Benzene Structure and Stability

This page delves into the unique structure of benzene, explaining why it differs from what one might expect based on its molecular formula.

The stability of benzene is discussed through three key points:

1. Lack of reactivity: Benzene does not undergo typical alkene reactions like decolorizing bromine water, indicating its unique structure.

2. Intermediate bond lengths: X-ray diffraction studies show that all bonds in benzene are of equal length, falling between single and double bond lengths.

3. Hydrogenation enthalpies: The actual hydrogenation enthalpy of benzene is 152 kJ/mol less than expected, demonstrating its enhanced stability.

Vocabulary: Kekulé's model refers to the cyclic structure of benzene proposed by August Kekulé, representing it as a six-membered ring with alternating single and double bonds.

Highlight: Benzene's stability is 152 kJ/mol greater than what would be expected for a cycloalkene structure, indicating its unique aromatic character.

Nitration of Benzene

This page details the benzene electrophilic substitution mechanism for nitration, a classic example of aromatic electrophilic substitution.

The nitration reaction occurs under the following conditions:

• Temperature: 50°C
• Catalyst: Concentrated H₂SO₄

The mechanism proceeds in three steps:

1. Formation of the electrophile (NO₂⁺): HNO₃ + H₂SO₄ → H₂O + NO₂⁺ + HSO₄⁻
2. Attack of the electrophile on the benzene ring
3. Loss of a proton to restore aromaticity: H⁺ + HSO₄⁻ → H₂SO₄

Example: The nitration of benzene mechanism is a prime illustration of electrophilic aromatic substitution, where the nitro group (NO₂) replaces a hydrogen atom on the benzene ring.

Highlight: The use of concentrated sulfuric acid as a catalyst is crucial for generating the electrophilic nitronium ion (NO₂⁺).

Nitration of Phenol and Reactivity Comparison

This page covers the nitration of phenol and compares the reactivities of benzene and phenol.

Nitration of phenol:

• Occurs with dilute nitric acid, unlike benzene which requires concentrated acid and a catalyst
• Produces a mixture of 2-nitrophenol and 4-nitrophenol

Reactivity comparison:

1. Benzene requires harsh conditions (concentrated acids, catalysts) for electrophilic substitution.
2. Phenol reacts under milder conditions without catalysts.

Vocabulary: Ortho and para directors are substituents that direct incoming electrophiles to positions 2 and 4 on the benzene ring, respectively.

Highlight: The lone pair of electrons on the oxygen atom in phenol's -OH group is partially donated into the π-system, increasing the electron density and reactivity of the aromatic ring.