Name: dissolving metal reduction of alkynes
Uploaded: 2023-04-30T18:28:38.000Z
Duration: 2 min 10 s
Channel: JoVE

Alkynes can be reduced to trans alkenes using alkali metals, like sodium or lithium, in liquid ammonia at low temperatures via a reaction called dissolving-metal reduction.
When alkali metals dissolve in liquid ammonia, they lose their single valence electron to the solvent, producing solvated electrons and turning the solution blue at low to moderate concentrations.
Solvated electrons act as strong reducing agents and readily add to the alkyne triple bond.
The mechanism begins with the addition of a solvated electron to the alkyne, forming a vinylic radical anion.
Note that the movement of a single electron is depicted by a single-headed curved arrow, often called a fishhook arrow. In contrast, a double-headed curved arrow signifies the movement of two electrons.
The radical anion formed in the first step is an intermediate. Here, the negative charge associated with the lone pair makes it an anion, while the unpaired electron lends a radical character.
Further, the radical anion can adopt two different configurations where the paired and unpaired electrons are oriented cis or trans to each other. The trans configuration is favored since it minimizes electronic repulsions, which influences the stereochemistry of the final product.
Next, the vinylic radical anion, a strong base, abstracts a proton from ammonia, forming a stable trans vinylic radical.
The addition of another electron to the vinylic radical gives a trans vinylic anion, which upon protonation by ammonia generates the trans alkene.
The reaction consistently involves anti addition of two hydrogen atoms across the carbon&#8211;carbon triple bond and thus is stereospecific.
However, while the dissolving-metal reduction method generates a trans alkene, reduction in the presence of Lindlar&#39;s catalyst yields a cis alkene.

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

Reduction of Alkynes to trans-Alkenes: Sodium in Liquid Ammonia

Alkynes can be reduced to trans-alkenes using sodium or lithium in liquid ammonia. The reaction, known as dissolving metal reduction, proceeds with an anti addition of hydrogen across the carbon&#8211;carbon triple bond to form the trans product. Since ammonia exists as a gas (bp = &#8722;33&#176;C) at room temperature, the reaction is carried out at low temperatures using a mixture of dry ice (sublimes at &#8722;78&#176;C) and acetone. 
When dissolved in liquid ammonia, an alkali metal, such as sodium, dissociates into a cation and a free electron. Ammonia molecules surround the free electrons, creating solvated electrons that impart a blue color to the solution. Solvated electrons are strong reducing agents and readily add to the alkyne triple bond.
<img alt="Sodium dissolution in ammonia, reaction equation, creating solvated electron, chemical process." src="/files/ftp_upload/11842/11842_PT_Figure_1.svg" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
<img alt="Chemical reaction: 2-Butyne reduction to trans-2-Butene, equation diagram with sodium, ammonia." src="/files/ftp_upload/11842/11842_PT_Figure_2.svg" style="text-align: center; height: 70px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
<h4>Limitation:</h4>
The reduction of terminal alkynes with sodium in liquid ammonia does not proceed as efficiently as the reduction of internal alkynes. This is because terminal alkynes have acidic protons that readily react with the sodium&#8211;liquid ammonia mixture to form sodium acetylide. Stoichiometrically, three moles of a terminal alkyne undergo metal-dissolved reduction to give only one mole of the corresponding alkene and two moles of sodium acetylide.
<img alt="Alkyne to Alkene conversion, sodium in liquid ammonia, chemical reaction formula diagram." src="/files/ftp_upload/11842/11842_PT_Figure_3.svg" style="text-align: center; height: 37px;" data-altupdatedat="1761830439000" data-alt="Figure3" />
Therefore, the reaction conditions need to be modified to completely convert terminal alkynes to alkenes. A common approach involves adding ammonium sulfate to the reaction mixture. The ammonium ion released into the solution protonates the acetylide, thus preserving the terminal alkyne for subsequent reduction.

Alkynes can be reduced to trans-alkenes using sodium or lithium in liquid ammonia. The reaction, known as dissolving metal reduction, proceeds with an ...

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

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JoVE Core

Organic Chemistry

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

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 surround those electrons and create solvated electrons. These electrons make the solution blue and act as strong reducing agents.

Solvated electrons add to the alkyne triple bond and drive the reduction. The process works better for internal alkynes than for terminal alkynes. Terminal alkynes have acidic protons, so they can react with the sodium-liquid ammonia mixture and form sodium acetylide instead.

Because of that side reaction, three moles of a terminal alkyne give only one mole of the alkene and two moles of sodium acetylide. To fully convert terminal alkynes to alkenes, the reaction conditions are adjusted. One common fix is to add ammonium sulfate. The ammonium ion protonates the acetylide and helps preserve the terminal alkyne for the reduction step.

Dissolving Metal Reduction of Alkynes

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...

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...

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

See how alkynes are reduced to trans-alkenes with sodium in liquid ammonia, why the solution turns blue, and why terminal alkynes need extra care.

Dissolving Metal Reduction of Alkynes | JoVE K12

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Dissolving Metal Reduction of Alkynes

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