Name: alkene hydrogenation and alkane stability
Uploaded: 2023-04-30T18:28:37.000Z
Duration: 2 min 13 s
Channel: JoVE

Hydrogenation of alkenes is a reduction process wherein the addition of molecular hydrogen breaks the weak &#960; bond of an alkene to form two C&#8211;H &#963; bonds of an alkane.
Intrinsically, hydrogenation of alkenes has a large energy barrier, making the reaction unfavorable at room temperature.
The reaction can be altered to give a low-energy pathway using a transition-metal catalyst &#8212; usually containing palladium, platinum, and nickel.
The catalyst is heterogeneous; that is, finely divided solid dispersed on the surface of inert support, such as charcoal.
Hydrogenation begins with the adsorption of molecular hydrogen to the surface of the metal catalyst.
Interaction of molecular hydrogen with the metal cleaves the H&#8211;H bond to give individually adsorbed hydrogen atoms.
Next, an alkene adsorbs by coordinating one of the faces of its &#960; bond to the metal surface.
This is followed by the sequential insertion of two hydrogen atoms into the &#960; bond, giving the reduced product that simultaneously releases from the catalyst surface.
As the hydrogens transfer to the same face of the &#960; bond, hydrogenation has syn stereochemistry.
Consider the hydrogenation of an alkene generating two new chiral centers.&#160;
Though the chirality in the molecule reveals the possibility of four stereoisomers, due to syn addition, only one pair of enantiomers is predominantly formed, making the reaction stereospecific.
Additionally, the steric environment around the double bond governs its approach towards the catalyst.
For instance, the double bond in &#945;-pinene is under the steric influence of the methyl group attached to the four-membered ring, inhibiting hydrogen insertion from the sterically hindered side.
Hence, the hydrogen insertion into the &#960; bond of &#945;-pinene takes place exclusively from its bottom face, forming a single product.

reduction of alkenes: catalytic hydrogenation

Reduction of Alkenes: Catalytic Hydrogenation

Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms &#8212; fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the surface of the metal catalyst. It begins with the adsorption of the hydrogen onto the metal surface, followed by the cleavage of the H&#8211;H bonds to give individual metal&#8211;hydrogen bonds. The alkene then complexes with the catalyst surface by using its p orbitals to overlap with the empty metal orbitals of the catalyst. The two hydrogen atoms then insert into the &#960; bond sequentially through syn addition (addition to the same face of the &#960; bond) to give the reduced product &#8212; the alkane. The alkane formed is no longer bound to the metal and diffuses away from the catalyst&#39;s surface.
The process of hydrogenation is exothermic. The heat released is called the heat of hydrogenation (&#916;H&#176;), and it helps predict&#160;the relative stabilities of alkenes. For example, although the hydrogenation of both cis-2-butene and trans-2-butene gives the same product &#8212; butane, trans-2-butene is more stable than cis-2-butene. This can be explained based on the heat of hydrogenation of the two isomers. The cis isomer (&#916;H&#176;&#160;= &#8722;28.6 kcal/mol) has a slightly higher heat of hydrogenation compared to the trans isomer (&#916;H&#176;&#160;= &#8722;27.6 kcal/mol). In cis-2-butene, the steric repulsion between the two methyl groups lying on the same side of the double bond makes it less stable, which is reflected in its larger heat of hydrogenation.

Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a ...

reduction-of-alkenes-catalytic-hydrogenation

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Organic Chemistry

Unit 1

Reactions of Alkenes

KEY TERMS AND CONCEPTS

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 bond breaks to form metal-hydrogen bonds.

Next, the alkene binds to the catalyst surface. Its p orbitals overlap with empty metal orbitals. The two hydrogen atoms then add step by step to the pi bond through syn addition, which means both hydrogens add to the same face of the double bond. The alkane product is no longer attached to the metal and diffuses away from the surface.

Hydrogenation releases heat, so the process is exothermic. The heat given off is called the heat of hydrogenation, written as delta H degree. This value helps compare alkene stability. For example, cis-2-butene and trans-2-butene both form butane, but trans-2-butene is more stable than cis-2-butene.

The heat of hydrogenation supports that trend. Cis-2-butene has a slightly higher heat of hydrogenation, at -28.6 kcal/mol, than trans-2-butene, at -27.6 kcal/mol. In cis-2-butene, the two methyl groups sit on the same side of the double bond, which creates steric repulsion and makes the molecule less stable.

Alkene Hydrogenation and Alkane Stability

alkene-hydrogenation-and-alkane-stability

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...

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

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...

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 alkene hydrogenation uses metal catalysts to make alkanes and how heat of hydrogenation helps compare cis and trans alkene stability.

Alkene Hydrogenation and Alkane Stability | 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.

Alkene Hydrogenation and Alkane Stability

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