Aromaticity Flashcards
What are the three key structural features of an aromatic compound?
Planar (flat) structure
Cyclic (ring) structure
Delocalized π-system (a conjugated system where electrons are shared across the ring)
What is an example of an aromatic compound, and what is its bonding structure?
Benzene (C₆H₆)
- Benzene has alternating double and single bonds.
- Each carbon atom forms:
- Three sigma bonds (one with a hydrogen and two with adjacent carbons)
- One pi bond (formed through overlapping p orbitals)
What are the key properties of aromatic compounds?
Must have a p orbital on every atom in the ring, perpendicular to the plane.
These orbitals overlap to form a delocalized π-system, providing stability.
Why are aromatic compounds important?
- Stability – Highly stable due to delocalized electrons.
- Role in fragrances and flavors – Found in essential oils and perfumes.
- Applications in synthetic drugs, materials, and industrial production.
Who discovered benzene, and what structures were proposed?
- Discovered by Faraday in 1825 during the study of gas combustion products.
- Molecular formula: C₆H₆
- Proposed structures:
1. Ladenburg structure
2. Dewar structure
3. Kekulé structure (alternating double bond resonance model)
How did diffraction studies confirm the structure of benzene?
All C–C bonds in benzene are equivalent.
Bond lengths:
- Single bond: 150 pm
- Double bond: 134 pm
What is the hybridization of benzene’s carbon atoms?
- Each carbon in benzene is sp² hybridized.
- Each forms three sigma bonds and has one unhybridized p orbital.
What forms the π-system in benzene?
- Overlapping 2p orbitals form a delocalized π-system.
- This delocalization stabilizes the molecule.
What are the two molecular orbital energy levels in benzene?
- Bonding orbitals (Lower energy, stabilize the molecule)
- Antibonding orbitals (Higher energy)
What are the computational models that explain benzene’s stability?
- Wavefunctions: Molecular orbitals formed from atomic/hybrid orbitals.
- Delocalization: Electron bonding influence extends over the entire molecule.
- Resonance: Electrons are not confined to specific bonds but are delocalized over the entire ring.
What is Hückel’s Rule, and how is it used to determine aromaticity?
- Hückel’s Rule: A compound is aromatic if it has 4n + 2 π-electrons (where n is an integer).
- Acceptable π-electron counts: 2, 6, 10, 14, etc.
Benzene: (4×1) + 2 = 6
Naphthalene: (4×2) + 2 = 10
Anthracene: (4×3) + 2 = 14
What is the difference between aromatic, anti-aromatic, and non-aromatic compounds?
- Aromatic: 4n + 2 π-electrons, planar, cyclic, delocalized π-electrons, stable.
- Anti-aromatic: 4n π-electrons, planar, cyclic, delocalized π-electrons, highly reactive.
- Non-aromatic: Either non-cyclic or non-planar.
What criteria define an aromatic ion?
- Cyclic structure
- Planar geometry
- 4n + 2 π-electrons
- Delocalization of π-electrons
Examples:
- Cyclopropene: Aromatic as a cation (n=0 in Hückel’s Rule).
- Cyclopentadienyl anion: Aromatic (6 π-electrons).
- Cycloheptatrienyl cation: Aromatic (6 π-electrons).
What are heterocyclic aromatic compounds?
- Aromatic compounds with heteroatoms (N, O, S) in the ring.
- Pyridine (6 π-electrons)
- Pyrrole (6 π-electrons, lone pair contributes)
- Furan (6 π-electrons)
- Thiophene (6 π-electrons)
What are the three key experimental proofs of aromaticity?
No addition reactions – Benzene does not react like alkenes.
Resistance to hydrogenation & oxidation – Difficult to hydrogenate or oxidize.
Dominance of substitution reactions – Electrophilic substitution instead of addition.
What does the heat of hydrogenation reveal about benzene’s stability?
- Expected value for a cyclic triene: -360 kJ/mol
- Observed value for benzene: -208 kJ/mol
- Resonance energy: 151 kJ/mol (extra stability due to delocalization)
How does NMR spectroscopy confirm aromaticity?
Delocalized electrons create a magnetic field.
Ring current causes a unique chemical shift.
NMR Peaks:
- ¹H NMR: All protons equivalent, single peak at 7.26 ppm
- ¹³C NMR: All carbons equivalent, single peak at 128 ppm
