Name: keto-enol tautomerism in alkyne hydration
Uploaded: 2023-04-30T18:28:38.000Z
Duration: 2 min 40 s
Channel: JoVE

Acid-catalyzed hydration of alkenes involves the addition of water across the &#960; bond in the presence of a strong acid, like sulfuric acid, to form alcohols.
Similarly, alkynes also undergo acid-catalyzed hydration; however, they form aldehydes and ketones instead.
For example, acetylene gives acetaldehyde, whereas other terminal alkynes form methyl ketones.
Unsymmetrical internal alkynes undergo an acid-catalyzed hydration to give a mixture of ketones; in contrast, symmetrical internal alkynes yield a single ketone.
The addition of water to terminal alkynes follows Markovnikov&#39;s rule. However, for internal alkynes the addition is non-regioselective.
Since alkynes are less reactive than alkenes towards the addition of water, a mercuric salt, like mercuric sulfate, is used as a catalyst to facilitate the reaction.
Analogous to the oxymercuration of alkenes, the mechanism of acid-catalyzed hydration of alkynes begins with the nucleophilic attack by one of the alkyne &#960; bonds on the mercuric ion to form a bridged-mercurinium ion intermediate.
Next, water, being a nucleophile, attacks the more substituted carbon from the opposite side of the bridge and opens the ring, accounting for the observed Markovnikov&#39;s regioselectivity.
Thereafter, a proton transfer to the solvent leads to the formation of an organomercuric enol, which rapidly converts into a more stable keto tautomer via the keto-enol tautomerism.
Tautomers are constitutional isomers that interconvert and differ in the locations of a hydrogen atom and a double bond. The equilibrium favors the more stable isomer, in this case, the keto form.
Protonation of the carbonyl group of the keto tautomer forms an oxonium ion, which loses a mercuric ion to give the enol form of the final product.
The last step involves tautomerization of the enol into the desired ketone.
Lastly, acid-catalyzed hydration is most useful for terminal and symmetrical internal alkynes since they produce one final product. Unsymmetrical alkynes form a mixture of products that need to be separated, thus lowering the overall yield.

alkynes to aldehydes and ketones: acid-catalyzed hydration

Alkynes to Aldehydes and Ketones: Acid-Catalyzed Hydration

<h4>Introduction</h4>
Analogous to alkenes, alkynes also undergo acid-catalyzed hydration. While the addition of water to an alkene gives an alcohol, hydration of alkynes produces different products such as aldehydes and ketones. 
<img alt="Alkyne hydration mechanism, H&#8322;SO&#8324;/HgSO&#8324; catalysis, reaction equations." src="/files/ftp_upload/11838/11838_PT_Figure%201.svg" style="text-align: center; height: 180px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
Since the rate of acid-catalyzed hydration of alkynes is much slower than alkenes, a mercuric salt like mercuric sulfate (HgSO4) is usually added to facilitate the reaction. Hydration of terminal alkynes follows Markovnikov's rule; however, for internal alkynes, the addition of water is non-regioselective.
<h4>Mechanism</h4>
The mechanism begins with a nucleophilic attack by the alkyne &#960; bond on the Hg2+ ion resulting in the formation of a cyclic mercurinium ion intermediate. A second nucleophilic attack by water on the more substituted carbon forms an organomercuric enol that rapidly converts into a stable keto form via keto-enol tautomerism. Protonation of the keto intermediate followed by the loss of an Hg2+ ion yields the enol form of the product. The final step proceeds with the tautomerization of the enol to the desired ketone.
<img alt="Mercury-catalyzed hydration mechanism diagram for converting alkyne to ketone via enol intermediate." src="/files/ftp_upload/11838/11838_PT_Figure%202.svg" style="text-align: center; height: 280px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
<h4>Keto-Enol Tautomerism</h4>
Unlike alkenes, acid-catalyzed hydration of alkynes is irreversible. This is because the enol intermediate formed during the hydration of alkynes is unstable and rapidly isomerizes to a more stable keto form. The chemical equilibrium that exists between the two forms is referred to as keto-enol tautomerism. Since the C=O bond is considerably stronger than the C=C bond, the equilibrium favors the keto isomer. Keto-enol tautomerism is characterized by the migration of a proton and the change in the location of a double bond.
Acid-catalyzed tautomerization is a two-step process:
Step 1: Addition of proton across the enol double bond
<img alt="Enol to keto tautomerization mechanism; chemical reaction diagram with molecular structures and arrows." src="/files/ftp_upload/11838/11838_PT_Figure%203.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure3" />
Step 2: Loss of a proton to yield the keto form
<img alt="Tautomerization process, chemical equation diagram, enol to keto conversion mechanism." src="/files/ftp_upload/11838/11838_PT_Figure%204.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure4" />
<h4>Example</h4>
Acid-catalyzed hydration of 1-propyne initially forms the less stable enol isomer, propen-2-ol, which tautomerizes into a more stable keto product, propan-2-one.
<img alt="Chemical reaction diagram of hydration of propyne to propan-2-one using sulfuric acid catalysts." src="/files/ftp_upload/11838/11838_PT_Figure%205.svg" style="text-align: center; height: 70px;" data-altupdatedat="1761830439000" data-alt="Figure5" />
<h4>Hydration of Terminal And Internal Alkynes</h4>
Acid-catalyzed hydration is most useful for terminal and symmetrical internal alkynes because they form only one final product. In contrast, unsymmetrical internal alkynes yield a mixture of products that need to be separated. This lowers the overall yield and makes the process less efficient.

Introduction
Analogous to alkenes, alkynes also undergo acid-catalyzed hydration. While the addition of water to an alkene gives an alcohol, hydration of ...

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

Education

JoVE Core

Organic Chemistry

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

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.

The reaction starts when the alkyne pi bond attacks Hg2+. This forms a cyclic mercurinium ion intermediate. Water then attacks the more substituted carbon to give an organomercuric enol, which quickly changes into a more stable keto form through keto-enol tautomerism.

Keto-enol tautomerism is the equilibrium between an enol and a keto form. The enol is unstable and shifts to the keto product because the C=O bond is stronger than the C=C bond. In this acid-catalyzed process, a proton adds across the enol double bond, then another proton is lost to form the keto form.

The hydration of alkynes is irreversible for this reason. For example, hydration of 1-propyne first gives the enol propen-2-ol, which tautomerizes to propan-2-one. This example shows how the final ketone can differ from the first product formed.

This reaction is most useful for terminal alkynes and symmetrical internal alkynes because they give one final product. Unsymmetrical internal alkynes can form a mixture of products, so the products must be separated. That lowers the overall yield and makes the process less efficient.

Keto-Enol Tautomerism in Alkyne Hydration

keto-enol-tautomerism-in-alkyne-hydration

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...

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 acid-catalyzed hydration of alkynes forms ketones and aldehydes, why HgSO4 is used, and how keto-enol tautomerism drives the final product.

Keto-Enol Tautomerism in Alkyne Hydration | JoVE K12

organic chemistry

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Keto-Enol Tautomerism in Alkyne Hydration

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