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Orbital Picture, Resonance in Benzene, Aromatic Characters, Huckel’s Rule

Observations of the benzene molecule, which are all planar, bond to three other atoms, and all bond angles are 120 degrees.

Orbital Picture

Observations of the benzene molecule, which are all planar, bond to three other atoms, and all bond angles are 120 degrees, indicate that sp 2 hybridization occurs within the molecule. A carbon atom with sp2 hybridization has an unhybridized atomic p orbital on its surface. By overlapping the sp 2 hybrid orbitals, the ring would be held together with σ atom bonds, but by overlapping the atomic p orbitals, the system goes through complete delocalization. The molecule is highly stable as a result of this complete delocalization.

Six atomic orbitals that overlap will generate six orbitals in the molecule, according to molecular orbital theory. The bonding orbitals makes up three of these, while the antibonding orbitals makes up the other three.

A bonding combination is one with π 1, π 2, and π 3, while an antibonding combination, denoted by π 4 , π 5 , and π 6 , is one with no bonding. Similarly, the bonding orbitals of atoms (π2 and π 3) as well as the antibonding orbitals of atoms (π 4 and π 5) have the same energy. They are called degenerate orbitals.

Molecular stability is very high due to the electrons in bonding orbitals. Furthermore, all the bonding orbitals are filled and all the π electrons have paired spins, so the stability is increased. A molecule which possesses all of these properties is said to have a closed bond shell of delocalized electrons. Delocalized electrons in closed bond molecules such as benzene give these molecules excellent stability and a large resonance energy.

Resonance in Benzene

In modern instrumental studies, it has been confirmed that benzene bonds are all of the same length, or approximately 1.40 pm. Exactly halfway between the lengths of a carbon-carbon single bond (1.46 pm) and carbon-carbon double bonds (1.34 pm), a picometer = 1 × 10 −12 meter. In addition, these studies demonstrate that all bond angles on the benzene molecule are equal (120°) and that its structure is planar (flat). Benzene's structure can be described by resonance theory and molecular orbital theory.

For the same molecule, there are many possible structures, but according to resonance theory, none are correct. The drawn structures are all made up of the true structure, which is the most stable. The more structures that a molecule can have, the more stable the hybrid structure will be. In a hybrid structure, resonance energy is the difference between the computed energy and the actual energy. The resonance energy of compounds increases with their stability. Two Kekulé structures can be used to represent the benzene molecule. Both structures are based on strong resonances.

This hybrid structure could be represented by the following

Electrons move throughout the entire molecule as shown by the circle. Also, conjugated diene systems exhibit delocalized electrons (electrons within molecular orbitals). Diene conjugated systems are stable like benzene. Due to its 1,3,5-cyclohexatriene structure, the benzene molecule is more stable than it would otherwise appear. The resonance energy of a molecule (36 kcal/mole) comes from its extra stability.

Aromatic Characters

Compounds that exhibit resonance delocalization of π -electron resonances, such as benzene, which exhibit special stability.

Naming the Aromatic Characters

A large number of names are not systematic

It is usually called -benzene as a parent name for mono-substituted benzenes, similar to hydrocarbons

It is referred to as a phenyl group when the benzene ring is attached to a parent chain. In the presence of more than six carbons in the parent chain, the benzene ring is considered a substitute. Where the longest alkyl chain substitute has six or fewer carbons, the benzene ring is the parent

Huckel's Rule

According to Huckel's Rule, you can determine if a molecule is aromatic, antiaromatic, or nonaromatic by using the number of π electrons (N) and the physical structure of the ring system.

In an aromatic system, you can determine the number of π electrons by using the following algorithm:

N = 4n + 2

n here is an integer.

Based on the following algorithm, one can determine how many AB electrons there are in an antiaromatic system:

N = 4n

n here is an integer

Nonaromatic compounds are those that have no continuous ring of conjugated p orbitals in a plane conformation. In determining if a ringed molecule is aromatic, Huckel's Rule is a useful first step. It is worth investigating whether the ring must be planar to be aromatic.

Aromatic Ions

A molecule is neutral or charged as long as it has 4n+2 π π electrons. Cyclopentadienyl anion, for instance, belongs to the aromatic ion family. Can it be fully conjugated if it is an aromatic ion? So how do we know there is one p orbital in each atom of this molecule? Among carbons 2-5, sp2 hybridization is a result of their three attached atoms and lack of lone electron pairs. But how about carbon 1? A simpler way to determine if a molecule is sp2 hybridized is to look for the presence of lone pairs. If an atom is attached to another molecule with lone pairs, that molecule is also sp2 hybridized. In accordance with the 4n+2 rule, cyclopentadienyl anion has 6 electrons.

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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