S2.2.12 Benzene (Higher Level only)

Structure of Benzene: Resonance and Delocalization

Picture a city where traffic moves so smoothly in a circular roundabout that no single car stays in one spot for long.

This is similar to how electrons behave in benzene, a molecule essential in organic chemistry due to its unusual stability and reactivity patterns.

Resonance and Delocalization in Benzene

Benzene is best described using resonance theory.

For benzene, two resonance structures exist where the double bonds shift positions around the ring.

However, benzene's true structure is better described as a hybrid of these forms where the electrons are delocalized across all six carbon atoms. This means the π-electrons from the double bonds are shared equally over the entire ring, rather than being confined to specific bonds.

Experimental Evidence for Benzene's Structure

The concept of delocalization is supported by experimental data:

• Bond Length Uniformity: X-ray diffraction reveals all carbon-carbon bonds in benzene have the same length of approximately 0.139 nm, intermediate between typical single (0.154 nm) and double (0.134 nm) bonds.
• Enthalpy of Hydrogenation: Hydrogenation of cyclohexene releases around -120 kJ/mol. If benzene contained three double bonds, its hydrogenation would be expected to release approximately -360 kJ/mol. However, the actual enthalpy change is around -208 kJ/mol, indicating extra stability due to delocalization.
• Spectroscopy Data: Benzene exhibits a single peak in the proton NMR spectrum, confirming that all hydrogen atoms are equivalent due to the symmetric electron distribution.

Resonance Energy and Benzene's Relative Unreactivity

What is Resonance Energy?

Benzene’s actual energy is lower than that predicted for a hypothetical molecule with alternating single and double bonds, indicating it is more stable than expected.

How Resonance Energy Reduces Reactivity

In reactions involving double bonds, the π-electrons are localized and available for reaction. However, in benzene, the delocalized π-electron cloud is more stable and less reactive.

This stability reduces benzene's tendency to undergo reactions typical of alkenes, such as electrophilic addition.

Evidence: Benzene vs. Alkenes

• Benzene does not decolorize bromine water under normal conditions, unlike alkenes.
• Benzene requires a catalyst (e.g., FeBr₃) for reactions with bromine, unlike alkenes that react readily.

Structural Features Favoring Electrophilic Substitution

Benzene is not completely inert. It undergoes electrophilic substitution rather than addition. But why?

Key Structural Features Supporting Electrophilic Substitution:

1. Delocalized Electron Cloud: The π-electrons create a region of high electron density, attracting electrophiles (electron-seeking species).
2. Stability Preservation: Substitution allows benzene to retain its delocalized structure, whereas addition would disrupt it.

Mechanism of Electrophilic Substitution (EAS)

Electrophilic substitution involves the following steps:

1. Generation of the Electrophile:
   • For bromination, $B{r}_2$ reacts with a catalyst like $FeB{r}_3$ to generate a positively charged $B{r}^{+}$ electrophile.
2. Electrophile Attack:
   • The electrophile attacks the benzene ring, forming a carbocation intermediate where delocalization is temporarily lost.
3. Restoration of Aromaticity:
   • The intermediate rapidly loses a proton ($H^{+}$) to restore aromaticity.

Typical Electrophilic Substitution Reactions:

• Nitration: Benzene + $HN{O}_3$ (catalyst: $H_{2}S{O}_4$) → Nitrobenzene
• Halogenation: Benzene + $B{r}_2$ (catalyst: $FeB{r}_3$) → Bromobenzene
• Friedel-Crafts Alkylation: Benzene + $C{H}_{3}Cl$ (catalyst: $AlC{l}_3$) → Methylbenzene

Reflection