Reactions of Alkenes

In alkenes, there is one or more double bonds. There is a high density of electrons around the double bond. Therefore, when negatively charged groups (nucleophiles) come closer to the pi bond, they will be repulsed.

But when positively charged groups (electrophiles) come closer, they will be attracted by the pi bond. Then the electrons in the pi bond will be attracted to the electrophile and form a bond. Therefore, the ideal reaction type of alkenes is electrophilic addition reactions.

Reactions of alkenes

1. Bromination of alkenes
2. Reaction with hydrogen halides
3. Reaction with dilute sulfuric acid
4. Reaction with concentrated sulfuric acid
5. Reaction with basic potassium permanganate

01. Bromination of alkene

In a medium of CCl₄ (Carbon tetrachloride), Br₂ reacts with alkenes and results in alkyl halides. Two bromine atoms will be added to the two carbon atoms that are included in the double bond, respectively. In this reaction, the red color of the bromine liquid turns colorless. Therefore, this reaction can be used to identify double bonds.

Reaction mechanism of bromination of alkenes

Br₂ is a non-polar molecule. When it comes closer to the pi bond in the alkene, the electrons in the pi bond repulse the electrons in the Br₂ molecule. Thus, a dipole is induced on the Br₂ molecule (δ⁺ and δ^-). Then the pi electrons will attract the positive pole (δ⁺) of the B₂ molecule.

Here, the pi bond between the two carbon atoms (C-C) and the sigma bond between the two Bromine atoms (Br-Br) are broken down. The Bromine that was attracted by the pi electrons forms two sigma bonds with the respective two carbon atoms. The other Bromine is removed as a negatively charged ion (Br^-).

After bromine has formed two sigma bonds, it creates a positive charge on the bromine atom. Usually, bromine is a highly electronegative atom. Therefore, it takes electrons from one C-Br bond and forms an intermediate carbonium ion. (Carbocation). The carbonium ion is generated in a way that gives it the highest stability.

Finally, the positively charged carbon atom is attacked by the bromide ion in the medium. The first bromine atom that is attached to the alkene is positively charged (an electrophile). Therefore, this reaction is considered an electrophilic addition reaction.

02. Reactions of alkenes with hydrogen halides

Alkenes react with hydrogen halides (HCl, HBr, or HI) to form alkyl halides. H⁺ acts as an electrophile, and it comes from the hydrogen halide. Two carbon atoms that are included in the double bond are attached to a hydrogen and the halide, respectively.

Reaction mechanism of the addition of hydrogen halide

A hydrogen halide molecule, as an example, a hydrogen bromide molecule, is a polar molecule itself. Due to the electronegativity difference of hydrogen and bromine, hydrogen possesses a δ⁺ charge while bromine possesses a δ^- charge.

When an HBr molecule comes closer to the double bond in an alkene, the pi electrons in the double bond will attack the hydrogen atom (δ⁺ pole) of the HBr molecule. A one-carbon atom that was included in the C=C bond forms a C-H bond. Here, the H-Br bond is broken, and Br is removed as a bromide ion (Br^-).

Thus, the pi bond is broken. The other carbon gets a positive charge on it. Thus, the intermediate carbocation is formed. Then the positive carbon is attacked by the bromide ion in the medium. Finally, it results in an alkyl halide.

Markovnikov’s rule

Markovnikov's rule predicts the structure of the final product of the addition of hydrogen halide to an asymmetric alkene or alkyne. When a hydrogen halide is added to an asymmetric alkene, the pi bond is broken down, and the hydrogen atom and the halogen are added to those carbon atoms in the double bond.

Here, the hydrogen attaches to the carbon with more hydrogen atoms already present. And the halogen attaches to the carbon with fewer hydrogen atoms.

This happens due to the formation of the more stable carbocation intermediate during the reaction. A more substituted (tertiary or secondary) carbocation is more stable than a primary one, leading to the preferential formation of the major product.

As an example, let’s consider the addition of HBr to propene (CH₃-CH=CH₂)

After the pi electrons have attached to the Hydrogen atom, it could be attached to either carbon atom, which was in the double bond. If the hydrogen is attached to the middle carbon (the carbon with fewer hydrogen atoms), a positive charge is formed on the terminal carbon.

This will generate a primary carbonation, which is relatively unstable. However, if this happens, the Br^- ion will be attached to the terminal carbon (carbon with more hydrogen) and result in CH₃-CH₂-CH₂Br (1-bromopropane) as a minor product.

If the hydrogen is attached to the terminal carbon (the carbon with the most hydrogen), a positive charge is formed on the middle carbon. This will generate a secondary carbocation, which is relatively stable than a primary carbocation. Then the Br^- ion will be attached to the middle carbon atom and result in CH₃-CHBr-CH₃ (2-bromopropane) as the major product.

Anti-Markovnikov Addition

If this reaction is done in the presence of a peroxide like H₂O₂ and ROOR, we can obtain CH3-CH2-CH2Br (1-bromopropane) as the major product. Here, hydrogen attaches to the carbon with fewer hydrogen atoms. This reaction happens in a free radical mechanism.

03. Reactions of alkenes with dilute sulfuric acid (H₂SO₄)

In the presence of dilute sulfuric acid, alkenes result in alcohols. In this reaction, a water molecule is added to the alkene. Sulfuric acid acts as a catalyst. Therefore, this reaction is also known as the addition of water to the alkenes.

Reaction mechanism of the addition of water

First, the sulfuric acid dissociates and results in H⁺ ions in the medium as follows.

These H⁺ act as electrophiles. When these electrophiles come closer to the pi bond in the alkene, the electrons in the pi bond attack the H⁺ ion. Then the pi bond breaks.

According to Markovnikov’s rule, this hydrogen will be attached to the carbon with the most hydrogen atoms. Then the other carbon gets a positive charge. Thus, the intermediate carbocation is formed.

In this reaction medium, there are two types of nucleophiles. One is the sulfate ion that comes from the dissociation of sulfuric acid. The other one is water molecules in the medium. A water molecule can act as a nucleophile due to the lone pair of electrons on the oxygen atom.

So, the carbocation is attacked by the lone pair electrons in the oxygen atom in a water molecule. Thus, a dative bond is formed with oxygen and carbon. Here, the oxygen has a positive charge on it. Oxygen is a highly electronegative atom.

Therefore, it is difficult for oxygen to bear a positive charge. So, oxygen takes electrons from one of the O-H bonds and results in alcohol. In this process, a H⁺ ion is released into the medium. This H⁺ is used to regenerate the sulfuric acid.

04. Reactions of alkenes with concentrated sulfuric acid (H₂SO₄)

The reaction mechanism of the alkenes with concentrated sulfuric acid is almost similar to the reaction with dilute acid. First, the sulfuric acid dissociates as follows and results in H⁺ ions in the medium.

The H⁺ ions come from the sulfuric acid acts as an electrophile, and it is attacked by the pi electrons in the alkene. Then this hydrogen is attached to the carbon with fewer hydrogen atoms, according to Markovnikov’s rule. Then, the intermediate carbonium cation is formed.

Unlike dilute sulfuric acid, concentrated sulfuric acid has less water. Therefore, the second nucleophile that is going to attach to the positively charged carbon is the HSO₄^- ion. Therefore, in the presence of concentrated sulfuric acid, alkenes result in products as follows.

If this is added to water, it results in alcohol.

05. Reactions of alkenes with basic Potassium permanganate (OH^-/KMnO₄)

Manganate ion (MnO₄^-) is a strong oxidizing agent. In the presence of a base, alkenes are oxidized into glycols. MnO₄^- ion is reduced to MnO₂, which is brown in color. So, it can be observed that the purple color solution turns into a brown color. Therefore, this reaction is used to identify alkenes.