Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

The kind of delocalization involving sigma bond orbitals is called (a) inductive effect (b) hyperconjugation effect (c) electromeric effect (d) mesomeric effect

Step-by-Step Solution

Answer

The correct answer is (b) hyperconjugation effect.

Step 1: Identify key terms

Let's first understand the meaning of each term: 1. **Inductive effect** is the polarization of sigma bonds due to electronegativity difference between atoms. 2. **Hyperconjugation effect** refers to the delocalization of electrons in sigma bonds with adjacent empty or partially filled p-orbitals or pi-bonds. 3. **Electromeric effect** involves complete transfer of a pair of electrons under the influence of a reagent in the presence of double or triple bonds. 4. **Mesomeric effect** refers to the delocalization of electrons in pi-bond systems in conjugated systems.

Step 2: Match the concept to the definition

The exercise asks for the type of delocalization involving sigma bond orbitals. Reviewing the definitions: - Hyperconjugation involves the delocalization of sigma bond electrons into adjacent orbitals, specifically with empty or partially filled p-orbitals or pi-bonds. - The other effects involve different forms of electron distribution that do not match the delocalization of sigma bond orbitals.

Step 3: Choose the correct answer

Based on the description, the delocalization involving sigma bond orbitals is known as **hyperconjugation effect** as it precisely involves the interaction between sigma bonds and adjacent orbitals.

Key Concepts

Inductive Effect

The inductive effect is a fascinating phenomenon in organic chemistry where the electronic influence is transmitted through sigma bonds. It stems from the electronegativity differences between atoms connected in a molecule. When atoms have differing electronegativities, the more electronegative atom pulls electron density towards itself, creating a dipole. This shift results in a slight positive charge on one end and a slight negative charge on the other. Consequently, this polarization affects the chemical behavior of molecules.
There are two types of inductive effects:

• **+I Effect**: Electron-donating groups push electrons through bonds, decreasing electronegativity.
• **-I Effect**: Electron-withdrawing groups pull electrons towards themselves, increasing electronegativity.

Understanding this effect helps us predict how different substituents will affect the reactivity of functional groups in a molecule.

Electromeric Effect

The electromeric effect is a temporary effect occurring when a molecule with conjugated pi-bonds is exposed to an external reagent. This effect involves the complete transfer of a pair of electrons from one atom to another. It's essential to understand the distinction between this effect and permanent effects like the inductive or mesomeric effect.
Typically, the electromeric effect is observed in:

• **+E Effect**: The electron pair is transferred to a more electronegative atom or group.
• **-E Effect**: The electron pair is taken away from the donor atom or group and transferred to another, leading to a temporary but complete charge separation.

Recognizing when and how this effect arises is critical for predicting reaction mechanisms, especially in electrophilic addition reactions.

Mesomeric Effect

The mesomeric effect is closely related to electron delocalization across pi-bonds in a molecule's structure, which is commonly seen in conjugated systems. While similar to resonance, the mesomeric effect focuses on the electron withdrawing or donating properties of substituents.
This effect can be of two types:

• **+M Effect**: Delocalization where electrons are pushed from substituents into the conjugated system, increasing electron density.
• **-M Effect**: Delocalization where electrons are withdrawn from the conjugated system towards the substituents, decreasing electron density.

The mesomeric effect significantly influences the stability and reactivity of molecular structures, helping chemists to quickly determine resonance structures and predict the stability of intermediates in reactions.