Name: alcohol synthesis from alkenes
Uploaded: 2023-04-30T18:28:39.000Z
Duration: 2 min 15 s
Channel: JoVE

Alcohols can be prepared through the reaction of alkenes with water using three synthetic routes: acid-catalyzed hydration, oxymercuration&#8211;demercuration, and hydroboration&#8211;oxidation.
In the acid-catalyzed hydration of alkenes, a proton is added to the less-substituted carbon in the double bond, forming a more stable carbocation. Subsequently, hydroxyl is added to the more substituted end of the double bond, which follows Markovnikov&#39;s rule.
Since a trigonal planar carbocation intermediate is involved in the reaction pathway, the addition of the water molecule on either face is equally probable, and a racemic mixture of alcohols is formed.
Alcohol synthesis via oxymercuration&#8211;demercuration of alkenes does not have a conventional carbocation intermediate. Instead, the nucleophilic attack of the double bond on the positively-charged mercury acetate creates a three-membered mercurinium ion intermediate, which can be considered a resonance hybrid with minor carbocation character.
The subsequent hydration of the mercurinium ion is consistent with Markovnikov&#39;s rule. Following proton transfer, sodium borohydride replaces the mercury species with a proton to give the final product &#8212; alcoho

preparation of alcohols via addition reactions

Preparation of Alcohols via Addition Reactions

<h4>Overview</h4>
The acid-catalyzed addition of water to the double bond of alkenes is a large-scale industrial method used to synthesize low-molecular-weight alcohols. An acidic atmosphere is required to allow the hydrogen in the water molecule to act as an electrophile and attack the double bond in an alkene. The addition of a proton to the double bond creates a carbocation intermediate. The proton preferentially bonds to the less substituted end of the double bond to create a more stable carbocation intermediate. Subsequently, water attacks the carbocation to yield a protonated alcohol.
Here, the reaction is regioselective and prefers Markovnikov's addition of water to the alkene. The proton is added to the less substituted end of the double bond, and the -OH group is added to the more substituted carbon on the other end. The reaction is not stereoselective since the water can approach the planar carbocation from both sides, which leads to a racemic mixture of alcohols. Water addition can happen from the same side (syn addition) or the opposite side (anti addition) with respect to the direction from which the proton is added to the double bond. In alkenes with a more substituted carbon adjacent to the double bond, the positive charge in the carbocation intermediate may rearrange (1,2 hydride shift or 1,2 methyl shift) to gain stability. In this situation, a mixture of constitutional isomers of alcohols is formed as products.
An alternative synthetic route for preparing alcohols from alkenes is via oxymercuration-demercuration reaction. In the oxymercuration reaction, the electrophilic mercury in mercury acetate is added to the double bond and forms a three-membered ring structure called mercurinium ion. Water then attacks the mercurinium ion to form organomercurial alcohol. The subsequent step is demercuration, where sodium borohydride replaces the mercury species with hydrogen.
The hydroxyl group bound to the more substituted carbon in the mercurinium ion guides the hydrogen towards the less substituted carbon, eventually resulting in a Markovnikov's product. Also, attack by water is anti with respect to mercury acetate addition. However, the reaction that replaces mercury species with hydrogen is not stereoselective, which results in the formation of both enantiomers of the alcohol as the final product. Due to the absence of a conventional carbocation and its rearrangement, oxymercuration-demercuration has better yields than direct acid-catalyzed hydration.
Anti-Markovnikov's addition of water to an alkene can be performed by the hydroboration-oxidation method. Hydroboration is the single-step addition of a B-H bond from a borane to the two carbons in the double bond via a four-membered transition state. Electrophilic boron attracts the &#960; orbital electrons from the least hindered carbon in the double bond. The resulting partial positive charge is stabilized on the other more substituted carbon atom. Steric factors also favor the addition of boron to the least hindered carbon and hydrogen to the other. The alkyl borane intermediate further reacts with two more alkene molecules to yield a trialkyl borane intermediate. Oxidation of trialkyl borane with hydrogen peroxide in sodium hydroxide replaces the boron with a hydroxyl group. The simultaneous addition of boron and hydrogen from borane ensures a syn addition over the double bond. However, the syn addition can happen from either face of the alkene.
Due to the difference in regiochemistry and reaction mechanisms, these three different synthetic pathways result in different alcohols from the same alkene substrate, as shown in
<img alt="Chemical reaction diagram, 3,3-dimethyl-1-butene hydration; acid, mercuration, hydroboration routes." src="/files/ftp_upload/11923/11923Figure_1.svg" style="text-align: center; height: 200px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
Figure 1. Addition of water over 3,3-dimethyl-1butene under acid-catalysis (top), oxymercuration-demercuration (middle) and hydroboration-oxidation conditions (bottom).

Overview
The acid-catalyzed addition of water to the double bond of alkenes is a large-scale industrial method used to synthesize low-molecular-weight ...

preparation-of-alcohols-via-addition-reactions

Education

JoVE Core

Organic Chemistry

Unit 1

Alcohols and Phenols

KEY TERMS AND CONCEPTS

alcohol synthesis from alkenes

Alcohol synthesis from alkenes can follow three addition reactions: acid-catalyzed hydration, oxymercuration-demercuration, and hydroboration-oxidation. These methods all add across the double bond, but they give different alcohol products from the same alkene.

In acid-catalyzed hydration, water adds to the double bond in an acidic atmosphere. The acid lets the hydrogen in water act as an electrophile and attack the alkene. A proton first adds to the double bond and forms a carbocation, which is a positively charged intermediate.

The proton usually bonds to the less substituted end of the double bond. This forms the more stable carbocation. Water then attacks the carbocation to form a protonated alcohol. The reaction is regioselective and follows Markovnikov's rule, so the -OH group ends up on the more substituted carbon.

This reaction is not stereoselective because the planar carbocation can be attacked from either side. That can lead to a racemic mixture of alcohols. Water addition can occur by syn addition or anti addition relative to the proton that added first. In some alkenes, the carbocation can rearrange by a 1,2 hydride shift or 1,2 methyl shift. When that happens, a mixture of constitutional isomers can form.

Oxymercuration-demercuration is another route for making alcohols from alkenes. In the oxymercuration step, mercury acetate adds electrophilic mercury to the double bond and forms a three-membered mercurinium ion. Water attacks this ring to give an organomercurial alcohol. In the demercuration step, sodium borohydride replaces the mercury species with hydrogen.

This pathway also gives a Markovnikov product. The hydroxyl group is placed on the more substituted carbon in the mercurinium ion, and the hydrogen ends up on the less substituted carbon. Water attacks anti to the mercury acetate addition. The final replacement of mercury with hydrogen is not stereoselective, so both enantiomers can form. Because it avoids a normal carbocation and rearrangement, oxymercuration-demercuration usually gives better yields than direct acid-catalyzed hydration.

Hydroboration-oxidation gives the anti-Markovnikov alcohol. Hydroboration adds a B-H bond from a borane across the double bond in one step through a four-membered transition state. Boron is electrophilic and adds to the least hindered carbon. Steric effects also favor that site, while hydrogen adds to the other carbon.

The alkyl borane can react with two more alkene molecules to form a trialkyl borane. Oxidation with hydrogen peroxide in sodium hydroxide then replaces boron with a hydroxyl group. Because boron and hydrogen add together, the reaction is syn addition. This syn addition can occur from either face of the alkene. Together, these three pathways show how one alkene can lead to different alcohols based on regiochemistry and mechanism.

Alcohol Synthesis from Alkenes

structure and nomenclature of alcohols and phenols

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Alcohols and Phenols: Naming Basics

Alcohols and phenols are important organic compounds with simple naming rules. Alcohols contain a hydroxyl group, or –OH group, directly bonded to carbon. Phenols also contain an OH group, but it is attached directly to a benzene ring. Alcohols are usually colorless. Phenol is a white crystalline compound with a characteristic hospital smell. These compounds are named using formal and standard systems. The most widely used system is the International Union of Pure and Applied Chemistry, or...

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Alcohols and Phenols: Boiling Point Trends

Alcohols and phenols are organic compounds whose physical properties depend on structure. In alcohols, a hydroxy group is attached to a saturated carbon. In phenols, the hydroxy group is attached to an aromatic ring. Their boiling points and water solubility are shaped by hydrogen bonding and dispersion forces. The oxygen-hydrogen dipole in the hydroxy group allows intermolecular hydrogen bonding. This gives alcohols a higher boiling point than aliphatic hydrocarbons with similar molecular...

acidity and basicity of alcohols and phenols

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Why Phenols Are More Acidic Than Alcohols

Alcohols are weak acids and weak bases because the O–H bond is polar and the oxygen atom has lone pairs. The partially positive hydrogen can be lost as a proton, and the oxygen can also accept a proton. When an alcohol is protonated, hydroxide changes into water, which is a much better leaving group. Alcohols vary a little in acidity. Methanol, with a pKa of 15.5, is slightly stronger as an acid than water, which has a pKa of 15.7. Ethanol, tert-butanol, and many other alcohols are weaker...

Alcohol synthesis from alkenes can follow three addition reactions: acid-catalyzed hydration, oxymercuration-demercuration, and hydroboration-oxidation. These methods all add across the double bond, but they give different alcohol products from the same alkene. In acid-catalyzed hydration, water adds to the double bond in an acidic atmosphere. The acid lets the hydrogen in water act as an electrophile and attack the alkene. A proton first adds to the double bond and forms a carbocation, which...

acid-catalyzed dehydration of alcohols to alkenes

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Alcohol Dehydration Pathways to Alkene Products

Alcohol dehydration forms alkenes and water by removing a hydroxyl group and a hydrogen from a neighboring carbon. This reaction is usually done by heating the alcohol with an acid catalyst. Primary alcohols need higher temperatures and stronger acid conditions, while secondary and tertiary alcohols can react under milder conditions. Secondary and tertiary alcohols usually dehydrate through an E1 mechanism. In the first fast step, the alcohol is protonated and becomes an alkyloxonium ion. Then...

alcohols from carbonyl compounds: reduction

alcohols-from-carbonyl-compounds-reduction

Making Alcohols by Carbonyl Reduction

Making alcohols by carbonyl reduction is a key reaction in organic chemistry. It converts a carbonyl group into a hydroxyl group. The main routes are catalytic hydrogenation, hydride reduction, and borane reduction. Catalytic hydrogenation works by adding H2 across the pi bond of the carbonyl group. It uses transition metal catalysts such as Raney Ni, Pd-C, Pt, or Ru. Aldehydes and ketones can be reduced this way under mild to moderate heat, usually 25–100°C, and H2 pressure of about 1–5 atm.

alcohols from carbonyl compounds: grignard reaction

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Grignard Reagents in Alcohol Synthesis

Grignard reagents are key tools for making alcohols from carbonyl compounds. They are organomagnesium halides with a highly polar carbon–magnesium bond. Because this bond is partly ionic, the carbon acts as a strong nucleophile. It attacks the carbonyl carbon, which is the electrophilic part of a carbonyl group. Magnesium in the reagent also helps the reaction move forward. It coordinates with the carbonyl oxygen and lowers the electron density on the carbonyl carbon. This makes the carbonyl...

protection of alcohols

Alcohol Protecting Groups in Synthesis

Alcohol protecting groups help control which part of an organic molecule reacts during synthesis. A protecting group is a masking agent that makes a more reactive species inert under a chosen set of conditions. The lesson uses a pipe-flow analogy to show how this improves reaction selectivity. It also shows how protecting an alcohol can allow an organolithium alkylation of a halide to occur when the alcohol would otherwise compete. Common protecting groups for alcohols include trialkylsilyl...

preparation of diols and pinacol rearrangement

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Pinacol Rearrangement of Vicinal Diols

Vicinal diols are compounds with two hydroxyl groups on adjacent carbon atoms. They are also called glycols. Under acidic conditions, these diols can undergo a pinacol rearrangement. The reaction starts when the acid catalyst transfers a proton to one hydroxyl group. This forms an oxonium ion. The oxonium ion then loses water and creates a tertiary carbocation intermediate. Next, a methyl group migrates from one carbon to the neighboring carbon. This step forms a resonance-stabilized cation...

conversion of alcohols to alkyl halides

Alcohols to Halides: Reagents and Mechanisms

Alcohols can be turned into alkyl halides by using reagents that make the hydroxyl group a better leaving group. The hydroxyl group is first protonated, which helps the reaction move forward. From there, the pathway can follow either an SN1 or SN2 substitution, depending on the starting alcohol and the reaction conditions. Tertiary alcohols usually form tertiary alkyl halides through a two-step SN1 mechanism. This route goes through a carbocation intermediate, and hyperconjugation helps...

oxidation of alcohols

Alcohol Oxidation Reagents and Products

Alcohol oxidation changes primary and secondary alcohols into carbonyl compounds. The reaction can mean loss of electrons, loss of hydrogen, or addition of oxygen. Oxidation is the opposite of reduction, so carbonyls can be reduced to alcohols, and alcohols can be oxidized back to carbonyls. The product depends on the alcohol type and the hydrogens on the alpha-carbon, the carbon next to the hydroxyl group. Primary alcohols can become aldehydes and then carboxylic acids. Secondary alcohols can...

preparation of alcohols via substitution reactions

preparation-of-alcohols-via-substitution-reactions

Alcohol Formation from Alkyl Halides

Alcohols can be made from alkyl halides by nucleophilic substitution. In these reactions, the carbon-halogen bond is highly polar, so the halide can leave the molecule. A nucleophile such as hydroxide ion or water then replaces the halide and forms an alcohol. The reaction path depends on the alkyl halide. Primary alkyl halides usually form primary alcohols through the SN2 mechanism. In SN2, the nucleophile attacks the halogen-bearing carbon from the side opposite the carbon-halogen bond. With...

key terms and concepts

Compare acid-catalyzed hydration, oxymercuration-demercuration, and hydroboration-oxidation to see how alkenes form alcohols with different regiochemistry.

Alcohol Synthesis from Alkenes | 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.

Alcohol Synthesis from Alkenes

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