Download Drawing Organic Structures Functional Groups Constitutional Isomers

AROMATIC COMPOUNDS
Dr. Sheppard
CHEM 2412
Summer 2015
Klein (2nd 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.
II.
III.
IV.
V.
Nomenclature Review
Physical Properties
Spectroscopy
Benzene Structure
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 F‒)
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:
-CO2H > -CHO > -OH > -NH2 > -CH3
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)
-17
-25
54
bp (°C)
181
173
170
III. Spectroscopy of Aromatics: IR
• sp2 C-H absorption at 3030 cm-1
• Ring absorptions at 1450-1600 and 1660-2000 cm-1
• Also peaks in fingerprint region can differentiate
substitution pattern
III. Spectroscopy of Aromatics: NMR
•
13C-NMR:
• 1H-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 structures
Hybrid 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
n
4n + 2
0
2
1
6
2
10
3
14
4
18
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 sp2 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 pKa = 16; cyclopentane pKa > 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?