Nomenclature of Alkanes (IUPAC Rules)

 

Nomenclature of Alkanes (IUPAC Rules):

The IUPAC (International Union of Pure and Applied Chemistry) system provides systematic rules for naming alkanes and other organic compounds. Here are the basic rules for naming alkanes:

Rule 1: Identify the longest continuous carbon chain in the molecule. This chain is referred to as the "parent chain," and its length determines the base name of the molecule (e.g., methane, ethane, propane, etc.).

Introduction to Alkanes and their general properties

Hydrocarbons

The branch of chemistry which deals with the carbon and hydrogen derivatives is called hydrocarbons.

They can be classified into two broad categories:

Aliphatic: These are open-chain molecules, meaning the carbon atoms are arranged in straight or branched chains. They do not contain aromatic rings.

Aromatic: These hydrocarbons have at least one benzene ring (a ring of six carbon atoms with alternating double and single bonds). Benzene rings provide special stability due to the delocalization of $-$electrons.

Introduction to aromaticity and Huckel's Rule

 Introduction to Aromaticity

Aromaticity is a concept in organic chemistry that explains the unusual stability and reactivity of certain cyclic compounds. Aromatic compounds exhibit unique electronic properties due to the delocalization of π-electrons across the ring structure. This delocalization imparts extra stability, making these compounds less reactive compared to their non-aromatic compounds.

Key Properties of Aromatic Compounds:

1.      Cyclic Structure:

The compound must be cyclic for the delocalization of electrons.

2.      Conjugated System:

There must be a continuous overlap of p-orbitals, allowing for the delocalization of π-electrons across the ring.

3.       Planarity:

The structure must be planar or nearly planar to maintain proper orbital overlap (all carbons are sp² hybridized).

4.      Obeying Huckel's Rule:

The compound must follow Huckel’s Rule (4n+2) , which is a key criterion for aromaticity.

Huckel's Rule

Huckel's Rule provides a mathematical framework to determine whether a compound is aromatic or not. According to the rule, a cyclic compound is aromatic if it contains (4n+2) π-electrons, where n is a non-negative integer (0, 1, 2, 3,...).

This rule applies to monocyclic conjugated systems and is essential in predicting aromatic stability.

Explanation of Huckel's Rule:

If a compound contains 2,6,10,14,… π-electrons, it follows Huckel’s Rule and is aromatic.

The most common example of a compound that follows Huckel’s Rule is benzene (C₆H₆), which has six π-electrons (n = 1).

Differentiating Aromatic, Anti-Aromatic, and Non-Aromatic Compounds

1. Aromatic Compounds:

Aromatic compounds are characterized by their high stability due to delocalized π-electrons. They follow Huckel’s Rule and are planar, cyclic compounds with a conjugated π-system.

Examples:

Benzene (C₆H₆): A six-membered ring with alternating single and double bonds. It has 6 π-electrons, satisfying Huckel’s Rule for n=1n = 1n=1.

Naphthalene (C₁₀H₈): A fused aromatic system with 10 π-electrons, following Huckel’s Rule for n=2n = 2.

2. Anti-Aromatic Compounds:

Anti-aromatic compounds are cyclic, planar, conjugated π-systems, but they contain 4n π-electrons, which leads to destabilization. Anti-aromatic compounds are highly reactive and less stable compared to both aromatic and non-aromatic compounds. If a compound contains 4,8,12,16,… π-electrons, it is anti-aromatic compounds

Examples:

Cyclobutadiene (C₄H₄): A four-membered cyclic compound with 4 π-electrons. It follows the 4n rule (n = 1) and is anti-aromatic, exhibiting instability and high reactivity. 

Cyclooctatetraene (C₈H₈): In its planar form, it has 8 π-electrons, making it anti-aromatic according to Huckel’s Rule. However, cyclooctatetraene prefers to adopt a non-planar "tub" shape to avoid anti-aromaticity.

3. Non-Aromatic Compounds:

Non-aromatic compounds do not have delocalized π-electrons, are either non-cyclic, non-planar, or non- conjugated system. They do not exhibit special stability or instability related to electron delocalization.

Examples:

Cyclohexane (C₆H₁₂): A saturated six-membered ring with no π-electrons. Since it lacks conjugation and planarity, it is non-aromatic.

Cyclooctatetraene (non-planar form): As mentioned earlier, in its non-planar form, cyclooctatetraene avoids anti-aromaticity and behaves like a non-aromatic compound.

For Double bond =   2πe­^-    For lone pair=   2πe­^-    For negative charge =   2πe­^-    For positive charge =   2πe­^-    

Table Summary of Aromaticity

Compound	π-Electrons	Type	Reason
Benzene (C₆H₆)	6	Aromatic	Follows (4n+2) Rule (n = 1)
Cyclobutadiene (C₄H₄)	4	Anti-Aromatic	Follows 4n Rule (n = 1)
Cyclohexane (C₆H₁₂)	0	Non-Aromatic	No π-electrons
Cyclooctatetraene (C₈H₈)	8	Anti-Aromatic (Planar), Non-Aromatic (Non-planar)	Depends on conformation

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: