Chapter 16 – Aromatic Compounds (Lecture Summary)

Vocabulary flashcards summarizing major terms, structures, and concepts from Chapter 16 on aromatic compounds, including nomenclature, physical properties, MO theory, aromaticity rules, heterocycles, spectroscopy, and notable aromatic molecules.

Aromatic Compound

A cyclic, planar molecule with a continuously conjugated π system containing (4n + 2) π-electrons, giving it extra thermodynamic stability.

Benzene

The prototype aromatic compound, molecular formula C6H6, all C–C bonds 1.397 Å, exhibits unusual stability and reacts mainly by substitution rather than addition.

Kekulé Structure

A representation of benzene as a six-membered ring with alternating single and double bonds; now understood as a resonance hybrid.

Resonance Energy of Benzene

≈151 kJ mol⁻¹ stabilization compared with three isolated C=C bonds, accounting for benzene’s low heat of hydrogenation.

Bond Length in Benzene

All C–C bonds are equal (1.397 Å), intermediate between typical C–C single (1.48 Å) and double (1.34 Å) bonds.

Hückel’s Rule

For a planar, cyclic, conjugated system: (4n + 2) π-electrons = aromatic; 4n π-electrons = antiaromatic.

Criteria for Aromaticity

Molecule must be cyclic, planar, fully conjugated, and obey Hückel’s (4n + 2) π-electron rule.

Antiaromatic Compound

Cyclic, planar, fully conjugated system with 4n π-electrons, destabilized relative to similar nonaromatic compounds.

Nonaromatic Compound

A cyclic species that is not fully conjugated or not planar; avoids both aromatic stabilization and antiaromatic destabilization.

Annulene

Monocyclic hydrocarbon with alternating single and double bonds; named [N]annulene where N = ring size, e.g., benzene = [6]annulene.

Cyclobutadiene

[4]Annulene with 4 π-electrons; square form would be antiaromatic and highly unstable, behaves as a diradical.

Cyclooctatetraene

[8]Annulene; adopts a non-planar ‘tub’ conformation to become nonaromatic and avoid antiaromaticity.

[10]Annulene

Can adopt non-planar shapes (nonaromatic) or, in certain conformations, aromatic if planarity and 10 π-electrons are achieved.

Polygon Rule

For monocyclic conjugated rings, inscribe the polygon in a circle with one vertex down; vertices show relative MO energies.

π Molecular Orbitals of Benzene

Six π-MOs: three bonding (filled by 6 π-e⁻), three antibonding; energy separation explains aromatic stability.

Phenyl Group (Ph-)

A C6H5- substituent obtained by removing one hydrogen from benzene.

Benzyl Group (Bn-)

The CH2C6H5- substituent derived from toluene; contains a benzylic methylene.

Ortho, Meta, Para

Positional prefixes for 1,2-; 1,3-; and 1,4-disubstituted benzenes, respectively (o-, m-, p-).

Cyclopentadienyl Anion

Five-membered ring with 6 π-electrons; planar and aromatic, basis of many organometallic compounds.

Cyclopentadienyl Cation

Four π-electron system; would be planar and antiaromatic, therefore extremely unstable and not observed.

Tropylium Ion

Cycloheptatrienyl cation (C7H7⁺) with 6 π-electrons; planar, aromatic, gives strong m/z 91 peak in mass spectra.

Cyclooctatetraene Dianion

C8H8²⁻ with 10 π-electrons; planar and aromatic after metal reduction (e.g., 2 K).

Pyridine

Six-membered aromatic heterocycle; nitrogen lone pair is sp² and basic, not part of the π system (6 π-electrons total).

Pyrrole

Five-membered aromatic heterocycle; nitrogen lone pair is in a p-orbital, part of the 6 π-electron system, making it weakly basic.

Furan

Oxygen-containing five-membered aromatic ring; one O lone pair participates in aromatic sextet, the other remains localized.

Thiophene

Sulfur analogue of furan; aromatic with 6 π-electrons including one S lone pair (uses 3p orbital).

Imidazole

Five-membered heteroaromatic with two nitrogens; one lone pair in aromatic sextet, the other basic and external.

Purine

Fused bicyclic heteroaromatic (imidazole + pyrimidine), core of adenine & guanine bases.

Indole

Fused benzene + pyrrole system; aromatic with 10 π-electrons; found in tryptophan and many natural products.

Naphthalene

Fused-ring aromatic hydrocarbon (C10H8) with 10 π-electrons; two fused benzene rings.

Anthracene

Linear fusion of three benzene rings (C14H10); 14 π-electrons.

Phenanthrene

Angular fusion of three benzene rings; structural isomer of anthracene, 14 π-electrons.

Polynuclear Aromatic Hydrocarbon (PAH)

Hydrocarbons containing multiple fused benzene rings; many are carcinogenic (e.g., benzo[a]pyrene).

Benzo[a]pyrene

Five-ring PAH activated by liver enzymes to arene oxides that bind DNA, leading to cancer.

Fullerene (C60)

Spherical carbon allotrope composed of fused five- and six-membered rings (soccer-ball structure).

Carbon Nanotube

Cylindrical allotrope formed by rolling graphene sheet; exhibits remarkable mechanical and electronic properties.

Graphene

Single atomic layer of graphite; two-dimensional sheet of fused benzene rings.

IR of Aromatics

Characteristic C–H stretch near 3030 cm⁻¹ and C=C ring stretch near 1600 cm⁻¹.

1H NMR of Aromatics

Protons on an aromatic ring appear at δ 7–8 ppm due to induced ring current.

13C NMR of Aromatics

Aromatic carbons resonate around δ 120–150 ppm, similar to alkenes but wider range.

Mass Spec m/z 91 Peak

Indicates formation of benzyl-to-tropylium cation; diagnostic for alkylbenzenes.

Melting Point Trend of Disubstituted Benzenes

o- < m- < p- when only steric/dipole factors considered; para is most symmetric, highest m.p.

Density of Aromatics

Greater than most alkanes (~0.8 g mL⁻¹) but less than water; halogenated aromatics are heavier.

Water Solubility of Aromatics

Generally insoluble due to non-polar character; limited solubility increases with polar substituents.

Spectroscopic Identification of Aromatics

Combination of IR (3030/1600 cm⁻¹), NMR (δ 7–8 ppm), and MS (tropylium ion) confirms aromatic rings.

Phenol

Benzene with an OH group; strongly hydrogen-bonding, mp 43 °C, bp 182 °C.

Aniline

Amino-substituted benzene (C6H5NH2); basic aromatic amine used in dyes.

Toluene

Methylbenzene; industrial solvent and starting material, bp 111 °C.

Styrene

Vinylbenzene; monomer for polystyrene plastics.

Acetophenone

Methyl phenyl ketone; fragrant solvent and precursor in pharmaceuticals.

Benzaldehyde

Aromatic aldehyde with almond odor; precursor to benzoic acid.

Benzoic Acid

Carboxylic acid derivative of benzene; preservative and precursor in synthesis.

Nitrobenzene

Benzene with NO2; dense (d = 1.20 g mL⁻¹), used to make aniline.

Benzenesulfonic Acid

SO3H-substituted benzene; strong acid used in detergents and sulfonation reactions.