Chemical Bonding: Localized and Delocalized Bonds & Electrons

 

Chemical Bonding: Localized and Delocalized Bonds & Electrons

Introduction to Chemical Bonding

Chemical bonding is a fundamental concept in chemistry that explains how atoms combine to form molecules. Bonds are formed when atoms share or transfer electrons to achieve a stable electronic configuration, often resembling the electron configuration of noble gases.

Hyperconjugation (No-bond Resonance)

 

Hyperconjugation (No-bond Resonance)

There is another type of delocalization, which involves σ-electrons called Hyperconjugation.

It may be regarded as a σ-π orbital overlap analogous to the π-π orbital overlap.

In hyperconjugation, the sigma C-H bond on the alpha carbon is delocalized with the empty p-orbital of a C= or a carbocation. As a result, H⁺ does not change its position.

Resonance/Mesomeric Effect:

 

Resonance/Mesomeric Effect:

The resonance or mesomeric effect refers to the polarity produced in a molecule as a result of interaction two π-bonds or between a π-bond and a lone pair of electrons. This effect operates through π-electron delocalization and is transmitted along a chain of conjugated bonds.

Or

This is the movement of electrons in a molecule where the electrons can shift between atoms or bonds. This shift of electrons creates resonance structures, leading to stabilization.

 

Resonance Concept and its rules

 

Resonance Concept:

The process in which different Lewis structures can be written for a compound which involve identical positions of atoms is called resonance. The actual structure of a compound is considered to be a weighted average of all the contributing structures. The representation of real structures as a weighted average of two or more contributing structures is called resonance. These structures are also called resonance contributing structures or canonical forms. The actual structure is a resonance hybrid of all these structures. The resonance hybrid resembles each of the contributing structures but is identical to none of them.

Representation

A double-headed arrow (↔) is placed between each pair of contributing structures. For example, there are various contributing structures of benzene:

*Introduction to Electromeric Effect*

 

Electromeric Effect

*Introduction to Electromeric Effect*

- The *Electromeric Effect* is a temporary effect that occurs in organic compounds when an attacking reagent interacts with the molecule. It leads to the *complete transfer of a pair of π electrons* from one atom to another in the molecule.

- This effect is significant in the presence of multiple bonds (such as C=C or C=O), and is reversed once the attacking reagent is removed.

Inductive effect applications

 

Negative Inductive Effect (-I Effect):

   - When an *electron-withdrawing group (EWG)* is attached to a carbon chain, it pulls electron density away from the chain. This leads to a decrease in electron density along the chain.

   - Example: ( C - C ^δ+++ -- C ^δ++ -- C ^δ+-- A ^δ-) 

     - Here, A is an electron-withdrawing group, causing a partial positive charge buildup on the adjacent carbons as electron density is pulled towards A.

 *-I Effect Order:* 

*Halogens:* 

- F > Cl > Br > I 

*Inductive Effect in Organic Chemistry*

 

General Concepts of Organic Chemistry

Organic Chemistry:

“Branch of chemistry which deals with carbon compounds and its derivatives.”

Carbon (abundant ) → Tetravalent (4 bonds)

Organic Reactions:

Chemical reactions that are undergone by organic compounds .

Factors that determine the reactivity of organic reactions:

• Inductive effect
• Hyperconjugation
• Resonance (Mesomeric effect)
• Electromeric effect