Problem 122 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\left(\mathrm{H}_{3} \mathrm{C}\right)_{2} \mathrm{C}=\mathrm{CHCH}_{3}+\mathrm{NOBr} \longrightarrow \mathrm{A}\) The structure of the product (a)is given as (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{Br})-\mathrm{CH}(\mathrm{NO}) \mathrm{CH}_{3}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{NO})-\mathrm{CH}(\mathrm{Br}) \mathrm{CH}_{3}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\mathrm{C}(\mathrm{NO})(\mathrm{Br}) \mathrm{CH}_{3}\) (d)

Step-by-Step Solution

Answer

The correct product is (a) \((\mathrm{CH}_{3})_{2}\mathrm{C}(\mathrm{Br})-\mathrm{CH}(\mathrm{NO})\mathrm{CH}_{3}\)."

Step 1: Identify the Reaction Type

The given reaction is between an alkene and an addition reagent. This suggests that the reaction is an electrophilic addition. In such reactions, the electrophile (NOBr) attacks the double bond of the alkene.

Step 2: Determine the Electrophile and Nucleophile

In NOBr, the nitrogen atom with a slight positive charge serves as the electrophile, while the bromine behaves as the nucleophile. The electron-rich double bond of the alkene will attack the electrophile.

Step 3: Markovnikov's Rule Application

According to Markovnikov's rule, in the addition of HX (or equivalent) to an asymmetric alkene, the hydrogen (or the equivalent electrophile, in this case, NO) adds to the carbon with more hydrogen atoms. Here, NO will attach to the carbon atom with the most hydrogens.

Step 4: Determine the Product Structure

Applying Markovnikov's rule, NO adds to the carbon with the more hydrogen atoms, which in this case is the terminal carbon of the double bond, resulting in the bromine substituting on the other carbon atom.

Step 5: Finding the Correct Product

With NO on the terminal carbon and Br on the internal carbon, we look at the given options:- Option (a) has Br attached to the internal carbon and NO to the terminal carbon, matching our prediction.- Option (b), (c), and (d) do not fit this arrangement.So the correct product is (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}(\mathrm{Br})-\mathrm{CH}(\mathrm{NO}) \mathrm{CH}_{3}\).

Key Concepts

Markovnikov's Rule

Markovnikov's Rule is a fundamental principle in organic chemistry. It helps predict the outcome of addition reactions involving alkenes. In simpler terms, when an addition reaction occurs with an unsymmetrical alkene, the electrophile (often represented as the positive part of a reacting molecule) will attach to the carbon atom that has the greater number of hydrogen atoms.
This rule stems from the stability of carbocations that form during the reaction. A more substituted carbocation is typically more stable, meaning it will form more readily. Thus, the rule guides us in determining which atom will attract the additional electrophile, like in the reaction involving alkenes and NOBr.

• Predicts where electrophiles add to alkenes.
• Based on carbocation stability.
• Electrophile attaches to the carbon with more hydrogens.

Alkene Reactions

Alkenes are hydrocarbons characterized by at least one carbon-to-carbon double bond. This double bond is a hub of activity, making alkenes highly reactive in certain conditions, particularly during electrophilic addition reactions.
With electrophilic addition, the double bond opens up to allow new atoms or groups to add across the former double bond. This process occurs because the electrons in the double bond are attracted to an incoming electrophile. The reaction often follows Markovnikov's Rule to determine the resulting structure of the molecule.
Typical reactions involving alkenes include:

• Hydrogenation (adding hydrogen)
• Halogenation (adding halogens like bromine or chlorine)
• Hydrohalogenation (adding HX, where X is a halogen)

Nucleophile and Electrophile Identification

An essential aspect of understanding reactions, especially with alkenes, is identifying nucleophiles and electrophiles. A nucleophile is an electron-rich species that is attracted to positively charged or electron-deficient areas, making it a donor in chemical reactions. On the other hand, an electrophile is an electron-poor species that seeks out electrons, acting as an acceptor.
In the given reaction, NOBr serves as both electrophile and nucleophile. The nitrogen part of the molecule, with a partial positive charge, acts as the electrophile. It seeks out the electron-rich double bond in the alkene. Meanwhile, the bromine in NOBr, which can share its electrons, works as the nucleophile after the initial electrophilic attack occurs. Identifying these roles is crucial for predicting the course and products of chemical reactions.

• Nucleophiles are electron-rich species.
• Electrophiles are electron-poor and seek electrons to form bonds.
• Correct identification helps in predicting how reactions progress.