Name: carbocation stability in alkene hydration
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 2 min 34 s
Channel: JoVE

The relative rates of an acid-catalyzed hydration of ethene, propene, or 2-methylpropene, show that alkyl substituents at the double bond accelerate the rate significantly.
Here, the protonation of an alkene by a hydronium ion leads to the formation of a carbocation, which is the rate-determining step.
The reaction that proceeds through a tertiary carbocation is faster than a reaction proceeding via a secondary or primary carbocation. The difference in the reaction rates is explained by comparing the stability of carbocations. The tertiary carbocation, being more stable than the secondary or primary carbocation, is formed faster.
The preference for the formation of a more stable carbocation also influences the regiochemical outcome of the reaction.
When the more stable carbocation is formed as an intermediate, the nucleophilic addition of water occurs at the more substituted carbon, followed by deprotonation of the oxonium ion yielding the Markovnikov&#39;s product.
Furthermore, the protonation of 1-butene yields a planar, achiral secondary carbocation having both faces equally accessible to the nucleophile.
The attack of water from the top face leads to (S)-2-butanol, while the attack from the bottom face produces (R)-2-butanol. Thus, when a new chiral center is generated, a racemic mixture is formed.
However, the addition of water to a chiral alkene forms a chiral carbocation with no plane of symmetry. The two faces of this intermediate are not equally accessible by a nucleophile due to different steric setups, and hence the diastereomeric products are formed in unequal amounts.
The utility of the acid-catalyzed hydration is limited due to the rearrangement of the carbocation intermediate. For instance, the protonation of 3-methyl-1-butene forms a secondary carbocation intermediate.
The less stable secondary carbocation rearranges to a more stable tertiary carbocation by the shift of hydrogen with its bonding pair of electrons through a 1,2-hydride shift. Thus, the nucleophilic attack by water at the tertiary carbocation with subsequent deprotonation yields the more substituted product.

regioselectivity and stereochemistry of acid-catalyzed hydration

Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration

The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
<img alt="Acid-catalyzed hydration reaction rate diagram showing relative rates for three alkenes." src="/files/ftp_upload/11777/11777_PT_Figure_1.svg" style="text-align: center; height: 225px;" data-altupdatedat="1754904545000" data-alt="Hydration reaction rates table, chemical equations, relative reactivity comparison, organic chemistry." />
The reaction proceeds with the slow protonation of an alkene by a hydronium ion to form a carbocation, which is the rate-determining step.
The reaction involving a tertiary carbocation intermediate is faster than a reaction proceeding through a secondary or primary carbocation. This can be justified by comparing their relative stabilities and the delocalization of the positive charge. Tertiary carbocations are the most stable and thus formed faster.
<h4>Regiochemical Outcome</h4>
The formation of a stable carbocation intermediate determines the regiochemical outcome as it directs the nucleophilic addition of water to the more substituted carbon following Markovnikov's orientation.
<h4>Stereochemical Outcome of Achiral Alkenes</h4>
For an achiral alkene such as 1-butene, protonation results in a secondary carbocation.
<img alt="Acid-catalyzed hydration reaction, chemical equations diagram, carbocation formation process." src="/files/ftp_upload/11777/11777_PT_Figure_2.svg" style="text-align: center; height: 65px;" data-altupdatedat="1754904545000" data-alt="Acid-catalyzed hydrohalogenation reaction; chemical equation diagram; mechanism visualization." />
The trivalent carbon is sp2-hybridized with a plane of symmetry. It can react with water either from the top or the bottom face with equal probability.
The reaction from the top face leads to (S)-2-butanol, while the reaction from the bottom face leads to (R)-2-butanol. Thus, the formation of a new chiral center leads to a racemic mixture of enantiomeric products.
<img alt="Stereochemistry reaction diagram; carbocation formation and (S/R)-2-butanol synthesis pathways." src="/files/ftp_upload/11777/11777_PT_Figure_3.svg" style="text-align: center; height: 225px;" data-altupdatedat="1754904545000" data-alt="Stereochemistry SN2 reaction diagram, showing pathway to (S)-2-Butanol and (R)-2-Butanol." />
<h4>Stereochemical Outcome of a Chiral Alkene</h4>
The protonation of a chiral alkene forms a chiral carbocation with no plane of symmetry. The carbocation does not react equally from the top and bottom faces because one of the faces is more accessible than the other due to different steric setups, leading to a mixture of R and S products. Thus, two diastereomeric products are produced in unequal amounts, and the mixture is optically active.
<img alt="Stereochemistry diagram: nucleophilic addition, reaction mechanism, chiral outcome, major/minor products." src="/files/ftp_upload/11777/11777_PT_Figure_4.svg" style="text-align: center; height: 225px;" data-altupdatedat="1754904545000" data-alt="Stereochemistry diagram showing reaction pathway for enantiomers of 5-Methyl-2-heptanol." />
<h4>Rearrangement of a Carbocation</h4>
In some cases, the carbocation formed in the first step can rearrange to a more stable carbocation. For example, the protonation of 3-methyl-1-butene forms a 2&#176; carbocation intermediate, which rearranges to a more stable 3&#176; carbocation via a 1,2-hydride shift.
<img alt="Isomerization reaction diagram; alkyl shift mechanism with carbocation rearrangement." src="/files/ftp_upload/11777/11777_PT_Figure_5.svg" style="text-align: center; height: 65px;" data-altupdatedat="1754904545000" data-alt="Hydride shift, alkyl carbocation rearrangement, molecular structure, equilibrium arrows, chemistry diagram." />

The rate of acid-catalyzed hydration of alkenes depends on the alkene&#39;s structure, as the presence of alkyl substituents at the double bond can ...

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

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Organic Chemistry

Unit 1

Reactions of Alkenes

KEY TERMS AND CONCEPTS

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 because tertiary carbocations are more stable. Their positive charge is spread out more, so they form more easily.

Carbocation stability also controls where water adds. After the stable carbocation forms, water adds to the more substituted carbon. This gives the Markovnikov product, where the OH group ends up on the more substituted side of the double bond.

Stereochemistry depends on the alkene. For an achiral alkene such as 1-butene, protonation gives a secondary carbocation with a plane of symmetry. Water can attack from the top face or the bottom face with equal chance. The top-face attack gives (S)-2-butanol, and the bottom-face attack gives (R)-2-butanol. The result is a racemic mixture of enantiomers.

A chiral alkene behaves differently. Protonation forms a chiral carbocation with no plane of symmetry. Water does not attack both faces equally because one face is more open than the other. This gives two diastereomeric products in unequal amounts, and the mixture remains optically active.

In some cases, the first carbocation rearranges before water adds. For example, protonation of 3-methyl-1-butene first gives a secondary carbocation. That intermediate can shift a hydride in a 1,2-hydride shift to form a more stable tertiary carbocation.

Carbocation Stability in Alkene Hydration

carbocation-stability-in-alkene-hydration

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

free-radical-chain-reaction-and-polymerization-of-alkenes

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...

acid-catalyzed hydration of alkenes

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...

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

regioselectivity-and-stereochemistry-of-hydroboration

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

oxidation-of-alkenes-syn-dihydroxylation-with-potassium-permanganate

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

Learn how carbocation stability affects alkene hydration, Markovnikov addition, racemic mixtures, and carbocation rearrangement in high school chemistry.

Carbocation Stability in Alkene Hydration | 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.

Carbocation Stability in Alkene Hydration

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