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Organic Chemistry Unit 6: Aromatic compounds

Aromatic compounds are especially stable and react by electrophilic aromatic substitution, guided by substituents on the ring.

Covers: Benzene and aromaticity, Hückel's rule, electrophilic aromatic substitution, activating and deactivating groups, ortho/para and meta directors, nucleophilic aromatic substitution

Study guide

Benzene (C₆H₆) is a planar ring with six delocalized π electrons, making it unusually stable; it undergoes substitution rather than addition, keeping its aromaticity. A compound is aromatic if it's cyclic, planar, fully conjugated (every ring atom has a p orbital) and has 4n + 2 π electrons (Hückel's rule: 2, 6, 10, ...). Compounds meeting the first three criteria with 4n π electrons are antiaromatic and unstable; those failing the criteria are nonaromatic. Examples of aromatic species include pyridine, pyrrole, furan, the cyclopentadienyl anion and the tropylium cation. Benzene derivatives are named with ortho (1,2), meta (1,3) and para (1,4) positions.

In electrophilic aromatic substitution (EAS), an electrophile attacks the ring, forming a resonance-stabilized carbocation (the sigma complex or arenium ion), and a proton is lost to restore aromaticity. Key EAS reactions are halogenation (Br₂ with FeBr₃), nitration (HNO₃ with H₂SO₄, forming NO₂⁺), sulfonation (SO₃ with H₂SO₄), Friedel–Crafts alkylation (RCl with AlCl₃, prone to rearrangement and polyalkylation) and Friedel–Crafts acylation (RCOCl with AlCl₃, no rearrangement). Friedel–Crafts reactions fail on strongly deactivated rings. Nitro groups can be reduced to amines, and acyl groups can be reduced to alkyl groups.

Substituents affect both rate and position. Activating groups donate electron density and speed up EAS; they direct ortho/para (–OH, –OR, –NH₂, alkyl groups). Deactivating groups withdraw electron density and slow EAS; most direct meta (–NO₂, –CN, –SO₃H, carbonyl groups, –CF₃). Halogens are the exception: they're deactivating but ortho/para directing. When planning a synthesis, the order of steps matters because existing groups direct the next substitution. Nucleophilic aromatic substitution can occur on rings with strong electron-withdrawing groups ortho or para to a leaving group.

Treating an aryl amine with NaNO₂ and HCl at about 0 °C gives an arenediazonium salt (ArN₂⁺). N₂ is an excellent leaving group, so it can be replaced by Cl, Br or CN using copper(I) salts (Sandmeyer reactions), by OH by warming in water, by F using HBF₄ and heat, or by H using H₃PO₂. This lets chemists make substitution patterns that directing effects alone can't, such as meta-halophenols. SNAr goes by addition–elimination: the nucleophile adds to the ring carbon bearing the leaving group, forming a resonance-stabilized anion (a Meisenheimer complex), and then the leaving group leaves. Electron-withdrawing groups ortho or para to the leaving group speed it up. Fluoride is the best leaving group here, because the addition step is rate-determining.

In pyridine, the nitrogen lone pair sits in an sp² orbital outside the aromatic π system, so pyridine is a weak base. In pyrrole, the lone pair is part of the aromatic sextet, so protonating it would destroy aromaticity and pyrrole is not basic. Imidazole is protonated on its pyridine-like nitrogen. Ortho/para directors give mixtures, but bulky substituents or bulky electrophiles block the ortho positions, so the para product dominates.

Hot KMnO₄ oxidizes any alkyl side chain with a benzylic hydrogen to a carboxylic acid (–COOH). NBS with light or peroxide brominates the benzylic position selectively, because the benzylic radical is resonance-stabilized.

Key ideas

• Aromatic rings are cyclic, planar, conjugated and have 4n + 2 π electrons.
• EAS replaces an H while keeping the ring aromatic.
• Activating groups speed EAS and direct ortho/para.
• Most deactivating groups direct meta; halogens direct ortho/para.
• The order of steps matters in aromatic synthesis.

Formulas and rules

Common mistakes

• Counting electrons in an sp³ atom; it breaks conjugation, so the ring isn't aromatic.
• Running Friedel–Crafts on a nitrobenzene ring; strongly deactivated rings don't react.

Words to know

Aromatic

Cyclic, planar, conjugated with 4n + 2 π electrons.

Antiaromatic

Cyclic, planar, conjugated with 4n π electrons; unstable.

Hückel's rule

Aromatic rings have 4n + 2 π electrons.

Ortho

Positions 1 and 2 on a benzene ring.

Meta

Positions 1 and 3 on a benzene ring.

Para

Positions 1 and 4 on a benzene ring.

Electrophilic aromatic substitution

An electrophile replaces a hydrogen on an aromatic ring.

Sigma complex

The carbocation intermediate in EAS.

Nitration

Adding an NO₂ group using HNO₃ and H₂SO₄.

Friedel–Crafts alkylation

Adding an alkyl group with RCl and AlCl₃.

Friedel–Crafts acylation

Adding an acyl group with RCOCl and AlCl₃.

Activating group

A substituent that speeds up EAS.

Deactivating group

A substituent that slows down EAS.

Ortho/para director

A substituent that directs new groups to ortho and para positions.

Meta director

A substituent that directs new groups to the meta position.

Pyridine

An aromatic six-membered ring containing nitrogen.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.