Name: alkyne hydration via borane and peroxide
Uploaded: 2023-04-30T18:28:38.000Z
Duration: 2 min 47 s
Channel: JoVE

The addition of water to alkynes can proceed via two complementary pathways, namely acid-catalyzed hydration, and hydroboration-oxidation.
Recall that acid-catalyzed hydration of terminal alkynes yields methyl ketones following Markovnikov&#39;s regioselectivity.
In contrast, the hydroboration-oxidation of terminal alkynes gives aldehydes with anti-Markovnikov&#39;s regioselectivity.
For example, the addition of borane to 1-propyne followed by oxidation with hydrogen peroxide in the presence of sodium hydroxide yields an enol that rearranges into a stable carbonyl compound to form propanal as the final product.
The hydroboration mechanism begins with a concerted syn addition of borane across the carbon&#8211;carbon triple bond forming an alkenyl borane. In terminal alkynes, the addition is regioselective with boron adding to the less substituted carbon of the triple bond.
Three successive hydroboration steps eventually convert borane into a trialkenylborane.
Next, the trialkenylborane undergoes oxidation in the presence of alkaline hydrogen peroxide to form an enol.
The final step involves the tautomerization of the enol into a stable aldehyde.
Although the mechanism is similar to that of alkenes, there is a vital difference in the hydroboration step.
Unlike alkenes, alkynes have two &#960; bonds that are equally suited to react with BH3. Therefore, the alkenylborane formed from the first addition of BH3 can undergo a second hydroboration reaction.
Terminal alkynes are less hindered compared to internal alkynes, and therefore, more susceptible to a second addition of BH3.
However, this can be prevented by using a bulky disubstituted borane such as di-sec-isoamylborane, where the branched alkyl substituents replace the two hydrogen atoms of BH3.
Under these conditions, the first hydroboration step forms a sterically hindered alkenyl borane that resists further additions, thereby facilitating the conversion of terminal alkynes to aldehydes.
Analogous to acid-catalyzed hydration, hydroboration-oxidation of internal alkynes gives ketones as the final product. Symmetrical internal alkynes give a single ketone product, whereas unsymmetrical alkynes yield a mixture of ketones.

alkynes to aldehydes and ketones: hydroboration-oxidation

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

<h4>Introduction</h4>
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
<img alt="Organic synthesis equation; hydroboration-oxidation of alkynes; chemical reaction diagram." src="/files/ftp_upload/11839/11839_PT_Figure_1.svg" style="text-align: center; height: 200px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
<h4>Mechanism</h4>
The hydroboration-oxidation reaction is a two-step process. It begins with the hydroboration step, which involves a concerted syn addition of BH3 across the carbon&#8211;carbon triple bond to form an alkenylborane. The concerted nature of the reaction also accounts for the anti-Markovnikov regiochemistry, where the BH2 group adds to the less substituted carbon and H to the more substituted carbon of the triple bond.
<img alt="Alkenylborane formation diagram showing chemical reaction and electron flow mechanism." src="/files/ftp_upload/11839/11839_PT_Figure_2.svg" style="text-align: center; height: 120px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
 Three successive hydroboration reactions convert an alkene into a trialkenylborane intermediate. The second part of the sequence is oxidation, where the trialkenylborane is treated with alkaline hydrogen peroxide to form an enol. The enol eventually converts into a stable carbonyl product via keto-enol tautomerism.
<img alt="Alkyne hydroboration oxidation chemical reaction diagram with trialkenylborane intermediate, aldehyde product." src="/files/ftp_upload/11839/11839_PT_Figure_3.svg" style="text-align: center; height: 250px;" data-altupdatedat="1761830439000" data-alt="Figure3" />
<h4>Hydroboration of Alkynes with Disubstituted Boranes</h4>
Unlike alkenes, hydroboration of alkynes does not stop at the first addition of BH3. This is because alkynes have two &#960; bonds, each capable of reacting with BH3. The first addition forms an organoborane, which is an alkene derivative that can react further with another equivalent of BH3.
Terminal alkynes being less hindered than internal alkynes are more susceptible to a second BH3 addition. With internal alkynes, the addition of BH3 stops after the first stage and proceeds in a direction to give the trialkenylborane.
<img alt="Hydroboration reaction of alkynes with BH3; chemical formulas; diagram; organic chemistry process." src="/files/ftp_upload/11839/11839_PT_Figure_4.svg" style="text-align: center; height: 250px;" data-altupdatedat="1761830439000" data-alt="Figure4" />
 Nevertheless, hydroboration of terminal alkynes can be stopped at the first step by using bulky disubstituted boranes (R2BH) such as disiamylborane and 9-BBN instead of BH3.
<img alt="Chemical structure diagrams of disiamylborane and 9-BBN compounds, illustrating boron hydride." src="/files/ftp_upload/11839/11839_PT_Figure_5.svg" style="text-align: center; height: 130px;" data-altupdatedat="1761830439000" data-alt="Figure5" />
The first addition of the bulky reagent forms a sterically hindered alkenylborane that resists any further additions and helps in the efficient conversion of alkynes to stable carbonyl compounds.
<img alt="Hydroboration process; alkyne to alkenylborane conversion; chemical reaction diagram." src="/files/ftp_upload/11839/11839_PT_Figure_6.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure6" />

Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is ...

alkynes-to-aldehydes-and-ketones-hydroboration-oxidation

Education

JoVE Core

Organic Chemistry

Unit 1

Alkynes

KEY TERMS AND CONCEPTS

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 triple bond at the same time. This also gives anti-Markovnikov regiochemistry, so the BH2 group adds to the less substituted carbon and hydrogen adds to the more substituted carbon. Three hydroboration additions can turn the alkyne into a trialkenylborane intermediate.

The second step is oxidation. The trialkenylborane reacts with alkaline hydrogen peroxide to form an enol. That enol then shifts to a stable carbonyl compound through keto-enol tautomerism.

Hydroboration of alkynes does not always stop after one addition of BH3. Alkynes have two pi bonds, and each one can react with BH3. The first addition forms an organoborane, which can react again with another equivalent of BH3. Terminal alkynes are less hindered than internal alkynes, so they are more likely to undergo a second BH3 addition.

For terminal alkynes, bulky disubstituted boranes can stop the reaction after the first step. Reagents such as disiamylborane and 9-BBN are examples of R2BH compounds used for this purpose. Their size creates a sterically hindered alkenylborane that resists further addition and helps convert alkynes efficiently into stable carbonyl compounds.

Alkyne Hydration via Borane and Peroxide

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...

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 form aldehydes or ketones through hydroboration-oxidation, including syn addition, anti-Markovnikov regiochemistry, and keto-enol tautomerism.

Alkyne Hydration via Borane and Peroxide | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Alkyne Hydration via Borane and Peroxide

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