Benzene & Phenols

You're about to dive into one of chemistry's most fascinating molecular structures. Benzene might look simple with its ring of six carbons, but its behaviour is anything but ordinary.

This unit will show you why benzene defied chemists for years and how understanding its structure revolutionised organic chemistry. You'll also discover how adding just one -OH group creates phenol, dramatically changing the molecule's reactivity.

Quick Win: Master benzene's stability concept early - it's the key to understanding everything else in this topic!

Kekulé's Model and Benzene's Stability

Kekulé originally proposed benzene as cyclo-1,3,5-hexatriene with alternating single and double bonds. Three pieces of evidence proved this model wrong and revealed benzene's true nature.

Lack of reactivity was the first clue - benzene doesn't decolourise bromine water like alkenes do. If it really had C=C bonds, it would undergo addition reactions instantly.

X-ray diffraction showed all carbon-carbon bonds are identical in length, sitting perfectly between single and double bond lengths. Finally, hydrogenation enthalpies revealed benzene is 152 kJ/mol more stable than expected - this extra stability comes from delocalisation.

Exam Tip: Remember the three pieces of evidence against Kekulé's model - they're exam favourites!

Delocalised Structure Formation

Each carbon atom uses three electrons to bond with two carbons and one hydrogen, leaving one electron in a p-orbital perpendicular to the ring. These p-orbitals overlap above and below the carbon plane, creating regions of electron density.

This overlapping forms delocalised electrons that spread across all six carbons rather than being stuck between two atoms. It's this delocalisation that gives benzene its remarkable stability and unique properties.

Think of it like spreading risk across multiple investments - the electrons are more stable when shared across the entire ring rather than confined to specific bonds.

Nitration of Benzene

Benzene undergoes electrophilic substitution rather than addition, preserving its stable ring structure. For nitration, you need concentrated nitric acid, concentrated sulfuric acid catalyst, and temperatures around 50°C.

The first step creates the electrophile: HNO₃ + H₂SO₄ → HSO₄⁻ + NO₂⁺ + H₂O. The NO₂⁺ ion is your electrophile that attacks the benzene ring.

After the electrophilic attack and substitution occurs, the catalyst regenerates: HSO₄⁻ + H⁺ → H₂SO₄. This mechanism pattern repeats for other benzene reactions.

Memory Hook: "Nitration needs NO₂⁺" - remember the positive nitronium ion is your electrophile!

Halogenation of Benzene

Benzene won't react with bromine alone because its delocalised electrons don't create enough electron density to polarise Br₂. You need a halogen carrier like FeBr₃ to generate the electrophile.

Step 1 creates the electrophile: Br₂ + FeBr₃ → FeBr₄⁻ + Br⁺. The Br⁺ ion attacks the benzene ring, substituting for a hydrogen atom.

The final step regenerates your catalyst: H⁺ + FeBr₄⁻ → FeBr₃ + HBr. Without this catalyst, benzene and bromine simply won't react - that's how stable benzene really is.

Friedel-Crafts Reactions

Friedel-Crafts alkylation adds alkyl groups using alkyl chlorides and AlCl₃ catalyst. The mechanism starts by forming a carbocation: CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻.

Friedel-Crafts acylation adds acyl groups using acyl chlorides. Step 1: CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻. The acylium ion (CH₃CO⁺) then attacks the benzene ring.

Both reactions end with catalyst regeneration when H⁺ combines with AlCl₄⁻. These reactions are brilliant for building complex molecules from simple benzene rings.

Practical Note: Acylation is often preferred over alkylation because it avoids carbocation rearrangement problems!

Alkenes vs Arenes Reactivity

Alkenes have localised π-electrons that create sufficient electron density to polarise bromine molecules directly. They undergo electrophilic addition without needing catalysts.

Arenes (benzene compounds) have delocalised π-electrons spread across the ring, creating insufficient electron density for direct bromine polarisation. They need halogen carriers and undergo electrophilic substitution to maintain ring stability.

This difference explains why alkenes readily decolourise bromine water whilst benzene doesn't react at all. The delocalisation that makes benzene stable also makes it much less reactive than alkenes.

Phenol Structure and Bromination

Phenol contains a benzene ring with an -OH group attached, making it a weak acid with Ka = 10⁻¹⁰. The oxygen's lone pair electrons interact with the benzene ring, dramatically increasing its reactivity.

Phenol readily undergoes bromination with bromine water - no catalyst needed! The reaction produces 2,4,6-tribromophenol as a white precipitate: C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr.

This reaction decolourises bromine water and forms a distinctive white precipitate, making it perfect for identifying phenol in practicals.

Lab Alert: The tribromophenol precipitate is your visual confirmation that phenol is present!

Phenol vs Benzene Reactivity

Phenol is dramatically more reactive than benzene because the oxygen's lone pair electrons donate into the π-system, increasing electron density around the ring.

For nitration, phenol only needs dilute HNO₃ at room temperature, producing both 2-nitrophenol and 4-nitrophenol. Benzene requires concentrated acids and heating.

This increased reactivity means phenol undergoes electrophilic substitution much more readily. The -OH group is an activating group that makes the benzene ring electron-rich and reactive - the complete opposite of benzene's usual reluctance to react.