Question

Resonance energy per benzene ring is maximum for: (a) Naphthalene (b) Anthracene (c) Benzene (d) Phenanthrene

Short answer

The maximum resonance energy per benzene ring is for benzene.

Step-by-step solution

Understand Resonance Energy

Resonance energy is the extra stability a molecule gains due to the delocalization of electrons in its conjugated system. It is often measured relative to a theoretical structure with localized electrons.

Identify Each Compound

Identify the structures of each compound indicated in the options: (a) Naphthalene is made up of two fused benzene rings, (b) Anthracene consists of three linearly arranged benzene rings, (c) Benzene is a single aromatic ring, (d) Phenanthrene contains three fused benzene rings in a non-linear arrangement.

Calculate Resonance Energy Per Ring

Resonance energy for benzene is approximately 36 kcal/mol. For naphthalene, the total resonance energy is about 61 kcal/mol for two rings, so 30.5 kcal/mol per ring. Phenanthrene has about 84 kcal/mol for three rings, totaling 28 kcal/mol per ring. Anthracene has 84 kcal/mol for three rings, also about 28 kcal/mol per ring.

Compare Resonance Energies

Compare the resonance energy per ring calculated for each compound: Benzene has the highest resonance energy per ring at 36 kcal/mol, followed by naphthalene, phenanthrene, and anthracene.

Key concepts

Aromatic Compounds

Aromatic compounds are a class of compounds known for their unique stability and classification by the presence of conjugated pi electrons. This characteristic makes them markedly more stable compared to other types of organic compounds. But what truly distinguishes these compounds is what is known as aromaticity.

• Aromaticity involves a specific type of electron delocalization within a cyclic structure that confers additional stability.
• For a compound to be considered aromatic, it must fit Hückel's rule, which specifies that only planar, cyclic structures with a certain number of pi electrons (4n+2, where n is an integer) can achieve this stabilizing effect.
• Another defining feature of aromatic compounds is their tendency to undergo substitution reactions rather than addition reactions, which helps preserve the aromatic ring's integrity.

Understanding these characteristics helps explain why aromatic compounds, such as benzene and its derivatives, maintain their structural framework under various chemical conditions.

Benzene Ring

The benzene ring is the prototypical example of an aromatic compound and serves as the cornerstone for understanding aromaticity. This six-carbon ring structure boasts a continuous loop of pi electrons above and below the plane of carbon atoms.

• The benzene ring is characterized by its ability to resist change. Its chemical formula is C₆H₆, everything is bonded equivalently, not alternating single and double bonds as initially thought.
• This equal bond measurement across the benzene ring is due to resonance, leading to what we describe as an overall stabilization or increased resonance energy.
• Because of its resonance, benzene can lend its stability to larger aromatic systems, spreading and thus diluting its remarkable properties when more rings are fused together.

The benzene ring's intrinsic features showcase what makes aromatic compounds so fascinating and are pivotal when discussing resonance energies across diverse aromatic structures.

Electron Delocalization

Electron delocalization in chemistry refers to the spread of electron density across adjacent atoms, particularly those involved in resonance structures. In aromatic systems, electron delocalization is a crucial component of their stability and results from overlapping p orbitals.

• This delocalization means the electrons are not confined to a single bond or atom but rather shared across multiple atoms within the aromatic ring, creating a unified electron cloud.
• The benefits of electron delocalization include enhanced stability of the molecule and reduced reactivity, which are particularly significant in aromatic compounds like benzene.
• When discussing the resonance energy of a molecule, it's the electron delocalization that plays a critical role in providing that extra measure of stability.

These effects are why structures exhibiting electron delocalization tend to be more energetically favorable than those with isolated pi bonds.

Chemical Bonding

Chemical bonding is the force that holds atoms together in any molecule, fundamentally driven by the sharing or transfer of electrons. In aromatic compounds, the nature of chemical bonding is distinct because of the electron delocalization and resonance.

• In benzene and other aromatic rings, the bonding isn't a simple alternation of single and double bonds, as it would appear in a structural representation.
• Instead, the bonds are of equal length due to resonance, signifying that the electrons involved in bonding are shared equally amongst all carbon atoms in the ring.
• This type of bonding results in unique properties like enhanced stability and particular reactivity patterns.

Understanding chemical bonding in aromatic compounds provides valuable insights into how these molecules behave and react under different chemical conditions, illustrating why aromatic compounds are so significant in organic chemistry.