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Organic Chemistry 2 Unit 3: Aromaticity and Benzene

Aromatic compounds are cyclic, planar, fully conjugated rings with 4n + 2 π electrons; this special stability explains benzene's unusual unreactivity, the acidity and basicity of many rings, and the chemistry of the benzylic position.

Covers: Structure and stability of benzene, Hückel's rule and aromatic criteria, Aromatic and antiaromatic ions, Aromatic heterocycles: pyridine, pyrrole, furan, imidazole, Naming benzene derivatives, Benzylic reactions: oxidation and bromination, Aromatic ring current in NMR

Study guide

Benzene, C₆H₆, is a planar hexagon of sp² carbons with six equal C–C bonds of about 139 pm, between a single and a double bond. Its six π electrons are delocalized over the whole ring, which is drawn as two resonance structures or as a circle in a hexagon. Benzene is much more stable than a hypothetical cyclohexatriene. Hydrogenating cyclohexene releases about 120 kJ/mol, so three isolated C=C bonds would release about 360 kJ/mol, but benzene releases only about 208 kJ/mol. The difference, about 150 kJ/mol, is benzene's resonance (aromatic stabilization) energy. As a result, benzene resists addition reactions such as Br₂ addition and instead undergoes substitution, which keeps the aromatic ring intact.

A compound is aromatic if it is (1) cyclic, (2) planar, (3) fully conjugated, with a p orbital on every ring atom, and (4) has 4n + 2 π electrons (2, 6, 10, 14, 18 …), which is Hückel's rule. If it meets the first three criteria but has 4n π electrons (4, 8 …), it is antiaromatic and especially unstable, like cyclobutadiene. If it fails any of the first three, it is nonaromatic. Cyclooctatetraene, with 8 π electrons, avoids antiaromaticity by puckering into a tub shape, so it behaves as an ordinary polyene. Ions follow the same rules: the cyclopentadienyl anion (6 π electrons) and the tropylium (cycloheptatrienyl) cation (6 π) are aromatic, while the cyclopentadienyl cation (4 π) is antiaromatic.

Aromaticity explains striking acid–base facts. Cyclopentadiene has a pKa near 16, as acidic as water, because losing H⁺ from its CH₂ gives the aromatic cyclopentadienyl anion. 7-Bromocycloheptatriene ionizes easily because it forms the aromatic tropylium cation. In heterocycles, decide whether each lone pair is part of the π system. In pyridine, the nitrogen contributes one electron to the π system through its p orbital, and its lone pair sits in an sp² orbital in the ring plane, so pyridine is a base (conjugate acid pKa about 5.2). In pyrrole, the nitrogen lone pair is part of the aromatic sextet, so pyrrole is a very weak base. Furan and thiophene use one heteroatom lone pair in the π system and keep the other in the plane. Imidazole has one pyrrole-like and one basic pyridine-like nitrogen.

Benzene derivatives are named as substituted benzenes (bromobenzene, nitrobenzene) or with common parent names: toluene (CH₃), phenol (OH), aniline (NH₂), anisole (OCH₃), benzaldehyde (CHO), benzoic acid (COOH), acetophenone (COCH₃) and styrene (CH=CH₂). For two substituents, ortho (o) means 1,2, meta (m) means 1,3 and para (p) means 1,4. With a common parent name, the parent group is C1 and the other substituents get the lowest possible numbers. As a substituent, C₆H₅– is phenyl and C₆H₅CH₂– is benzyl. Polycyclic aromatics such as naphthalene (10 π electrons) and anthracene (14 π electrons) are also aromatic.

The benzylic position, the carbon attached directly to the ring, is reactive because benzylic radicals, cations and anions are resonance-stabilized by the ring. NBS with light or peroxide brominates the benzylic carbon selectively, and the benzylic bromide can then undergo SN1, SN2 or E2 reactions. Hot KMnO₄ (or Na₂Cr₂O₇ with acid) oxidizes any alkyl side chain that has at least one benzylic hydrogen all the way to COOH, cutting off the rest of the chain; a tert-butyl group, with no benzylic H, is not oxidized. In ¹H NMR, the circulating π electrons of an aromatic ring create a ring current that deshields protons outside the ring, so aromatic H appear at 6.5 to 8 ppm, while protons held inside a large aromatic ring are strongly shielded.

Key ideas

• Benzene's six π electrons are delocalized, giving about 150 kJ/mol of aromatic stabilization.
• Aromatic rings are cyclic, planar, fully conjugated and have 4n + 2 π electrons.
• Rings meeting the first three criteria with 4n π electrons are antiaromatic and unstable.
• Ions such as the cyclopentadienyl anion and tropylium cation are aromatic, which drives their formation.
• A heteroatom lone pair counts toward aromaticity only if it occupies the p orbital in the π system.
• Benzylic positions are activated for radical bromination and side-chain oxidation.
• The aromatic ring current deshields protons outside the ring and shields protons inside it.

Formulas and rules

Common mistakes

• Counting every lone pair on a heteroatom as π electrons; only the one in the p orbital counts.
• Calling a ring aromatic when it has an sp³ carbon that breaks conjugation, as in neutral cyclopentadiene.
• Assuming cyclooctatetraene is antiaromatic; it is nonplanar and nonaromatic.
• Expecting KMnO₄ to oxidize a tert-butyl side chain, which has no benzylic hydrogen.
• Thinking benzene undergoes addition like an alkene; it prefers substitution.

Before the test, make sure you can

• Classify a ring or ion as aromatic, antiaromatic or nonaromatic using all four criteria.
• Count π electrons, deciding which heteroatom lone pairs participate.
• Calculate benzene's resonance energy from heats of hydrogenation.
• Explain the basicity of pyridine, pyrrole and imidazole.
• Explain unusual acidity or ionization driven by forming aromatic ions.
• Name disubstituted and trisubstituted benzenes with o/m/p and numbers.
• Predict products of benzylic bromination and side-chain oxidation.
• Explain aromatic ¹H chemical shifts using the ring current.

Words to know

Aromatic compound

A cyclic, planar, fully conjugated molecule or ion with 4n + 2 π electrons and special stability.

Antiaromatic compound

A cyclic, planar, fully conjugated species with 4n π electrons that is less stable than an open-chain analog.

Nonaromatic compound

A compound that fails at least one of the cyclic, planar or fully conjugated requirements.

Hückel's rule

Planar monocyclic fully conjugated systems are aromatic when they have 4n + 2 π electrons.

Resonance energy

The extra stability of a delocalized system compared with a localized structure; about 150 kJ/mol for benzene.

Cyclopentadienyl anion

C₅H₅⁻, an aromatic 6 π-electron anion formed when cyclopentadiene loses a proton.

Tropylium cation

C₇H₇⁺, the aromatic 6 π-electron cycloheptatrienyl cation.

Pyridine

A six-membered aromatic ring with one N whose sp² lone pair is outside the π system, making it basic.

Pyrrole

A five-membered aromatic ring with an N–H whose lone pair is part of the aromatic sextet, so it is weakly basic.

Furan

A five-membered aromatic ring containing oxygen that contributes one lone pair to the π system.

Imidazole

A five-membered aromatic ring with one pyrrole-like N and one basic pyridine-like N, found in histidine.

Phenyl group

C₆H₅–, a benzene ring used as a substituent.

Benzyl group

C₆H₅CH₂–, a benzene ring attached through a CH₂.

Benzylic position

The carbon directly bonded to an aromatic ring; its radicals and ions are resonance-stabilized.

Ring current

Circulation of aromatic π electrons in a magnetic field that deshields outer ring protons.

Annulene

A monocyclic, fully conjugated hydrocarbon, named [n]annulene for n ring carbons.

Sample practice questions

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