Addition of HCl to an Alkyne
Mechanism I &#8211; Vinylic carbocation Intermediate
The mechanism begins with a proton transfer from HCl to the alkyne. Here, the &#960; electrons attack the hydrogen atom of hydrogen chloride and displace the chloride ion. This gives a stable secondary vinylic carbocation, which is further attacked by the chloride ion to form a vinyl chloride.
<img alt="Chemical reaction mechanism diagram; free radical addition to alkyne, illustrating bond shifts." src="/files/ftp_upload/11837/11837_PT_Figure_2.svg" style="text-align: center; height: 55px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction mechanism, diagram, radical addition to alkyne, organic chemistry, electron movement." />
The addition of a second equivalent of hydrogen chloride proceeds with the protonation of vinyl chloride, giving two possible carbocations. In the secondary carbocation the positive charge is delocalized through resonance. As a result, it is more stable and favored over the primary carbocation.
<img alt="Radical polymerization process; chemical equation diagram; polymer formation from alkene and radical." src="/files/ftp_upload/11837/11837_PT_Figure_3.svg" style="text-align: center; height: 80px;" data-altupdatedat="1754904545000" data-alt="Polymerization reaction diagram, free-radical initiation, showing monomer to polymer conversion." />
<img alt="Chemical resonance structures, carbocation stability, diagram, organic chemistry, electron delocalization." src="/files/ftp_upload/11837/11837_PT_Figure_4.svg" style="text-align: center; height: 60px;" data-altupdatedat="1754904545000" data-alt="Organic reaction mechanism, radical and ionic forms, resonance structure, chemical diagram." />
A second nucleophilic attack by the chloride ion gives a geminal dichloride.
<img alt="Nucleophilic substitution mechanism diagram, displaying electron pair movement in organic reaction." src="/files/ftp_upload/11837/11837_PT_Figure_5.svg" style="text-align: center; height: 75px;" data-altupdatedat="1754904545000" data-alt="Nucleophilic substitution reaction, Lewis structures, SN1 mechanism, organic chemistry diagram." />
Mechanism II &#8211; Concerted Termolecular Process
In a vinylic carbocation, the positive charge resides on an electronegative sp-hybridized carbon making it unstable. The second mechanism avoids the formation of this carbocation. Instead, it proceeds via a termolecular (three molecules) process where the alkyne interacts simultaneously with two equivalents of a hydrogen halide like HCl. This leads to a transition state with a partially broken C&#8211;C &#960; bond and partially formed C&#8211;Cl and C&#8211;H &#963; bonds. The net result is a trans addition of hydrogen from one HCl and a chloride from the other HCl to form a chloroalkene. This further reacts with the displaced HCl to form a geminal dichloride as the final product.
<img alt="Alkyne hydrohalogenation mechanism diagram, showing reaction intermediates and molecular structures." src="/files/ftp_upload/11837/11837_PT_Figure_6v2.svg" style="text-align: center; height: 250px;" data-altupdatedat="1754904545000" data-alt="Alkyne addition mechanism; HCl reacts; structural diagram shows reaction steps; hydrohalogenation process." />
Halogenation of Alkynes with Peroxides
When treated specifically with HBr in the presence of peroxides, terminal alkynes undergo anti-Markovnikov's addition. The Br gets added to the less substituted carbon forming a mixture of E and Z alkenes.
<img alt="Alkyne hydrobromination mechanism diagram; E/Z alkene formation with ROOR catalyst." src="/files/ftp_upload/11837/11837_PT%20Figure%207.svg" style="text-align: center; height: 90px;" data-altupdatedat="1754904545000" data-alt="Hydrobromination of alkynes diagram with HBr, ROOR catalysts forming E and Z alkenes." />

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov&#39;s rule, where the hydrogen gets ...

electrophilic-addition-to-alkynes-hydrohalogenation

Education

JoVE Core

Organic Chemistry

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

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 vinyl chloride.

A second equivalent of HCl can add after the first step. Protonation of the vinyl chloride can give two possible carbocations. The secondary carbocation is favored because the positive charge is spread out by resonance, while the primary carbocation is less stable. Chloride attack on the favored carbocation produces a geminal dichloride.

A second mechanism avoids making the unstable vinylic carbocation. In this termolecular process, the alkyne interacts with two equivalents of hydrogen halide at the same time. The transition state has a partly broken C-C pi bond and partly formed C-H and C-Cl sigma bonds. The net result is trans addition to form a chloroalkene, which can react further with HCl to give the final geminal dichloride.

With HBr and peroxides, terminal alkynes follow anti-Markovnikov addition. In this case, bromine adds to the less substituted carbon. The products are a mixture of E and Z alkenes.

Alkyne Reactions with HX and HBr Peroxides

alkyne-reactions-with-hx-and-hbr-peroxides

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...

electrophilic addition to alkynes: halogenation

electrophilic-addition-to-alkynes-halogenation

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...

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

Learn how alkynes react with HX to form vinyl halides and geminal dihalides, plus anti-Markovnikov HBr addition with peroxides in terminal alkynes.

Alkyne Reactions with HX and HBr Peroxides | JoVE K12

organic chemistry

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Alkyne Reactions with HX and HBr Peroxides

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