RAYO AROMATIC HYDROCARBONS

• AROMATICS / ARENES

  • Stable, unsaturated hydrocarbons
  • Compounds that consist of a conjugated planar ring system accompanied by delocalized π electron clouds in place of alternating double and single bonds
  • Any compound that contains a benzene ring or has benzene-like properties
• Benzene: coal distillate (gaseous states)

  Benzaldehyde: from cherries, peaches, almonds

  Toluene: Tolu balsam
• • Steroids: estrone

  • Lipitor: atorvastatin (cholesterol-lowering drug).

  • Benzene: toxic; causes bone-marrow depression with prolonged exposure → leukopenia (low WBC count).
• Benzene: Should not be used as a laboratory solvent.

  o Major sources: Coal and Petroleum.
• Coal: Complex mixture with benzene-like rings.

  • Heated to 1000°C in absence of air for thermal breakdown.
  • Produces volatile coal tar.
• Some aromatic hydrocarbons found in coal tar:

  A) 1

  B) 2

  
• Petroleum: Contains few aromatic compounds.

  • Consists mainly of alkanes.
  • Aromatic molecules formed at 500°C, high pressure.
  • Example: Heptane converted to toluene by dehydrogenation and cyclization.
• DELOCALIZED BONDS

  • Covalent bond in which electrons are shared among more than 2 atoms
• RESONANCE EFFECT

  Donation/withdrawal of electrons through orbital overlap with neighboring π bonds
• CRITERIA FOR AROMATICITY

  • Planar Geometry
  • Monocyclic System
  • Continuous Conjugation
  • Obeys Huckel's Rule (contains 4n+2 π electrons)
• Planar Geometry

  o All carbon atoms are present in 1 single plane.

  o Visualized on a flat surface.

  o Along x & y axis.
• EXHIBITS CONTINUOUS CONJUGATION

  • Alternating double and single bonds
  • System of continuous and parallel p-orbitals with delocalized electrons
  • Conjugated pi system
  • Decreases overall energy
  • Promote stability
• Obeys Huckel’s Rule

  • Only planar, fully conjugated, cyclic polyenes having 4n + 2 π electrons, where n = positive integer including 0, possess aromaticit
  • by the German physicist Erich Hückel (1931).
  • contains 4n+2 π electrons
  • where n is a non-negative integer.
• Criteria for aromaticity

  A) Unusually stable

  B) unsually unstable

  C) 4n+2

  D) 4n

  E) tub-like

  
• Aromatic: molecules with 2, 6, 10, 14, 18, ... π electrons.

  • Benzene: 6 π
  • Anti-aromatic: molecules with 4n π electrons (4, 8, 12, 16, ...), even if they are cyclic, planar, and apparently conjugated.
  • Cyclobutadiene: 4 π
  • Cyclooctatetraene: 8 π

  
• Nonaromatic: cyclic, planar, fully conjugated is not met.
• Benzene

  A) yes

  
• The Benzene Anion

  A) yes

  
• The Cyclopentadiene Cation

  A) NO

  
• Pyrrole

  A) YES

  
• Pyrrole Conjugate Acid

  A) NO

  
• Pyridine

  A) YES

  
• Thiophene

  A) YES

  
• Cyclobutadiene

  A) NO

  
• Cyclobutene Di-Anion

  A) YES

  
• Naphthalene

  A) YES

  
• Pyrylium Ion

  A) YES

  
• Indole

  A) YES

  
• Azulene

  A) YES

  
• Molecular Orbital Theory: 4n+2 π electrons in a conjugated ring lead to aromatic stability due to delocalization and maximum overlap of π electrons.
• BENZENOID AROMATIC COMPOUNDS

  • At least 1 benzene ring
• NON-BENZENOID AROMATIC COMPOUNDS

  • Other unsaturated rings aside from benzene rings
  • Ring structure have about 5 to 7 carbon atoms
• HETEROCYCLIC AROMATIC COMPOUNDS

  • Atoms other than carbon is present in the ring

  
• Benzene (C6H6): Simplest aromatic hydrocarbon; Eight fewer hydrogens than the corresponding alkane (C6H14).
• Benzene

  • Undergoes substitution reaction rather than addition reaction
  • sp2 hybridized
  • Each carbon has a p-orbital perpendicular to the plane of a six-membered ring
• BOND ORDER OF 1.5 & BOND LENGTH OF 139 pm

  • All C-C bonds is equal and intermediate in length between single and double bonds
• BOND ANGLE OF 120°

  • Every carbon and hydrogen lie on the same plane
  • It is symmetrical, with each carbon atom lying at the angle of a regular hexagon
• RESONANCE THEORY

  • Benzene is a resonance of 2 kekule structures
  • Benzene doesn’t correspond to either structure I or II, but rather to a structure intermediate to I and II
• HEAT OF HYDROGENATION & COMBUSTION

  • Heats of hydrogenation and combustion is lower than expected
  • Has a heat of hydrogenation 150 kJ/mol less negative than expected for a conjugated cyclic triene.
  • Heat of Hydrogenation: Quantity of heat evolved when 1 mol of unsaturated compound is hydrogenated
• Stability of benzene

  A) 1