Aromatic Compounds
Chapter 16 Lecture: Organic Chemistry - Aromatic Compounds
Resonance Structures of Benzene
Benzene is represented as a resonance hybrid of two Kekulé structures.
C—C bond lengths:
Shorter than typical single-bond lengths
Longer than typical double-bond lengths
The bond order is 1.5.
Resonance of benzene can be depicted with a circle inside the six-membered ring, representing a combined structure.
Structure of Benzene (C₆H₆)
Each carbon in the benzene ring is sp² hybridized, possessing an unhybridized p orbital that is perpendicular to the ring.
Six pi electrons are delocalized over the six carbon atoms.
Unusual Addition of Bromine to Benzene
Addition of bromine to benzene requires a catalyst, such as FeBr₃.
The reaction proceeds through substitution of a hydrogen atom by a bromine atom.
Direct addition of Br₂ to the double bond of benzene is not observed.
Molar Heats of Hydrogenation
Energies associated with the hydrogenation process:
Predicted energy: -240 kJ
Actual energy: -232 kJ
Resonance energy: -120 kJ/mol
Actual values indicate the aromatic stabilization effect.
Annulenes
Definition: Annulenes are hydrocarbons characterized by alternating single and double bonds.
Examples:
Benzene is named [6]-annulene.
Cyclobutadiene is [4]-annulene.
Cyclooctatetraene is [8]-annulene.
Failures of the Resonance Picture
All cyclic conjugated hydrocarbons were once thought to be aromatic.
Cyclobutadiene is extremely reactive, dimerizing before it can be isolated.
Cyclooctatetraene readily adds Br₂ to its double bonds, indicating non-aromatic behavior.
MO Rules for Benzene
In benzene, six overlapping p orbitals create six molecular orbitals (MOs).
Of these:
Three are bonding MOs
Three are antibonding MOs
The lowest-energy MO encompasses all bonding interactions and contains no nodes.
Higher energy MOs possess an increasing number of nodes.
MOs for Benzene
Energy diagram demonstrates:
The first MO is entirely bonding with six bonding interactions.
Molecules fill the three bonding pi orbitals, achieving a stable configuration.
Intermediate and All-Antibonding MOs of Benzene
Intermediate MOs, such as p² and p³, are degenerate with one nodal plane each.
The all-antibonding p₆* has three nodal planes with destructive interference between adjacent p orbitals.
Energy Diagram for Benzene
The benzene molecule's six pi electrons fill the lowest three bonding pi orbitals, indicating a completely filled "closed shell"—a very stable arrangement.
Energy Diagram for Cyclobutadiene
Cyclobutadiene's electronic structure shows two electrons occupying nonbonding molecular orbitals, exhibiting a diradical character, leading to high reactivity.
Polygon Rule
The energy diagram for an annulene mirrors the cyclic compound shape, exhibiting a vertex at the bottom of the diagram.
Aromatic Requirements
Essential criteria for aromaticity include:
The structure must be cyclic with conjugated pi bonds.
Every atom in the ring needs an unhybridized p orbital (sp² or sp).
Continuous overlap of p orbitals around the ring is necessary.
The structure should be planar for effective overlap.
Delocalization of pi electrons must lower the electronic energy.
Nonaromatic Compounds
Nonaromatic compounds lack a continuous ring of overlapping p orbitals and may exhibit nonplanarity.
Antiaromatic Compounds
Antiaromatic compounds are defined as cyclic and conjugated, possessing overlapping p orbitals, but their electron delocalization increases electronic energy.
Hückel’s Rule
After meeting aromatic criteria, Hückel’s rule applies:
If the count of pi electrons equals (4N + 2), the system is aromatic (N is an integer).
If the count is (4N), the system is antiaromatic.
MO Derivation of Hückel’s Rule
Aromatic compounds' filled orbitals total (4N + 2) electrons.
Antiaromatic compounds possess only (4N) electrons and exhibit unpaired electrons in two degenerate orbitals.
Cyclopentadienyl Ions
The cation has an empty p orbital and four pi electrons, classifying it as antiaromatic.
The anion has a nonbonding electron pair in a p orbital, totaling six pi electrons, thus making it aromatic.
Deprotonation of Cyclopentadiene
By deprotonating the sp³ carbon of cyclopentadiene, a pair of electrons resides in one of the sp³ orbitals and can rehybridize to form a p orbital.
This rehybridization permits delocalization of six electrons over all five carbon atoms, resulting in an aromatic structure.
Orbital View of the Deprotonation of Cyclopentadiene
Deprotonation facilitates the overlap of all p orbitals.
Cyclopentadiene demonstrates lower stability compared to benzene and reacts readily with electrophiles.
Cyclopentadienyl Cation
Hückel’s rule suggests the cyclopentadienyl cation, with four pi electrons, is antiaromatic, explaining its formation difficulty.
Cycloheptatrienyl Cation
The cycloheptatrienyl cation features six pi electrons and an empty p orbital, easily formed by treating an alcohol with dilute (0.01N) aqueous sulfuric acid, known as the tropylium ion.
Cycloheptatrienyl Ions
The cation (tropylium ion) has six pi electrons, marking it as aromatic.
The anion, possessing eight pi electrons, is antiaromatic if planar; the resonance picture can mislead stability expectations.
Identifying Aromatic Compounds
For aromaticity, consider:
If all carbons are sp³ hybridized, aromaticity is negated due to lack of complete delocalization.
Recognize nonaromatic characteristics when sp³ carbons are present in the ring.
Pyridine Pi System
Pyridine holds six delocalized electrons in its pi system.
Two nonbonding electrons on nitrogen occupy an sp² orbital and do not interact with the ring's pi electrons.
Pyridine Properties
Pyridine is a basic compound, given that a pair of nonbonding electrons can abstract a proton.
The protonated form (pyridinium ion) retains aromaticity.
Pyrrole Pi System
In pyrrole, the nitrogen atom is sp² hybridized, providing a lone pair in the p orbital that overlaps with carbon's p orbitals to create a continuous ring.
Pyrrole is classified as aromatic with six electrons (N = 1).
Pyrrole Properties
The aromaticity of pyrrole stems from the delocalization of the nitrogen lone pair.
N-protonated pyrrole loses its aromatic characteristics as nitrogen transitions to sp³ hybridization.
Basicities of Heterocycles
Pyrimidine contains two basic nitrogens.
Imidazole has one basic nitrogen and one nonbasic.
In purines, only one nitrogen is nonbasic.
Summary of Other Heterocycles
Pyrrole, furan, and thiophene all have six pi electrons in their respective structures, demonstrating similar aromaticity principles in heterocycles.
Identifying Molecular Aromaticity
Questions about whether a structure is aromatic, antiaromatic, or nonaromatic can often be resolved by evaluating electron delocalization and orbital hybridization.
Common Names of Benzene Derivatives
Examples of common names associated with benzene derivatives:
Phenol (benzenol)
Toluene (methylbenzene)
Aniline (benzenamine)
Anisole (methoxybenzene)
Disubstituted Benzenes
Disubstituted benzenes are numbered to identify their positions:
Ortho- (1,2-disubstituted)
Meta- (1,3-disubstituted)
Para- (1,4-disubstituted)
Three or More Substituents in Benzenes
When numbering, assign the lowest possible numbers, with the carbon containing a functional group designated as carbon 1.
Common Names for Disubstituted Benzenes
Notable examples:
m-xylene or 1,3-dimethylbenzene
mesitylene or 1,3,5-trimethylbenzene
o-toluic acid or 2-methylbenzoic acid
p-cresol or 4-methylphenol
Phenyl and Benzyl Groups
The term "phenyl" indicates an attachment to a benzene ring, while the "benzyl" group contains an additional carbon.
Spectroscopy in Aromatic Compounds
Infrared (IR) Spectroscopy:
C═C stretch absorption occurs at around 1600 cm⁻¹.
sp² C—H stretches appear just above 3000 cm⁻¹.
Nuclear Magnetic Resonance (NMR) Spectroscopy:
Proton NMR shows signals between δ 7 and δ 8 for hydrogens on the aromatic ring.
Carbon-13 NMR displays signals from δ 120 to δ 150, similar to alkene carbons.