What is an Alkene Reaction? ^(1-4)

Alkenes are hydrocarbons that contain one or more double bonds. Their double bond consists of a sigma (σ) bond and a pi (π) bond. The carbon-carbon π bond is formed by side-by-side overlapping. It is relatively weak and exhibits high reactivity. The bond can be easily broken, and reagents can be added to carbon. The π-electrons also make the carbons atom electron-rich and attract an electrophile. Thus, alkenes can undergo a large variety of reactions. Alkenes can be used to synthesize a wide variety of compounds, such as halide, alcohol, ethers, and alkanes.

Types of Alkene Reaction ^(1-4)

1. Addition

The most common reaction for an alkene is the addition reaction. It involves the addition of two atoms to the molecule or compound. As already stated, breaking the π-bond of alkenes is relatively more straightforward than the σ-bond. So, one π-bond gets converted to two σ-bonds during the addition reaction, attaching to the incoming atoms.

In the addition reaction, π-bond is necessary. Unlike alkenes, alkanes do not undergo an addition reaction because they do not have a π-bond. They can participate in substitution reactions.

General Formula for Addition Reaction of Alkene

To understand the addition reaction, let us take an alkene R₁CH=HCR₂, which reacts with a hydrogen halide HX, yielding an alkyl halide, R₁CHBX-H₂CR₂. The reaction is as follows:

R₁CH=HCR₂ + HX → R₁CHBX-H₂CR₂

Types of Addition Reaction of Alkene

1.1 Hydrogenation

Hydrogenation is the addition of hydrogen (H₂) to the π-bond of alkenes, producing an alkane. The reaction must be catalyzed by metals such as Pd, Pt, Rh, and Ni.

Example

1. Ethene (C₂H₄) converts into ethane (C₂H₆) by reacting with hydrogen at 150  ͦC.

C₂H₄ + H₂ → C₂H₆

2. Cyclohexene (C₆H₁₀) reacts with hydrogen in the presence of palladium (Pd) catalyst to produce cyclohexane (C₆H₁₂).

C₆H₁₀ + H₂ → C₆H₁₂

1.2 Halogenation

Halogenation of alkene is an addition reaction where a halide, such as Cl₂ or Br_2, gets attached to the alkene breaking the double bond between the carbon atoms. The resulting product is a vicinal (neighboring) dihalide.

Example

1. Ethene (CH₂ = CH₂) reacts with chlorine (Cl₂) in the presence of carbon tetrachloride (CCl₄), producing dichloroethane (CH₂Cl – ClCH₂).

CH₂ = CH₂ + Cl₂ → CH₂Cl – ClCH₂

1.3 Hydrohalogenation

In hydrohalogenation of alkenes, the double bond between carbon atom breaks, followed by the electrophilic addition of a hydrogen atom and halogen. Following Markovnikov’s rule, the halide gets attached to the more substituted carbon. The resultant product is a haloalkane or alkyl halide.

Examples

1. Ethene (CH₂=CH₂) reacts with HBr in the presence of CCl₄, producing bromoethane or ethyl bromide (CH₃CH₂Br).

CH₂=CH₂ + HBr → CH₃CH₂Br

2. Propene (CH₃CH=CH₂) reacts with hydrogen chloride, producing 2-chloropropane or 2-propyl chloride (CH₃CHClCH₃) as the major product and 1-chloropropane (CH₃CH₂CH₂Cl) as the minor product.

CH₃CH=CH₂ + HCl → CH₃CHClCH₃ (major product, more stable) + CH₃CH₂CH₂Cl (minor product, less stable)

Here, the two products CH₃CHClCH₃ and CH₃CH₂CH₂Cl are constitutional isomers.

1.4 Hydration

The addition of water to alkenes is known as hydration. In this reaction, the π-bond of alkene and OH bond in water breaks, producing alcohol. The hydration process usually makes many simple alcohols of alkenes in the presence of an acid catalyst.

Example

Ethene (CH₂=CH₂) reacts with water in the presence of a strong acid such as sulfuric acid (H₂SO₄) to produce ethanol (CH₃CH₂OH).

CH₂=CH₂ + H₂O → CH₃CH₂OH

1.5 Polymerization

In this addition reaction, an alkene reacts with itself to form a high‐molecular‐weight compound composed of repeating units of the original compound. Here, the alkene acts as a monomer, producing polymer by joining its repeated units to give a single product.

Example

During the polymerization of ethene (CH₂=CH₂), thousands of ethene molecules join together to make poly(ethene) or polythene (-CH₂-CH₂-CH₂– CH₂– CH₂-)_n.

[CH₂=CH₂]n  → (-CH₂-CH₂-CH₂-CH₂-CH₂-)_n

2. Oxidation

Alkenes can easily undergo oxidation under the influence of oxidizing agents, such as potassium permanganate. The products formed depend on the reaction conditions. At low temperatures and low concentrations of oxidizing reagents, alkenes tend to form glycols.

Example

Ethene (CH₂=CH₂) reacts with 1-4 % potassium permanganate (KMnO₄) solution to produce ethylene glycol (HOCH₂-CH₂OH), manganese dioxide (MnO₂), and potassium hydroxide (KOH).

3C₂H₄ + 2KMnO₄ + 4H₂O → 3HOCH₂-CH₂OH + 2MnO₂ + 2KOH

3. Ozonolysis

Ozonolysis is the oxidation of alkenes by ozone. Here, both the σ and π bonds of the alkenes get destructed. Ozone (O₃) has a dipolar ion structure which first adds to the π-bond of alkene to form a highly unstable molozonide structure and then gets rearranged to another structure by breaking of C-C σ-bond to give ozonide. It is further cleaved under a reductive atmosphere using Zn dust/H₂O to form corresponding carbonyl compounds, like aldehydes and ketones. This oxidative cleavage of an alkene double bond generally accomplishes good yield.

Example

1. Ethene (CH₂=CH₂) undergoes ozonolysis, yielding two molecules of formaldehyde (HCHO). An ozonide is formed as an intermediate product. In the presence of zinc dust and H₂O, the ozonide gets transformed into formaldehyde.

CH₂=CH₂ + O₃ → Ozonide

Ozonide → CH₂O + CH₂O

The ozonolysis reaction is often used to find the position of the double bond in an alkene molecule. For instance, the isomers of C₄H₈, 2-butene, and 1-butene, can be distinguished via oxidative cleavage. By identifying the products, one isomer can be distinguished from another, and the position of the bonds in the original compound can be determined.

2. Ozonolysis of 2-butene: Here, 2-butene (CH₃CH=CHCH₃) yields two acetaldehyde molecules (CH₃CHO) after undergoing ozonolysis.

CH₃CH=CHCH₃ + O₃ → Ozonide

Ozonide → 2CH₃COH + H₂O + ZnO

3. Ozonolysis of 1-butene: In the ozonolysis reaction of 1-butene (CH₃CH₂CH=CH₂), it produces one molecule of propanal (CH₃CH₂CHO) and one molecule of formaldehyde (HCHO).

CH₃CH₂CH=CH₂ + O₃ → Ozonide

Ozonide → CH₃CH₂COH + CH₂O

4. Other Reactions of Alkene

4.1 Oxymercuration Demercuration

In this reaction, an alkene reacts with mercuric acetate [Hg(OAc)₂] in the presence of an oxygen nucleophile, such as water or alcohol, to form an organomercury intermediate, such as hydroxyl or ethoxy mercurial compounds. The bond present between the carbon and mercury of the intermediate product gets converted to a carbon-hydrogen bond after treatment with sodium borohydride (NaBH₄). As a result of the overall reaction, the alkene converts into alcohol or ether, depending on the nucleophile used.  If the nucleophile is water, the alkene gets converted into alcohol. When the nucleophile is an alcohol, the alkene gets converted into an ether. Thus, the hydroxyl or ethoxy mercurial compounds are further reduced to alcohols or ethers, respectively.

Example

1. Propene (CH₃CH=CH₂) reacts with water and mercuric acetate (Hg(OAc)₂) followed by NaBH₄, giving isopropyl alcohol or 2-propanol (CH₃CHOHCH₃). The overall reaction is the electrophilic addition of water to the alkene following Markovnikov’s rule.

4.2 Hydroboration-Oxidation

Hydroboration is an oxidation reaction in which alkenes transform into alcohol by reacting with water. It is a two-step reaction. In the first step, diborane (B₂H₆) is added to form trialkyl boranes. The reaction proceeds in an anti-Markovnikov manner. The hydrogen from B₂H₆ attaches to the more substituted carbon, and the boron attaches to the least substituted carbon in the alkene double bond. The trialkyl borane is oxidized using an alkaline hydrogen peroxide solution in the next step. It leads to the formation of primary alcohols.

Example

1. In the hydroboration of propene (CH₃CH=CH₂), diborane (B₂H₆) is added in the first step to form trialkyl boranes [(CH₃CH₂CH₂)₃B]. The next step is the oxidation of trialkyl borane, which is done using an alkaline hydrogen peroxide (H₂O₂) solution, leading to the formation of 1-propanol (CH₃CH₂CH₂OH) along with trisodium orthoborate (Na₃BO₃).

4.3 Dihydroxylation

Dihydroxylation is when an alkene is converted into a vicinal diol. The most common and direct process for this oxidation is using a high-oxidation-state transition metal, typically osmium or manganese. The metal is often used as a catalyst, with other oxidizing agents like hydrogen peroxide (H₂O₂).

Example

1. In the dihydroxylation of 2-butene (CH₃CH=CHCH₃), it reacts with osmium tetroxide (OsO₄) in the presence of hydrogen peroxide (H₂O₂). The products depend on the type of butane used. If it is cis-2-butene, the products are meso-2,3-butanediol (CH₃CH(OH)CH(OH)CH₃. On the other hand, if it is trans-2-butene, the products are a racemic mixture of enantiomers.

Mechanism of Alkene Reaction ⁽⁵⁾

To understand the mechanism of the alkene reaction, let us take the example of the hydrohalogenation reaction. It is an electrophilic addition reaction as an electrophile initially attacks the substrate.

The fundamental mechanism of hydrohalogenation of alkenes is as follows:

1. First, the nucleophilic π-bond of the alkene breaks down as it tries to grab the electrophilic H⁺ from hydrogen halides, such as HBr.
2. Then, following Markovnikov’s rule, H⁺ gets added to the less substituted carbon atom. The more substituted carbon is now deficient in electrons, thus turning into a carbocation.
3. Finally, the halide ion (Br^–) present in the solution attacks the carbocation, forming alkyl bromide.

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