Aromatic Compounds

What is an aromatic compound?

• organic compound

• conjugated ring system

• delocalized π-electrons

• Follows Hückel’s rule for aromaticity

What is the simplest aromatic compound?

Benzene (C₆H₆)

What is Hückel’s Rule?

• A molecule is aromatic if it has

• (4n + 2) π-electrons

Are aromatic compounds stable?

• Yes, they are stable

• Stability comes from electron delocalization

Give two common examples of aromatic compounds

Benzene • Toluene

What is the structure of benzene?

• Six-membered carbon ring

• Alternating double bonds

  • (delocalized π-system)

• All C–C bond lengths are equal

What is meant by delocalized electrons in aromatic rings?

• Electrons are spread over the whole ring

• Not confined to a single bond

• Contributes to resonance stability

• electrons are delocalized (spread out)

  • molecule is lower in energy + more stable

• spreading out of electrons reduces repulsion

What happens when benzene undergoes a reaction?

• undergoes electrophilic aromatic substitution (EAS)

  • Substitution preserves aromaticity

• Doesn’t favor addition reactions

What are ortho-, meta-, and para- positions?

• Ortho: positions 1,2

• Meta: positions 1,3

• Para: positions 1,4

What are some common electrophilic aromatic substitution reactions?

• Nitration:

  • benzene + HNO₃ → nitrobenzene (in presence of H₂SO₄)

• Halogenation:

  • benzene + Cl₂/Br₂ → chlorobenzene / bromobenzene (w/ FeCl₃/FeBr₃)

• Sulfonation:

  • benzene + SO₃ → benzenesulfonic acid (in H₂SO₄)

• Friedel–Crafts alkylation:

  • benzene + R–Cl → alkylbenzene (with AlCl₃)

• Friedel–Crafts acylation:

  • benzene + RCOCl → acylbenzene (with AlCl₃)

Why is benzene more stable than expected from its structure?

• Due to resonance energy

  • Delocalized π-electrons lower energy

• More stable than a typical triene

What makes a compound aromatic, antiaromatic, or non-aromatic?

• Aromatic: cyclic, planar, fully conjugated

  • (4n + 2) π-electrons

• Antiaromatic: cyclic, planar, fully conjugated

  • 4n π-electrons → unstable

• Non-aromatic: doesn’t meet all of this criteria

What is heteroaromaticity?

• Aromatic ring contains non-carbon atoms (like N, O, or S)

  • Eg. pyridine

Can aromatic compounds have substituents?

• Yes

• Substituents affect reactivity + orientation of new groups

• Can be activating or deactivating

• Can direct to ortho/para or meta positions

Why doesn’t benzene undergo typical alkene addition reactions?

• Aromatic rings are super stable

  • they have a special kind of electron sharing (called delocalization)

    • makes them very hard to break.

• Addition reactions break double bonds

  • atoms added by breaking 2ble bonds in the ring.

• Breaking the double bonds ruins the stability

  • stops the electrons from being shared around the whole ring

  • ring loses its aromaticity (and stability).