Mechanism of electrophilic addition in alkenes with Markovnikov rule and examples
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 on Electrophilic Addition Reactions of Alkenes Explained
1. What is electrophilic addition in alkenes?
Electrophilic addition is a reaction in which an electrophile adds across the carbon–carbon double bond (C=C) of an alkene to form a saturated product. In this reaction:
- The π bond of the alkene acts as a region of high electron density.
- An electrophile (electron pair acceptor) attacks the double bond.
- The double bond breaks and two new single bonds are formed.
2. Why do alkenes undergo electrophilic addition reactions?
Alkenes undergo electrophilic addition because their π bond is electron-rich and attracts electrophiles. The C=C double bond consists of:
- One strong σ bond
- One weaker, exposed π bond
3. What is the mechanism of electrophilic addition to alkenes?
The mechanism of electrophilic addition to alkenes occurs in two main steps: formation of a carbocation intermediate followed by nucleophilic attack. The steps are:
- Step 1: The electrophile attacks the π bond, forming a carbocation.
- Step 2: A nucleophile attacks the carbocation, forming the final product.
- CH3–CH=CH2 + H+ → CH3–C+H–CH3
- CH3–C+H–CH3 + Br- → CH3–CHBr–CH3
4. What is Markovnikov’s rule in electrophilic addition?
Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms already attached. This occurs because:
- The reaction proceeds via the most stable carbocation.
- More substituted carbocations are more stable.
5. What happens when bromine reacts with an alkene?
When bromine reacts with an alkene, it undergoes electrophilic addition to form a vicinal dibromide. The reaction involves:
- Formation of a cyclic bromonium ion intermediate.
- Attack by Br- on the intermediate.
6. How does hydrogen halide (HX) add to an alkene?
Hydrogen halides (HCl, HBr, HI) add to alkenes by electrophilic addition to form haloalkanes. The process involves:
- Protonation of the double bond to form a carbocation.
- Attack by the halide ion (Cl-, Br-, I-).
7. What is the product of acid-catalyzed hydration of an alkene?
Acid-catalyzed hydration of an alkene produces an alcohol by addition of water across the double bond. The reaction requires a strong acid such as H2SO4 as a catalyst and follows Markovnikov’s rule.
- Alkene + H2O (in presence of H+) → Alcohol
8. What is the difference between electrophilic addition and nucleophilic addition?
The key difference is that electrophilic addition involves attack by an electrophile on an electron-rich double bond, while nucleophilic addition involves attack by a nucleophile on an electron-deficient carbon. In summary:
- Electrophilic addition: Common in alkenes (C=C).
- Nucleophilic addition: Common in carbonyl compounds (C=O).
- Electrophilic addition often forms carbocations; nucleophilic addition involves partial positive carbon atoms.
9. What factors affect the rate of electrophilic addition in alkenes?
The rate of electrophilic addition depends mainly on alkene stability, carbocation stability, and the strength of the electrophile. Important factors include:
- Substitution level: More substituted alkenes react faster due to stable carbocations.
- Electrophile strength: Stronger electrophiles increase reaction rate.
- Solvent effects: Polar solvents stabilize intermediates.
10. Can you give an example of an electrophilic addition reaction with a balanced equation?
A classic example of electrophilic addition is the reaction of ethene with hydrogen bromide to form bromoethane. The balanced equation is:
- CH2=CH2(g) + HBr(g) → CH3–CH2Br(l)
