Question

The kind of delocalization involving sigma bond orbitals is called: (a) Inductive effect (b) Hyperconjugation effect (c) Electromeric effect (d) Mesomeric effect

Short answer

The answer is (b) Hyperconjugation effect.

Step-by-step solution

Identify the Concept

Review the concept of orbital delocalization related to sigma (\( \sigma \)) bond electrons. Delocalization occurs when electrons are not confined to one bond or atom but can move over multiple areas.

Understand the Choices

Examine each of the given options: (a) Inductive effect involves electron delocalization due to differences in electronegativity along a sigma bond. (b) Hyperconjugation effect involves delocalization involving sigma bond electrons adjacent to a pi bond or a carbocation. (c) Electromeric effect refers to electron delocalization in pi bonds in response to a reagent. (d) Mesomeric effect involves pi electron delocalization over several atoms, typically in resonance.

Match Concept with Choices

Hyperconjugation involves the delocalization of electrons from a sigma bond, which fits the description of the exercise question. This makes option (b) the most likely candidate as it specifically involves sigma bond electrons.

Key concepts

Sigma Bond Electrons

Sigma bond electrons are the electrons that reside in the sigma bonds, which are the strongest type of covalent bonds between atoms. A sigma bond (\( \sigma \)) is formed by the head-to-head overlap of atomic orbitals, typically s orbitals, p orbitals, or a combination of both. These types of bonds are crucial in organic molecules because they form the sturdy backbone that holds different atoms together.
They allow for the free rotation of bonded atoms around the bond axis, as their electron clouds overlap directly along the internuclear axis. While sigma bonds provide a stable connection, their electrons can play an interesting role in certain phenomena such as hyperconjugation, where they appear to provide stability to molecules like carbocations.
Hyperconjugation involving sigma bond electrons occurs when these electrons, typically from C-H bonds, engage in a sort of indirect interaction with a neighboring pi system or an empty p orbital adjacent to a carbocation. This interaction provides extra stabilization, which is a fascinating example of how sigma bond electrons can be "delocalized" to some extent in response to molecular structures nearby.

Delocalization

Delocalization refers to the phenomenon where electrons are not confined to a single bond or atom but instead are distributed over several atoms, giving rise to an overall stabilization effect in molecules. This is a key concept in understanding molecule stability, especially in organic chemistry.
In hyperconjugation, for example, delocalization involves sigma bond electrons, specifically from C-H bonds, interacting with pi bonds or vacant p orbitals.

• Delocalization reduces electron density in particular areas, spreading it across the molecule and lowering the energy.
• This makes the molecule more stable than it would be if all electrons were localized.
• Delocalization is often visualized using resonance structures in molecules.

Besides hyperconjugation, delocalization can also involve pi electrons, as seen in resonance, where pi electrons are shared across multiple atoms.

Carbocation

A carbocation is an organic ion that carries a positive charge on a carbon atom. This area of positive charge occurs when a carbon atom lacks a complete set of electrons, having only six instead of the usual eight needed to satisfy the octet rule. Carbocations are usually highly reactive intermediates in many organic reactions such as substitutions and eliminations.
They can gain stability through electrophilic interactions and hyperconjugation.

• In hyperconjugation, carbocations are stabilized through the delocalization of electrons from adjacent sigma bonds (often C-H bonds).
• This interaction involves the overlap of the empty p orbital on the positively charged carbon with the sigma bond electrons, lending some electron density into the carbocation, thereby lowering its instability.
• Recognizing carbocations is important in predicting the reactivity and outcome of chemical reactions.

The ability of carbocations to be stabilized through hyperconjugation highlights the fascinating connection and critical role these concepts play in organic chemistry.
This understanding helps in predicting reaction pathways and the stability of intermediate structures in various chemical processes.