Name: hydrating alkenes to form alcohols
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 2 min 45 s
Channel: JoVE

While the direct addition of water to an alkene leads to no reaction, in the presence of an acid, an alcohol is obtained by the net addition of a hydroxyl group and a hydrogen across the double bond.
Since strong acids, like sulfuric acid, dissociate completely in an aqueous solution, the acid participating in the reaction is the hydronium ion.
The first step is the slow protonation of the less substituted end of the double bond to form a more substituted carbocation.
In the second step, water, present in excess, acts as the nucleophile and attacks the carbocation to give the oxonium ion.
Lastly, water, with a pKa of 15.7, acts as a base and deprotonates the more acidic oxonium ion with a pKa of approximately -2, to yield the final product. The process is called acid-catalyzed, as a hydronium ion that initiates the reaction is recovered at the end.
The hydration of alkenes and the dehydration of alcohols are in equilibrium with each other, and the equilibrium position depends on the concentration of water and temperature.
According to Le Chatelier&#8217;s principle, the system in equilibrium adjusts to reduce the changes employed.
For instance, in the reaction of 2-methylpropene, water is present on the left side. The use of a dilute acid, which has more water, shifts the equilibrium to form more alcohol.
Alternatively, concentrated acid, which has less water, reverses the equilibrium to form the alkene and water.
Addition reactions are also temperature-dependent. Here, the enthalpy term is negative because the bond formation is exothermic. The entropy term, a product of a negative entropy change and a positive temperature value, is positive.
At low temperatures, the entropy term becomes smaller compared to the enthalpy term. Therefore, the Gibbs free energy becomes negative and the equilibrium constant being greater than one favors the formation of the alcohol.
In contrast, at higher temperatures, the entropy term dominates the enthalpy term, and the change in Gibbs free energy becomes positive. Consequently, the equilibrium constant becomes less than one, favoring the formation of the alkene.

acid-catalyzed hydration of alkenes

Acid-Catalyzed Hydration of Alkenes

Alkenes react with water in the presence of an acid to form an alcohol. In the absence of acid, hydration of alkenes does not occur at a significant rate, and the acid is not consumed in the reaction. Therefore, alkene hydration is an acid-catalyzed reaction.
<img alt="Hydration reaction of propene equation, diagram of alkene reaction with water forming alcohol." src="/files/ftp_upload/11776/11776_PT_Figure_1.svg" style="text-align: center; height: 75px;" data-altupdatedat="1754904545000" data-alt="Hydration reaction of propene to isopropanol; organic chemistry, reaction equation diagram." />
Strong acids, such as sulfuric acid, dissociate completely in an aqueous solution, and the acid participating in the reaction is the hydronium ion.
<img alt="Sulfuric acid ionization equation, chemical reaction formula, reversible arrow representation." src="/files/ftp_upload/11776/11776_PT_Figure_2.svg" style="text-align: center; height: 30px;" data-altupdatedat="1754904545000" data-alt="Sulfuric acid dissociation equilibrium equation, H2SO4 + H2O &#8596; H3O+ + HSO4&#8722;." />
The first step is the slow protonation of an alkene at the less-substituted end to form the more-substituted carbocation.
<img alt="Organic reaction mechanism: hydrocarbon addition to hydronium, illustrating slow step, diagram." src="/files/ftp_upload/11776/11776_PT_Figure_3.svg" style="text-align: center; height: 60px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction mechanism; organic chemistry equation diagram; carbocation rearrangement reaction." />
The second step is the nucleophilic attack by water at the carbocation to give an oxonium ion.
<img alt="Acid-catalyzed hydration mechanism; chemical equation; organic chemistry reaction pathway." src="/files/ftp_upload/11776/11776_PT_Figure_4.svg" style="text-align: center; height: 75px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction mechanism: acid-base reaction, equilibrium, reaction intermediate, structural formula." />
In the last step, water, with a pKa of 15.7, acts as a base and deprotonates the acidic oxonium ion (protonated alcohol), which has a pKa of approximately &#8211;2, to yield the final product.
<img alt="Acid-base reaction, equilibrium, chemical equation diagram, hydronium transfer, reaction mechanism." src="/files/ftp_upload/11776/11776_PT_Figure_5.svg" style="text-align: center; height: 85px;" data-altupdatedat="1754904545000" data-alt="Acid-catalyzed hydration mechanism diagram; organic reaction structure, equilibrium, protonation process." />
The two processes, hydration of alkenes to form alcohols and the dehydration of alcohols to form alkenes, are in equilibrium with each other. The control over this equilibrium can be explained by Le Chatelier&#8217;s principle, which states that a system at equilibrium will adjust to minimize any stress placed on the system.
In the hydration of 2-methylpropene, water is on the left side of the reaction. When the amount of water increases, the equilibrium shifts towards the right, producing more alcohol. In contrast, eliminating water from the system shifts the equilibrium to produce more alkene. Thus, the presence of dilute acids favors the formation of alcohols from alkenes, while the reverse occurs in the presence of concentrated acids that contain very little water.
Addition reactions are temperature-dependent. The enthalpy term for these reactions is negative as new bonds are formed during the process. In contrast, the entropy term is positive as the two reactant molecules give one molecule of product.
At low temperatures, the entropy term is small and the enthalpy term dominates. Thus, the Gibbs free energy is negative, and the equilibrium constant being greater than one promotes the formation of product over reactants.
<img alt="Hydration reaction of propene with water, chemical equation diagram, low temperature conditions." src="/files/ftp_upload/11776/11776_PT_Figure_7.svg" style="text-align: center; height: 75px;" data-altupdatedat="1754904545000" data-alt="Hydration reaction, alkene to alcohol, chemical equation, low temperature condition, diagram." />
However, at high temperatures, the large entropy term dominates the enthalpy term and the Gibbs free energy is positive. The equilibrium constant being less than one reverses the reaction, implying that reactants will be favored over products.
<img alt="Alkene hydration reaction; chemical equation diagram showing isobutylene and water forming tert-butanol." src="/files/ftp_upload/11776/11776_PT_Figure_8.svg" style="text-align: center; height: 75px;" data-altupdatedat="1754904545000" data-alt="Hydration reaction formula; alkene to alcohol conversion under high temperature; chemical process diagram." />

Alkenes react with water in the presence of an acid to form an alcohol. In the absence of acid, hydration of alkenes does not occur at a significant rate, ...

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Organic Chemistry

Unit 1

Reactions of Alkenes

KEY TERMS AND CONCEPTS

hydrating alkenes to form alcohols

Alkenes can be converted into alcohols by adding water in the presence of an acid. Without acid, this hydration happens too slowly to matter in practice. The acid is not used up in the reaction, so this is an acid-catalyzed process.

Strong acids such as sulfuric acid dissociate completely in water. In that case, the reacting acid species is the hydronium ion. The first step is a slow protonation of the alkene at the less-substituted end. This forms the more-substituted carbocation, which is more stable.

Next, water acts as a nucleophile and attacks the carbocation. This gives an oxonium ion, which is a protonated alcohol. In the final step, water acts as a base and removes a proton from the oxonium ion. That produces the alcohol product.

The hydration of alkenes and the dehydration of alcohols are in equilibrium with each other. Le Chatelier’s principle explains how the balance shifts when conditions change. In the hydration of 2-methylpropene, water is on the left side of the reaction. Adding more water shifts the equilibrium to the right and makes more alcohol. Removing water shifts the equilibrium toward alkene formation. For this reason, dilute acids favor alcohol formation, while concentrated acids with very little water favor the reverse reaction.

Temperature also affects these addition reactions. Their enthalpy term is negative because new bonds form during the reaction. Their entropy term is positive because two reactant molecules become one product molecule. At low temperature, the entropy term is small, so enthalpy dominates and Gibbs free energy is negative. The equilibrium constant is greater than one, which favors product formation. At high temperature, the entropy term becomes large, Gibbs free energy turns positive, and the equilibrium constant drops below one. In that case, the reactants are favored over the products.

Hydrating Alkenes to Form Alcohols

regioselectivity of electrophilic additions-peroxide effect

regioselectivity-of-electrophilic-additions-peroxide-effect

Peroxide Effect in HBr Alkene Addition

Hydrogen bromide can add to an alkene in a way that goes against Markovnikov’s rule when organic peroxides are present. For example, 2-methylpropene reacts with HBr and peroxide to give 1-bromo-2-methylpropane. This change in product comes from a free radical mechanism, which reverses the usual regioselectivity. The reaction has three stages: initiation, propagation, and termination. In initiation, an oxygen-oxygen bond in the radical initiator breaks by homolytic cleavage, meaning each atom...

free-radical chain reaction and polymerization of alkenes

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How Alkene Monomers Form Polymers

Alkene monomers can join together to form polymers, which are large macromolecules used in many commercial products. In a polymer, the repeating unit defines the structure, while the terminal groups are considered insignificant. The average degree of polymerization shows how many repeating units are in a polymer molecule and is written as the subscript n. Alkenes form polymers through a free-radical chain reaction with three main stages: initiation, propagation, and termination. In the...

halogenation of alkenes

Anti Addition in Alkene Halogenation

Halogenation of alkenes adds chlorine or bromine across a carbon-carbon double bond to form a vicinal dihalide. This reaction is carried out in inert, non-nucleophilic solvents such as methylene chloride, chloroform, or carbon tetrachloride. The reaction starts when bromine is drawn toward the pi electrons of an alkene such as cyclopentene. One bromine atom acts as an electrophile and adds across the double bond. This forms a cyclic bromonium ion intermediate. The bromonium ion is more stable...

formation of halohydrin from alkenes

Bromohydrin Addition in Alkene Reactions

Bromohydrin addition shows how an alkene can react with bromine in water to form a halohydrin. In this reaction, the product has a halogen and a hydroxyl group on neighboring carbons. When the halogen is bromine, the product is called a bromohydrin. When the halogen is chlorine, it is called a chlorohydrin. The reaction begins when the pi electrons of the alkene react with electrophilic bromine. This step forms a bridged intermediate called a bromonium ion. Water then acts as a nucleophile. It...

Alkenes can be converted into alcohols by adding water in the presence of an acid. Without acid, this hydration happens too slowly to matter in practice. The acid is not used up in the reaction, so this is an acid-catalyzed process. Strong acids such as sulfuric acid dissociate completely in water. In that case, the reacting acid species is the hydronium ion. The first step is a slow protonation of the alkene at the less-substituted end. This forms the more-substituted carbocation, which is...

regioselectivity and stereochemistry of acid-catalyzed hydration

regioselectivity-and-stereochemistry-of-acid-catalyzed-hydration

Carbocation Stability in Alkene Hydration

Acid-catalyzed hydration of alkenes forms alcohols through a carbocation intermediate. The reaction starts when hydronium ion protonates the alkene. This step is slow and controls the overall rate. The alkene structure matters because alkyl substituents at the double bond can change how fast the reaction occurs. The kind of carbocation formed also affects the rate. A reaction that passes through a tertiary carbocation is faster than one that forms a secondary or primary carbocation. This is...

oxymercuration-reduction of alkenes

Markovnikov Hydration with Mercury

Oxymercuration-reduction is a way to turn an alkene into an alcohol. It hydrates the alkene using mercuric acetate in a mix of tetrahydrofuran and water. The reaction first makes an organomercury adduct and then reduces it to an alcohol with sodium borohydride. The solvent mix has two jobs. Tetrahydrofuran dissolves the alkene and the aqueous mercuric acetate solution. Water acts as both a reactant and a solvent for mercuric acetate. The mechanism can be seen in the conversion of...

hydroboration-oxidation of alkenes

Anti-Markovnikov Alcohols from Alkenes

Hydroboration-oxidation turns alkenes into alcohols with anti-Markovnikov orientation. It is a complementary route to oxymercuration-demercuration, which gives Markovnikov products. In this reaction, borane adds across the alkene first, and basic hydrogen peroxide finishes the conversion to an alcohol. Borane is very reactive because the boron atom has only six electrons in its valence shell. It also has an empty 2p orbital that sits perpendicular to the plane formed by boron and its three...

regioselectivity and stereochemistry of hydroboration

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Hydroboration Reaction Paths and Product Control

Hydroboration shows how alkene reactions control both where atoms add and how the product is shaped. In hydroboration–oxidation, borane attacks the alkene π bond in a concerted way, which means the bond changes happen together. This forms a cyclic, four-centered transition state. In this transition state, the BH2 group attaches to the less substituted carbon, while H attaches to the more substituted carbon. The addition happens across the same face of the alkene, so the reaction gives syn...

oxidation of alkenes: syn dihydroxylation with osmium tetraoxide

oxidation-of-alkenes-syn-dihydroxylation-with-osmium-tetraoxide

Alkene Dihydroxylation with Osmium Tetroxide

Alkene dihydroxylation with osmium tetroxide adds two hydroxyl groups across a double bond to make 1,2-diols, also called glycols. The reaction can give either anti or syn products, and this method follows syn stereochemistry. That means both new O atoms add to the same face of the alkene. The reaction occurs in two steps. First, osmium tetroxide adds to the alkene double bond in a concerted step, where both bonds form at the same time. This creates a five-membered cyclic osmate ester...

oxidation of alkenes: syn dihydroxylation with potassium permanganate

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Potassium Permanganate Tests for Alkene Bonds

Potassium permanganate can be used to test and change alkene double bonds. When an alkene reacts with cold, dilute potassium permanganate under basic conditions, it forms a cis-diol, which is a molecule with two hydroxyl groups on the same side. A brown precipitate of manganese dioxide also appears during the reaction. The reaction starts when the permanganate ion adds to the same side of the alkene pi bond. This syn addition forms a cyclic manganate ester intermediate. Water then hydrolyzes...

oxidation of alkenes: anti dihydroxylation with peroxy acids

oxidation-of-alkenes-anti-dihydroxylation-with-peroxy-acids

Epoxide Route to Trans Diols

Epoxides provide a route to trans diols from alkenes. In anti dihydroxylation, an alkene is treated with a peroxycarboxylic acid to first make an epoxide. A diol is a compound with two hydroxyl groups, and in this pathway the two OH groups end up on opposite sides. An epoxide is a highly strained three-membered ring. Oxygen and two carbon atoms sit at the corners of an equilateral triangle. The first step uses a peracid, which is a strong oxidizing agent related to a carboxylic acid but with...

oxidative cleavage of alkenes: ozonolysis

Alkene Bond Cutting by Ozonolysis

Ozonolysis is a reaction that cuts a carbon-carbon double bond in an alkene. Ozone attacks the double bond and breaks the molecule into smaller carbonyl products. Depending on the work-up, the products can be aldehydes, ketones, or carboxylic acids. Ozone is a symmetrical bent molecule with a resonance-stabilized structure. The reaction starts with electrophilic addition of ozone across the alkene double bond. This step forms an unstable molozonide, which then reacts further to give a carbonyl...

reduction of alkenes: catalytic hydrogenation

reduction-of-alkenes-catalytic-hydrogenation

Alkene Hydrogenation and Alkane Stability

Alkene hydrogenation turns an alkene into an alkane by adding molecular hydrogen. This reduction usually happens with a transition-metal catalyst, so the reaction is called catalytic hydrogenation. Palladium, platinum, and nickel are common catalysts in solid form as fine powder on an inert surface. These catalysts do not dissolve in the reaction mixture, so they are heterogeneous catalysts. The reaction takes place on the metal surface. First, hydrogen adsorbs onto the surface and the H-H...

reduction of alkenes: asymmetric catalytic hydrogenation

reduction-of-alkenes-asymmetric-catalytic-hydrogenation

Chiral Alkene Hydrogenation for Drug Synthesis

Catalytic hydrogenation adds molecular hydrogen to an alkene double bond and turns it into an alkane. A transition-metal catalyst drives the reaction, and the hydrogen atoms add in syn stereochemistry, meaning they enter from the same side. The catalyst can be heterogeneous or homogeneous. When hydrogenation creates a chiral center, the products are usually a pair of enantiomers, which are mirror-image molecules. An enantiomeric excess of one product can be obtained with an enantioselective...

key terms and concepts

See how alkenes react with water and acid to form alcohols, why hydronium ion drives the reaction, and how equilibrium and temperature shift the products.

Hydrating Alkenes to Form Alcohols | JoVE K12

organic chemistry

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Hydrating Alkenes to Form Alcohols

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