Name: making alkynes from dihalides
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 2 min 34 s
Channel: JoVE

In addition to the alkylation pathway, alkynes can also be prepared by dehydrohalogenation of vicinal or geminal dihalides.
Vicinal dihalides are compounds with halogens on adjacent carbons, whereas compounds with halogens on the same carbon are called geminal dihalides.
For example, a vicinal dichloride can be synthesized from an alkene by the addition of chlorine in the presence of an inert solvent like dichloromethane. While a geminal dichloride can be prepared by treating a ketone with phosphorous pentachloride.
In the presence of a strong base like sodium amide in liquid ammonia, the dihalides lose two equivalents of hydrogen halide through two successive E2 elimination reactions. Hence the name double dehydrohalogenation.
The first elimination reaction proceeds with the abstraction of a proton by sodium amide and simultaneous departure of the halide leaving group to form a haloalkene.
In the second elimination reaction, another equivalent of the base reacts with the haloalkene to yield the desired alkyne. Thus, at least two equivalents of sodium amide are required for the reaction to go to completion.
Similarly, treatment of geminal dihalides with sodium amide gives alkynes through two consecutive E2 elimination reactions.
However, if the product is a terminal alkyne, the acidic hydrogen is deprotonated by the strong base to form an acetylide ion. Thus, a third equivalent of the base is required to complete the dehydrohalogenation of the remaining haloalkene. Protonation of the acetylide ion with water or a weak acid drives the reaction to completion.
If the first elimination step gives a haloalkene with hydrogen on adjacent carbons, subsequent elimination can yield an allene as a side product in addition to the alkyne. However, the presence of adjacent double bonds in an allene makes them more unstable, thereby favoring the formation of alkynes.
Lastly, the reaction can be terminated at the first elimination step using weaker bases like sodium hydroxide to give an alkene as the final product.

preparation of alkynes: dehydrohalogenation

Preparation of Alkynes: Dehydrohalogenation

<h4>Introduction</h4>
Alkynes can be prepared by dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base like sodium amide in liquid ammonia. The reaction proceeds with the loss of two equivalents of hydrogen halide (HX) via two successive E2 elimination reactions.
<img alt="Alkyne synthesis via elimination reaction; chemical equation; organic chemistry diagram; R-C&#8801;C-R'." src="/files/ftp_upload/11835/11835_PT_Figure_1.svg" style="text-align: center; height: 200px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction diagram, alkyne synthesis, 2NaNH2/NH3, hydrogen elimination, organic chemistry." />
<h4>Reaction Mechanism &#8211; E2 pathway</h4>
Vicinal dihalides
In the first elimination step, the strong base abstracts the proton from the dihalide that is oriented anti to the leaving group. Since E2 reactions follow a concerted pathway, the abstraction of a proton and departure of the halide leaving group occur simultaneously to form a haloalkene.
<img alt="Birch reduction equation showing reaction mechanism with Na, NH3, alkyne to trans-alkene conversion." src="/files/ftp_upload/11835/11835_PT_Figure_2.svg" style="text-align: center; height: 90px;" data-altupdatedat="1754904545000" data-alt="Birch reduction mechanism diagram, showing electron flow in organic compound reaction." />
In the second elimination reaction, another equivalent of the strong base reacts with the haloalkene in a similar fashion to give the desired alkyne.
<img alt="Alkyne formation reaction mechanism diagram; sodium amide acts on alkene to form alkyne product." src="/files/ftp_upload/11835/11835_PT_Figure_3.svg" style="text-align: center; height: 90px;" data-altupdatedat="1754904545000" data-alt="Elimination reaction mechanism diagram, E2 mechanism, showing alkene formation from alkyl halide." />
Geminal dihalides
Likewise, geminal dihalides, when treated with two equivalents of a sodium amide, undergo double dehydrohalogenation to give alkynes.
<img alt="Hofmann elimination reaction diagram, showing chemical equation and mechanism steps." src="/files/ftp_upload/11835/11835_PT_Figure_4.svg" style="text-align: center; height: 90px;" data-altupdatedat="1754904545000" data-alt="Alkyne formation reaction; sodium amide abstracting proton; chemical equation diagram; synthesis method." />
<img alt="Alkyne synthesis equation diagram with sodium amide, depicting chemical reaction process." src="/files/ftp_upload/11835/11835_PT_Figure_5.svg" style="text-align: center; height: 90px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction mechanism, arrow-pushing diagram, alkyne formation using sodium amide." />
Terminal dihalides 
Dehydrohalogenation of terminal dihalides yields terminal alkynes as the final product. In the presence of a strong base like sodium amide, terminal alkynes get converted to acetylide ions. In such cases, a third equivalent of the base is required to complete the dehydrohalogenation of the remaining haloalkene.
<img alt="Alkyne synthesis via elimination, chemical reaction equation diagram, using sodium amide and alkenes." src="/files/ftp_upload/11835/11835_PT_Figure_6.svg" style="text-align: center; height: 70px;" data-altupdatedat="1754904545000" data-alt="Hofmann elimination reaction; chemical equations diagram; synthesis process using NaNH2; alkyne formation." />
Protonation of the acetylide ions with water or an aqueous acid completes the reaction.
<img alt="Chemical reaction diagram showing alkynide ion deprotonation, featuring alkyne synthesis." src="/files/ftp_upload/11835/11835_PT_Figure_7.svg" style="text-align: center; height: 50px;" data-altupdatedat="1754904545000" data-alt="Organic reaction mechanism with sodium acetylide, water; diagram illustrating nucleophilic attack." />
<h4>Application in Organic Synthesis</h4>
Dehydrohalogenation of vicinal dihalides is a useful intermediate step in the conversion of alkenes to alkynes. For example, chlorination of 1-propene gives 1,2-dichloropropane &#8211; a vicinal dihalide, which upon double dehydrohalogenation yields 1-propyne.
<img alt="Alkene conversion to alkyne; reaction diagram, chemical formulas, synthesis method, dichlorination." src="/files/ftp_upload/11835/11835_PT_Figure_8.svg" style="text-align: center; height: 60px;" data-altupdatedat="1754904545000" data-alt="Alkene to alkyne conversion diagram; chemical reaction mechanism, dichlorination, dehydrohalogenation." />
Similarly, alkynes can also be synthesized from ketones via dehydrohalogenation of geminal dihalides. For example, treatment of acetone with phosphorous pentachloride yields 2,2-dichloropropane &#8211; a geminal dihalide, which undergoes double dehydrohalogenation to give 1-propyne.
<img alt="Chemical reaction mechanism, acyl chloride to alkyne, diagram, involving PCl&#8325;, NaNH&#8322; in NH&#8323;." src="/files/ftp_upload/11835/11835_PT_Figure_9.svg" style="text-align: center; height: 80px;" data-altupdatedat="1754904545000" data-alt="Organic synthesis mechanism; ketone to alkyne via substitution and elimination; chemical reaction diagram." />

Introduction
Alkynes can be prepared by dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base like sodium amide in liquid ...

preparation-of-alkynes-dehydrohalogenation

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JoVE Core

Organic Chemistry

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

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 forms a haloalkene, which then reacts again with another equivalent of strong base to give the alkyne.

Geminal dihalides have both halogen atoms on the same carbon. When treated with two equivalents of sodium amide, they also undergo double dehydrohalogenation to form alkynes. Terminal dihalides give terminal alkynes as the final product. In the presence of a strong base, terminal alkynes can become acetylide ions, so a third equivalent of base may be needed to finish the reaction. Water or an aqueous acid then protonates the acetylide ion.

This reaction is useful in organic synthesis because it helps convert alkenes into alkynes. For example, chlorination of 1-propene gives 1,2-dichloropropane, a vicinal dihalide, and double dehydrohalogenation of this compound gives 1-propyne. Alkynes can also be made from ketones through geminal dihalides. Acetone can react with phosphorus pentachloride to form 2,2-dichloropropane, which then undergoes double dehydrohalogenation to give 1-propyne.

Making Alkynes from Dihalides

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...

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...

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

Learn how vicinal and geminal dihalides form alkynes by double dehydrohalogenation with sodium amide, plus examples from 1-propene and acetone.

Making Alkynes from Dihalides | JoVE K12

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Making Alkynes from Dihalides

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