Name: stopping alkyne hydrogenation at cis-alkenes
Uploaded: 2023-04-30T18:28:38.000Z
Duration: 2 min 24 s
Channel: JoVE

Catalytic hydrogenation of alkenes involves the syn addition of one mole of hydrogen across the &#960; bond in the presence of transition metal catalysts like platinum, palladium, or nickel to form alkanes.
Similarly, alkynes also undergo catalytic reduction to alkanes. However, since there are two &#960; bonds, two moles of hydrogen are required to saturate the triple bond.
The addition of the first equivalent forms an alkene, which upon further reduction gives an alkane. Both steps follow syn stereochemistry, with the hydrogen adding to the same side of the &#960; bond.
It is possible to stop the reaction at the cis-alkene stage using a poisoned transition metal catalyst synthesized by adding certain chemical compounds that deactivate the metal.
One such example is Lindlar&#39;s catalyst, a mixture of palladium deposited on calcium carbonate and deactivated by treatment with lead salts and quinoline. Another is a nickel&#8211;boron complex, known as the P-2 catalyst.
The modified catalyst lowers the activation energy and effectively reduces the first &#960; bond of the alkyne. However, it is not reactive enough to reduce the second &#960; bond. Thus, partial hydrogenation of alkynes using a Lindlar or P-2 catalyst forms cis-alkenes.
The mechanism begins with the adsorption of molecular hydrogen onto the surface of the metal catalyst, thereby breaking the H&#8211;H bond and forming new metal&#8211;H bonds.
Next, the alkyne binds to the catalytic surface, forming a &#960; complex with the metal. A sequential transfer of two hydrogens from the metal surface to the same face of the alkyne leads to the formation of the cis-alkene product. 
Hydroboration-protonolysis is an alternative non-catalytic approach for converting internal alkynes to cis-alkenes.
The reaction proceeds via a syn-stereoselective addition of borane to an internal alkyne to form a trialkenylborane. Subsequent treatment of the trialkenyl intermediate with acetic acid replaces the boron with hydrogen to give a cis-alkene.

reduction of alkynes to cis-alkenes: catalytic hydrogenation

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

<h4>Introduction</h4>
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
<img alt="Hydrogenation process diagram; 2-butyne to butane conversion using H2, Pd catalyst." src="/files/ftp_upload/11841/PT_Figure_1.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
<h4>Thermodynamic Stability</h4>
Catalytic hydrogenation reactions help evaluate the relative thermodynamic stability of hydrocarbons. For example, the heat of hydrogenation of acetylene is &#8722;176 kJ/mol, and that of ethylene is &#8722;137 kJ/mol. The higher exothermicity associated with the addition of hydrogen to acetylene suggests that it is thermodynamically less stable than ethylene.
<img alt="Alkyne hydrogenation equation with catalyst, displaying ethylene formation and enthalpy change diagram." src="/files/ftp_upload/11841/PT_Figure_2.svg" style="text-align: center; height: 45px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
<img alt="Hydrogenation reaction equation, alkene to alkane, exothermic enthalpy change, &#916;H=-137 kJ/mole." src="/files/ftp_upload/11841/PT_Figure_3.svg" style="text-align: center; height: 45px;" data-altupdatedat="1761830439000" data-alt="Figure3" />
<h4>Modified Catalyst</h4>
Catalytic reduction of alkynes can be stopped at the cis-alkene stage using a modified or poisoned catalyst such as Lindlar or P-2 catalyst. The Lindlar catalyst consists of palladium metal deposited on calcium carbonate and modified using lead acetate and quinoline; the P-2 catalyst is a nickel-boride complex.
<img alt="Catalytic hydrogenation: Lindlar vs P-2 catalyst, chemical equations and structural formulas diagram." src="/files/ftp_upload/11841/PT_Figure_4.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure4" />
A modified catalyst lowers the activation energy for the reduction of the first &#960; bond; however, it is not powerful enough to catalyze the reduction of the second &#960; bond. For example, hydrogenation of 2-pentyne over Lindlar catalyst gives cis-2-pentene.
<img alt="Hydrogenation of 2-pentyne to cis-2-pentene, chemical reaction with Lindlar catalyst diagram." src="/files/ftp_upload/11841/PT_Figure_5.svg" style="text-align: center; height: 95px;" data-altupdatedat="1761830439000" data-alt="Figure5" />
<h4>Hydroboration-Protonolysis</h4>
Hydroboration-protonolysis is a non-catalytic method for the conversion of internal alkynes into cis-alkenes. The reaction involves treating an internal alkyne with borane to form a trialkenylborane intermediate followed by treatment with acetic acid to yield the desired cis-alkene.
<img alt="4-Octyne hydroboration oxidation reaction diagram showing conversion to cis-4-octene via trialkenylborane." src="/files/ftp_upload/11841/PT_Figure_6.svg" style="text-align: center; height: 150px;" data-altupdatedat="1761830439000" data-alt="Figure6" />

Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves ...

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

Education

JoVE Core

Organic Chemistry

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

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 it is less stable than ethylene.

A poisoned, or modified, catalyst can stop the reaction at the cis-alkene stage. Lindlar catalyst contains palladium on calcium carbonate and is modified with lead acetate and quinoline. P-2 catalyst is a nickel-boride complex. These catalysts lower the activation energy for reduction of the first pi bond, but they are not strong enough to reduce the second pi bond.

For example, hydrogenation of 2-pentyne with Lindlar catalyst gives cis-2-pentene. Another way to make cis-alkenes from internal alkynes is hydroboration-protonolysis. In this non-catalytic reaction, borane forms a trialkenylborane intermediate, and acetic acid then gives the desired cis-alkene.

Stopping Alkyne Hydrogenation at cis-Alkenes

stopping-alkyne-hydrogenation-at-cis-alkenes

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...

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 can be stopped at cis-alkenes using Lindlar or P-2 catalyst, and how hydroboration-protonolysis also makes cis-alkenes.

Stopping Alkyne Hydrogenation at cis-Alkenes | JoVE K12

organic chemistry

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Stopping Alkyne Hydrogenation at cis-Alkenes

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