[Chemistry Class Notes] on Electrophilic Addition Alkenes Pdf for Exam

In chemistry, we have several kinds of elements like – alkenes, alkynes, etc. Each kind exhibits different behaviour and has distinct properties. Similarly, alkenes undergo various reactions. We will learn more about what are alkenes, how it undergoes various reactions etc.

An addition reaction is a reaction that forms a strong molecule by interacting with two or more molecules. The resultant molecule is called the adduct. Generally, we have two types of addition reactions in organic chemistry. One is an electrophilic addition reaction, and the other is a nucleophilic addition reaction.

Electrophilic Addition Reaction

Alkenes are a group of hydrocarbons where each molecule contains a double bond at least. Due to this double bond, the alkenes undergo an addition reaction. The addition reaction occurs when an electrophile attacks with the double bond of carbon atoms with the help of pi electrons present in the alkenes, then the reaction is said to be an electrophilic addition reaction of alkenes.  The electrophilic addition reaction has a mechanism that can be explained below. Also, it follows a free radical mechanism at times. 

Electrophilic Addition Reaction Mechanism

As we already understood, alkenes exhibit addition reactions to a great extent. The hydrogen bromide and hydrogen chloride in which the addition of hydrogen halides takes place was the simplest example for understanding the electrophilic addition reaction mechanism. Because the hydrogen halides have both protons and halides, we call these protons electrophiles and halides nucleophiles.

In the electrophilic addition, the initial step is to attack an electrophile on the carbon-carbon double bond, which exerts a set of electrons. This step is known as the deprotonation step. Hence the released electrons were attached to the molecule. Now, it has only a single carbon-carbon bond with a positive charge. This is what we call the process of carbocation. During the next step, the halide will get attached to the carbocation, which results in original hydrogen bonds. If we use the correct nucleophile, it will produce a new molecule.

Generally, we can represent the hydrogen halides as 

 HI >HBr> HCl. 

Markonikov Rule

A scientist called Markovnikov had introduced a prediction rule. This rule is known as the Markovnikov rule. According to this rule, one can predict the end product of the reaction. In most of the reactions, the resultant molecule will process less number of hydrogen atoms if it’s a negative part may get attached to the carbon atom. As the adding molecule contains both a negative part and a positive part, it explains only the negative part of the adding molecule. 

Using this rule, it is easier to predict the end product for symmetrical alkenes than unsymmetrical alkenes. Here, the symmetrical alkene is ethane, and the unsymmetrical alkene is propane.

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Few examples for electrophilic addition reactions of alkenes mechanism are – 

• Hydrogenation

• Cyclopropanation

• Halogenation

• Oxidative Cleavage

• Hydration

• Hydroxylation

• Epoxidation

• Halohydrin Formation

Illustration

Let us observe the electrophilic reaction with the help of hydrogen bromide. During the formation of the carbocation process, the hydrogen bromide attacks with the carbon-carbon double bond. It results in the generation of positive charge H+. 

As we already know that the secondary carbocation has more stability than that of the primary carbocation; the bromide ion attacks the carbocation atoms and results in the formation of alkyl halides. 

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Formation of Ketones and Alcohols Using Electrophilic Addition Reactions

Yes, the formation of Ketones and Alcohols will occur if the electrophilic addition reactions occur in the oxidizing state. We can use potassium permanganate to produce both ketones and alcohol. Let’s have a glance at them. 

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Conclusion

Hence these are the various chemical reactions that can be formed using alkenes, especially in electrophilic addition reactions. Every group of elements will react uniquely when it undergoes various reactions at various states. As we have observed in oxidization, it may vary in other states with other chemicals at different temperatures. So one should understand the concept and importance of each reaction before performing it.