Stepwise Guide: Electrophilic Addition Mechanism & Markovnikov’s Rule
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.
[Image will be Uploaded Soon]
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.
[Image will be Uploaded Soon]
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.
[Image will be Uploaded Soon]
If the potassium permanganate is taken under a cooling aqueous state, the alkenes will get oxidized with it and produce vicinal glycols.
[Image will be Uploaded Soon]
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 in Alkenes Made Easy
1. What is an electrophilic addition reaction in alkenes?
An electrophilic addition reaction is a characteristic chemical reaction of alkenes. In this process, the pi (π) bond of the carbon-carbon double bond is broken, and two new sigma (σ) bonds are formed by the addition of an electrophile. The electron-rich double bond attracts an electron-deficient species, which is the electrophile, thereby initiating the reaction.
2. What is Markovnikov's rule for electrophilic addition?
Markovnikov's rule governs the addition of unsymmetrical reagents (like HBr) to unsymmetrical alkenes. It states that the negative part of the adding molecule attaches to the carbon atom of the double bond that has fewer hydrogen atoms. This rule is based on the formation of the most stable carbocation intermediate during the reaction mechanism.
3. Can you give an example of an electrophilic addition mechanism?
A classic example is the addition of hydrogen bromide (HBr) to propene, which occurs in two main steps:
- Step 1: The electrophile (H⁺ from HBr) attacks the pi electron cloud of propene, forming a more stable secondary carbocation.
- Step 2: The nucleophile (Br⁻) then attacks the positively charged carbon atom of the carbocation.
The final product formed according to this mechanism is 2-bromopropane.
4. Why do alkenes undergo electrophilic addition reactions instead of nucleophilic ones?
Alkenes undergo electrophilic addition reactions because the carbon-carbon double bond (C=C) is a region of high electron density due to the presence of loose pi (π) electrons. This electron-rich nature makes them a target for electron-deficient species, known as electrophiles. Nucleophiles, which are also electron-rich, are repelled by the alkene's pi cloud and therefore do not typically initiate an addition reaction.
5. What is the key difference between Markovnikov's and Anti-Markovnikov's addition?
The key difference lies in the regioselectivity of the addition to an unsymmetrical alkene, which depends on the reaction conditions:
- Markovnikov's Addition: This occurs in standard additions (e.g., with HBr, HCl, H₂O). The hydrogen atom adds to the carbon with more hydrogen atoms.
- Anti-Markovnikov's Addition: The hydrogen atom adds to the carbon with fewer hydrogen atoms. This is specifically observed in the addition of HBr in the presence of peroxide, which follows a free-radical mechanism instead of an ionic one.
6. What is the significance of the carbocation intermediate in these reactions?
The carbocation intermediate is crucial because its stability determines the major product of the reaction. Electrophilic addition proceeds through the formation of the most stable possible carbocation (tertiary > secondary > primary). This principle is the electronic basis for Markovnikov's rule. If a carbocation rearrangement to a more stable form is possible, it will occur before the nucleophile attacks, leading to a rearranged product.
7. How does the electrophilic addition of bromine (Br₂) to an alkene test for unsaturation?
The addition of bromine water (an aqueous solution of bromine, which is reddish-brown) is a common chemical test for unsaturation. When added to an alkene, an electrophilic addition reaction occurs, breaking the C=C double bond and forming a vicinal dihalide (e.g., 1,2-dibromoethane). This product is colourless. The rapid disappearance of the reddish-brown colour of the bromine water indicates the presence of a double or triple bond in the compound.
8. What is the difference between an electrophilic reaction and a nucleophilic reaction?
The fundamental difference is based on the attacking species and the substrate:
- In an electrophilic reaction, an electron-deficient species (electrophile) attacks an electron-rich substrate, such as the pi bond in an alkene.
- In a nucleophilic reaction, an electron-rich species (nucleophile) donates an electron pair to an electron-deficient substrate, such as the carbon atom in an alkyl halide.
