AROMATIC COMPOUNDS Dr. Sheppard CHEM 2412 Summer 2015 Klein (2 nd ed.) sections: 18.1, 18.2, 18.8, 18.3, 18.4, 18.5

Aromatic Compounds Originally distinguished because of smell Then noticed trends in reactivity Now, highly unsaturated, stable compounds Unreactive to many reagents that react with alkenes Aromatic hydrocarbons = arenes (Ar-) Most famous is benzene

Aromatic Compounds I. Nomenclature Review II. Physical Properties III. Spectroscopy IV. Benzene Structure V. Aromaticity

I. Nomenclature (Review) Monosubstituted benzenes Substituent name + “benzene”

Common Benzene Compounds

Benzene Nomenclature If substituent has greater than 6 carbons, it becomes the parent, and benzene is called a phenyl group Benzene substituents: (Ph ‒ or  ‒ )

Disubstituted Benzenes ortho (1,2) meta (1,3) para (1,4)

Naming Disubstituted Benzenes If one substituent is part of a common name, that name is the parent and that substituent is at carbon 1 If neither substituent is part of a common name, list the substituents in alphabetical order (first alphabetically is at carbon 1) If both substituents are part of common name, use this order of priority to determine the parent name: -CO 2 H > -CHO > -OH > -NH 2 > -CH 3

Naming Polysubstituted Benzenes With 3 or more substituents do not use ortho, meta, para Number ring to give smallest set of numbers If a common name, use as parent (substituent at carbon 1) List substituents in alphabetical order

II. Physical Properties Melting point Based on “packing” Benzene packs easily, so has a higher mp than other hydrocarbons Substituted benzenes: para > ortho and meta due to packing Boiling point Polarity depends on substituents Higher polarity = higher boiling point mp (°C) bp (°C)

III. Spectroscopy of Aromatics: IR sp 2 C-H absorption at 3030 cm -1 Ring absorptions at and cm -1 Also peaks in fingerprint region can differentiate substitution pattern

13 C-NMR: 1 H-NMR: III. Spectroscopy of Aromatics: NMR

IV. Benzene Structure and Stability Cyclic, planar, hexagonal shape Conjugated Hybridization of carbons? Bond angles? All H’s are identical All C-C bonds are equivalent Bond order = 1.5

Benzene Reactivity Unsaturated, but doesn’t behave like alkene Alkenes: Benzene: Benzene will reduce at high pressure and temperature or with special catalyst:

Explanation for Benzene Stability 1. Molecular orbital model Bonding and antibonding molecular orbitals Skip this 2. Resonance model Lots of orbital overlap and conjugation = very stable Kekulé structuresHybrid structure

V. Aromaticity All aromatic structures are similar in stability and reactivity All have structural similarities Example: Benzene is aromatic But, 1,3-cyclobutadiene is not aromatic Even though it resembles benzene and is resonance-stabilized Reacts like an alkene (addition) not benzene (substitution)

Criteria for Aromaticity Hückel Based on molecular orbital calculations 1. Cyclic 2. Planar 3. Unhybridized p orbital on each atom of the ring 4. (4n + 2) electrons in the p orbitals n4n

Examples Benzene 1. Is it cyclic? 2. Is it planar? 3. Does it have an unhybridized p orbital on each atom of the ring? 4. How many electrons does it have in the p orbitals? Is that equal to (4n + 2)?

Examples [14] annulene [#] is number of atoms in ring Annulene = cyclic, conjugated hydrocarbon 1. Is it cyclic? 2. Is it planar? 3. Does it have an unhybridized p orbital on each atom of the ring? 4. How many electrons does it have in the p orbitals? Is that equal to (4n + 2)?

Aromaticity Molecules that are not aromatic (do not satisfy the 4 criteria listed above) are either: Antiaromatic Nonaromatic The reactivity of both antiaromatic and nonaromatic molecules will be like alkenes (addition reactions)

Antiaromatic 1. Cyclic 2. Planar 3. Unhybridized p orbital on each atom of the ring 4. (4n) electrons in the p orbitals Examples:

Nonaromatic Either not cyclic, or not conjugated, or not planar Examples: Not cyclic: Not conjugated: Not planar:

Aromaticity of Ions Cyclopentadienyl cation Cyclic Planar Unhybridized p orbitals 4 electrons = 4n Antiaromatic Cyclopentadienyl anion Cyclic Planar Atom with : can become sp 2 hybridized Lone pair in p orbital Unhybridized p orbitals 6 electrons = 4n+2 Aromatic Very stable, likely to form (H lost is more acidic than other hydrocarbon H’s) Cyclopentadiene pK a = 16; cyclopentane pK a > 50

Aromaticity of Heterocycles Heteroatoms with lone pairs Determine which electrons are part of aromatic system Examples: Pyridine Furan Remember aromatic amines are weaker bases than aliphatic amines Partially due to resonance delocalization Also due to lone pair electrons counting toward aromaticity Pyrrole loses aromaticity when protonated What about pyridine?

Polycyclic aromatic compounds Fused ring systems Naphthalene Indole Fused benzene rings

Aromatic, antiaromatic, or nonaromatic?