ch14 -Aromatic Compounds
Chapter 14: Aromatic Compounds
Nomenclature of Benzene Derivatives
Benzene is the parent name for monosubstituted benzenes.
For some monosubstituted benzenes, the substituent name is added as a prefix.
Other monosubstituted benzenes create a new parent name based on the presence of substituents.
Substituents and Their Positions
Two substituents may be indicated by prefixes ortho (o), meta (m), and para (p) or numerical positions.
Dimethyl-substituted benzenes are referred to as xylenes.
Multiple Substituents
Numbers must serve as locants for more than two substituents.
Aim for the lowest possible set of numbers for these substituents.
List substituents in alphabetical order.
If a substituent defines a parent other than benzene, it takes position 1.
Functional Groups and Naming
The group C6H5- is called phenyl when it is a substituent, abbreviated as Ph or F.
A hydrocarbon with a saturated chain and a benzene ring names the larger unit as parent, while an unsaturated chain names the unsaturated part as the parent and the benzene as a phenyl substituent.
The phenylmethyl group is referred to as benzyl (Bz).
Reactions of Benzene
Benzene is highly unsaturated but does not undergo reactions typical of alkenes like addition or oxidation.
It can react with bromine in the presence of a Lewis acid catalyst, but the reaction is substitution, not addition.
This indicates that all six carbon-hydrogen bonds in benzene are equivalent.
Kekule Structure for Benzene
Kekule proposed the first reasonable representation for benzene with alternating double and single carbon-carbon bonds.
Two different 1,2-dibromobenzenes would be expected from the Kekule structure, but only one exists; hence, no equilibrium between these compounds occurs.
Stability of Benzene
Benzene exhibits significantly greater stability than expected from theoretical calculations for cyclohexatriene.
The predicted heat of hydrogenation for cyclohexatriene is -360 kJ mol-1 compared to benzene's experimentally determined value of -280 kJ mol-1.
This difference, 152 kJ mol-1, is termed resonance energy.
Modern Theories of Benzene Structure
Resonance Explanation: Structures I and II contribute equally as resonance structures, stabilizing benzene.
Each carbon-carbon bond measures 1.39 Å, between lengths of single (1.47Å) and double bonds (1.33 Å); often represented by a hexagon with a circle.
Molecular Orbital Explanation
Carbons in benzene are sp2 hybridized, with overlapping p orbitals leading to a bonding molecular orbital with electron density above and below the ring.
There are six p molecular orbitals in benzene.
Huckel’s Rule: The 4n+2p Electron Rule
Planar monocyclic rings with continuous p orbitals and 4n + 2p electrons are aromatic.
Benzene qualifies as it is planar, cyclic, has p orbitals at every carbon, and contains 6 p electrons (n=1).
The polygon-and-circle method helps derive the energy levels of orbitals in cyclic p systems, showing benzene has 3 bonding and 3 antibonding orbitals, indicating strong stability and a closed shell of delocalized electrons.
Cyclooctatetraene
Contains two nonbonding orbitals with instability, necessitating a nonplanar structure to maintain stability.
Annulenes
Annulenes are compounds with alternating double and single bonds. Their naming includes a number to indicate ring size.
Benzene is [6]annulene, and cyclooctatetraene is [8]annulene. Aromatic annulenes have 4n + 2p electrons and planar carbon skeletons.
The [14] and [18] annulenes are aromatic, while the [16] annulene is non-aromatic.
[10]annulenes lack planarity or stability, leading to non-aromatic classifications.
Cyclobutadiene is [4]annulene and is not aromatic.
Evidence for Electron Delocalization: NMR Spectroscopy
When benzene is in a magnetic field, a p-electron ring current is induced, leading to deshielding of protons, observable as a singlet at d 7.27.
Peripheral protons are usually highly deshielded, reinforcing aromaticity evidence.
In large annulenes, internal protons become very shielded.
Aromatic Ions
Cyclopentadiene is acidic due to the formation of the aromatic cyclopentadienyl anion.
The cyclopentadienyl anion has 6 p electrons and applies to the 4n + 2 rule.
Cycloheptatriene is non-aromatic but produces the aromatic cycloheptatrienyl cation (tropylium cation) upon losing a hydride.
Aromatic, Antiaromatic, and Nonaromatic Compounds
Structural comparisons between cyclic annulenes and their acyclic counterparts measure aromatic stability by comparing their p-electron energies.
Benzene and cyclopentadienyl anion are aromatic, while cyclobutadiene is antiaromatic; cyclooctatetraene would also be antiaromatic if planar.
Other Aromatic Compounds
Benzenoid Aromatic Compounds
Polycyclic benzenoid compounds have two or more fused benzene rings.
Naphthalene and Pyrene
Naphthalene possesses three resonance structures, with significant electron delocalization.
Pyrene contains 16 p electrons, with a non-Huckel count deemed aromatic based on a Huckel-like exterior perimeter.
Nonbenzenoid Aromatic Compounds
Describes compounds lacking benzene rings; includes cyclopentadienyl anion and certain annulenes except [6] annulene.
Azulene exhibits resonance and charge separation in its structure.
Fullerenes
Buckminsterfullerene (C60) has a soccer ball-like structure and is aromatic, allowing for sp2 hybridization with bonds to 3 carbons.
Analogous structures include C70.
Heterocyclic Aromatic Compounds
Incorporates other elements besides carbon in the ring structure.
Examples include pyridine (sp2 N contributing to aromaticity) and pyrrole (N lone pair participating in the aromatic system).
Spectroscopy of Aromatic Compounds
1H NMR Spectra
Protons within benzene derivatives appear deshielded, typically ranging from d 6.0 to d 9.5.
13C NMR Spectra
Aromatic carbons generally lie within d 100-170 region; DEPT spectra reveal attachments to protons.
Infrared Spectra of Substituted Benzenes
Characteristic frequencies include C-H stretching near 3030 cm-1 and multiple bands between 1450-1600 cm-1.
Ultraviolet-Visible Spectra
Aromatic compounds exhibit moderate absorption bands near 205 nm and a less intense band at 250-275 nm.
Mass Spectra
The significant ion in alkyl benzenes' mass spectrum is m/z 91, corresponding to a benzyl cation, which rearranges to a tropylium ion (C7H7+).