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.).
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
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.
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 |
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:
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:
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:
