Electrophilic Addition Alkenes

Define Addition Reaction

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. 

  • If the potassium permanganate is in an acidic state, the alkenes produce ketones after undergoing oxidation. 

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  • If the potassium permanganate is taken under a cooling aqueous state, the alkenes will get oxidized with it and produce vicinal glycols.

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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. 

FAQs (Frequently Asked Questions)

Q1. Distinguish between Electrophilic Reaction and Nucleophilic Reaction.

Ans. Even though both electrophilic reactions and nucleophilic reactions are addition reactions, they have few differences. They are explained as follows - 

  • An atom that accepts the electron pair is called an electrophilic reaction. On the other hand, an atom that donates the electron pair is known as a nucleophilic reaction.

  • The electrophilic reaction has either positively charged electrons or neutrally charged electrons but not negatively charged electrons. On the other hand, the nucleophilic reactions of having either negatively charged electrons and neutral charger electrons.

  • The electrophilic reactions are known as acidic reactions and called Lewis acids. At the same time, the nucleophile reactions are known as Lewis bases because they undergo base reactions.

  • Both electrophilic reactions and nucleophilic reactions undergo addition reactions, but the substitution reactions may occur concerning the atom. It means the electrophilic reactions undergo electrophilic substitution reactions and the nucleophile reactions undergo nucleophilic substitution reactions.

These are the four major differences the scientists had observed so far between the electrophilic reactions and nucleophilic reactions.

Q2. What are the types of Chemical Reactions?

Ans. We have different types of chemical reactions. The classifications may vary on various considering factors. Some of the basic types of chemical reactions are - 

Combination Reaction: The name itself explains that the reaction can be formed by the combination of two or more products which results in a strong product.

Decomposition ReactionIn contrast to the combination reaction, if a product splits up into two different products, then this reaction is said to be a decomposition reaction.

Oxidation-Reduction ReactionThe transfer of electrons irrespective of their number from oxidizing agent to reducing agent and vice versa, then those reactions are called Oxidation-reduction Reaction.

Acid-Base ReactionThe transfer of hydrogen ions takes place between an acid and a base; it is known as acid-base reactions. These are widely used to find pH values.