Reaction of Alkenes and their Mechanisms

Alkenes are unsaturated hydrocarbons. They contain at least one double bond. The reactions of alkenes are explained on the basis of the availability of a delocalized pi-electrons cloud. These pi electrons can be easily attacked by electrophiles.

The reactions of alkenes are dominated by electrophile addition reactions. Markonikov’s rule governs the addition of unsymmetrical molecules e.g. (H-X), which states that the hydrogen of the HX molecule is added to the carbon that is less substituted.

Electrophiles other than HX can also be added to alkenes. For example, Br₂ and Cl₂ react with alkenes to form 1,2-dihalide addition products via the halonium ion mechanism. When alkene reacts with aqueous acid, water is added (hydration occurs).

Furthermore, hydrogenation, the addition of hydrogen (H₂) occurs in the presence of a metal catalyst such as platinum or palladium. Oxidation reactions of alkenes can be carried out using potassium permanganate, (KMnO₄). Alkenes can be converted into epoxides by treating them with a peroxy acid (RCO₃H), and conjugated dienes undergo a 1,4-addition of electrophiles known as Michael addition.

Some important reactions of alkenes are given below:

1. Addition reactions 

• Hydrogenation of alkenes
• Electrophilic addition of hydrogen halides by Markonikov’s rule
• Anti-Markovnikov (free-radical addition) of hydrogen bromide to alkenes
• Electrophilic addition of bromine and chlorine
• Addition of sulfuric acid

2. Oxidation

• By kMnO4
• By Ozone (O₃)

3. Hydrolysis

4. Halohydrin addition

5. Dihydroxalation

6. Epoxidation

7. Polymerization

Addition Reaction of Alkenes

Electrophilic addition of hydrogen halides to alkenes

Hydrogen halide is a polar molecule. 1,2 or alpha-beta addition occurs when it is added to alkenes. The general equation is:

The mechanism of the addition reaction of hydrogen halide is simple and consists of two steps. In the first step, electrophile adds to the double bond and produces a carbocation intermediate. In the second step, the nucleophile adds to the carbocation. In such reactions, the alkene double bond may act as a nucleophile as well as a base.

Hydrogenation (Addition of hydrogen)

Hydrogenation is widely used to synthesize vegetable ghee from vegetable oils. It is the simplest example of the addition reaction of alkene. The hydrogen molecules are added leading to saturation in an exothermic reaction. 

For example, this method is employed to convert vegetable oils into fats or margarine. At 200ºC and high pressure, hydrogen gas is charged in the presence of a nickel catalyst. However, various metal catalysts such as platinum (Pt), and palladium (Pd), can also be used in this reaction.

Chemistry of hydrogenation

The molecular hydrogen interacts with the surface of the catalyst and breaks apart to form hydrogen atoms and the alkene molecule also coordinates with the metal surface. This way, the surface chemistry allows the reaction between an alkene and two hydrogen atoms effectively adding H and H across the double bond. Both hydrogen atoms in this process add to the same face of alkene explaining the observed stereospecificity (syn addition).

Note that, trans fats are produced during the synthesis of vanaspati ghee. They might be the cause of an increase in LDL cholesterol levels which leads to cardiovascular diseases. Therefore, it is highly emphasized to remove trans fats properly from food products through virtual transfat-free (VTF) technology.

2. Electrophilic Addition to unsymmetrical alkenes

According to Markonikov’s rule, in unsymmetrical alkenes, the H atom adds to carbon which has more hydrogen atoms or fewer substituted. It is to be noted that when this reaction takes place in the laboratory, only the product predicted by markovnikov’s rule is observed.

However, peroxides are used to produce anti-markonikov products. This means, if peroxides are added in the same reaction, 1-bromo propane is predominately produced. This happens because free radicals produced by the peroxides change the mechanism of this reaction.

Addition of water (Hydration)

Acid-catalyzed hydration is an addition reaction of alkenes. This addition follows markonikov’s rule. The rate of reaction depends upon the type of intermediate carbonation from during the reaction. The carbonation stability is responsible for the rate of product formation. For example, tertiary carbonation is more stable the primary carbonation. Hence, the rate of reaction incorporating tertiary carbocation is 10 billion times greater than the one which includes primary carbonation.

Industrial application

Alcohol is widely used as a solvent in paints and as a fuel. It is primarily produced through the acid-catalyzed hydration of ethene and is separated through fractional distillation. Phosphoric acid is used here as a catalyst. Through this process only, America produces 5 billion tonnes of ethanol per year.

Halogenation

Bromine(Br₂) and chlorine (Cl₂) are added to alkenes using an electrophilic addition mechanism.

This reaction is also used to confirm the presence of unsaturation in a sample. When an unsaturated alkene, for instance, ethene, is bubbled through bromine water, it decolorizes the solution.

Stereospecificity of halogenation of alkenes

The anti-addition of halogen to alkenes results in either an anti- or a syn product. It depends on the type of starting material. For example, when 2-butene is treated with bromine, it gives mesomers, while enantiomers are formed with cis 2-butene.

Halohydrin formation with alkene

Alkene gives halohydrin as a major product rather than vicinal dihalide when alkenes react with halogens in the aqueous medium.

Oxidation reactions

Alkenes give an oxidation reaction with potassium permanganate (KMnO₄). If cold dilute KMnO₄ is added to alkenes in basic conditions, alkenes are converted to 1,2 diols. On the other hand, in acidic conditions, hot concentrated KMnO₄ rupture the double bond of alkenes to yield carbonyl compounds.

So, different type of products is formed by cleavage of alkenes with hot concentrated KMnO4 in acidic or neutral conditions.

1. Monosubstituted alkenes are cleaved to salts of carboxylic acids.
2. Unsubstituted alkenes after treatment give carbon dioxide.
3. Di-substituted alkenes give aldehydes and ketones.

Ozonolysis: Cleavage of alkene double bond with ozone

Ozonolysis is the most suitable method for the cleavage of the double bond of alkenes. It is also used to form the double bond in unsaturated long-chain alkenes. Ozone gas is bubbled through an alkene solution at -78ºC. Dichloromethane is the solvent while dimethyl sulfide is the catalyst being employed.

The above examples of ozonolysis clearly illustrate the cleavage of alkenes

Dihydroxylation of alkenes

Diols are synthesized from alkenes in a variety of ways. The method for syn and anti-dihydroxylation that gives high yields are explained.

The common reaction to produce trans diol by anti-dihydroxylation is:

Alkenes, when reacting with osmium tetraoxide, oxygen attacks both ends of the double bond on the same face, resulting in syn-dihydroxylation of alkenes. The ester is formed and then separated with sodium bisulfite in an aqueous medium to give diols.

Epoxidation: Formation of epoxides from alkenes

Epoxidation is a chemical reaction in which a carbon-carbon double bond is converted into a carbon-oxygen bond. The resulting compound is called an epoxide. This reaction can be catalyzed with peroxy acids and enzymes.

Epoxidation reactions are commonly used to synthesize epoxy resins, coatings, adhesives, and composite materials.

Polymerization 

Polymers are large and usually long-.chain molecules. They are built by a combination of small units, called monomers. Polymers are produced to be used as synthetic fiber, plastic, and rubber. This category of reaction comprises 50% of all industrial applications of chemistry.

Alkenes are used to synthesize these polymers. For example, the simplest polymer is polyethylene which is produced by the free radical addition polymerization of ethene.

In this reaction, n represents a large number usually thousands of monomers. The reaction mechanism for free radical addition polymerization is given below:

Teflon (PTFE) is another important industrial polymer prepared by the same mechanism. Some common synthetic polymers constructed by alkenes monomers and their applications are given below:

Related Topics

• How to Name Alkenes? Naming Alkenes with Examples
• Carbocation Rearrangement
• Wittig Reaction, Example, Mechanism, and Stereochemistry.
• Diels Alder reaction Example, Mechanism, and stereochemistry

Key Takeaways

• Unsaturated hydrocarbons with at least one double bond between carbon atoms are referred to as alkenes.
• The reactivity of alkenes depends on the availability of a delocalized electron cloud.
• Alkenes can undergo a variety of chemical reactions, including addition reactions, substitution reactions, and elimination reactions.
• They can react with hydrogen gas in the presence of a metal catalyst to form alkanes through a process called hydrogenation.
• They can also undergo addition reactions with halogens, such as chlorine and bromine, to form haloalkanes
• Alkenes can react with peroxy acid to form epoxides.
• Elimination reactions of alkenes involve the removal of hydrogen (H₂) to form alkynes
• Additionally, alkenes can give polymerization reactions, which is of enormous industrial significance.

Concepts Berg

What is the best way to understand and memorize these alkene addition reactions?

The best way to memorize how the addition reaction of alkenes causes an addition reaction due to unsaturation. It adds symmetrical and unsymmetrical molecules across a double bond. 

What’s the mechanism for the electrophilic addition of formic acid to alkenes?

Formic acid reacts with alkenes to form formate esters. The reaction is also known as hydroxycarbonylation.

Why, in the addition reaction of alkenes, does carbocation rearrange?

The carbocation rearranges itself to gain stability. The stability of carbocation depends upon the number of electron-donating groups attached to it. As we know, a carbohydrate has a positive charge, and electron-donating groups stabilize it by inductive effect.

What causes addition reactions in alkenes? What causes addition reactions in alkenes?

Addition reactions are given by the alkenes due to the presence of a carbon-carbon double bond. That is also the cause of the addition of molecules.

What are the four types of alkene addition reactions?

1. Electrophilic addition
2. Double bond cleavage
3. Hydrolysis
4. Ozonolysis

Which type of reaction is characteristic of alkenes?

Electrophilic addition is the characteristic reaction for alkenes.

What are the conditions for alkene addition reactions?

Addition reactions for alkenes are usually carried out at lower temperatures. At higher temperatures, the reversible reaction may result in elimination. In the reversible reaction, elimination is favored.

Why do alkenes give electrophilic addition reactions?

The presence of an electron cloud is due to the pi bond. These electrons are readily available for attack on nearby electrophiles.

Why do some addition reactions with alkenes produce more than one product?

Some alkenes produce more than one product by addition reactions due to regioselectivity. With a stable major product, a minor product forms in this phenomenon.

What is the mechanism for the addition of HCN to alkenes?

The addition of HCN to alkenes is done by electrophilic addition.

What is the mechanism for the addition of HOCl to an alkene?

The addition of HOCl to alkenes is done by electrophilic addition.

Can an alkene react with a nucleophile?

Nucleophiles cannot react with alkenes due to the same negative fields on both constituents.

How to prepare a gem dihalide from an alkene?

As the carbon-carbon double bond is present, only vicinal dihalide is possible in alkene addition reactions. 

What reaction will be used to differentiate an alkyne from an alkene?

Alkynes at 600 °C in carbon tubes give benzene, while alkenes do not react in this way. 

Alkynes are less reactive than alkenes towards electrophilic addition reactions. Why?

In alkynes, the carbon-carbon bond length is reduced due to the triple bond in alkynes, therefore they are not readily available to be attacked by electrophiles, in comparison to alkenes.

How does the typical reaction of benzene differ from those of the alkenes?

Alkene differs from benzene in its chemical behavior. For example, 1,2 addition of bromine is given by alkenes whereas, bromobenzene is the only product of benzene with the same.

What is the role of THF in the hydroboration oxidation reaction of an alkene?

Tetrahydrofuran (THF) acts as a solvent in the hydroboration oxidation reaction of alkene.

How does the addition of alkenes give diastereomers?

In stereoselective reactions of alkenes, there is the possibility of giving two products, which are diastereomers.

References

• https://alevelchemistry.co.uk/notes/reactions-of-alkenes/