CHEMISTRY LECTURE AND LABORATORY TRANSES

• Aromatic hydrocarbons


  Stable, unsaturated hydrocarbons
• Aromatic compounds


  • Consist of 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

  
• 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


  • Has planar geometry
  • Exhibits continuous conjugation
  • Obeys Huckel's rule

  
• Types of aromatic compounds


  • Benzenoid aromatic compounds
  • Non-benzenoid aromatic compounds
  • Heterocyclic aromatic compounds

  
• Stability of 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

  
• Heat of hydrogenation & combustion


  Heats of hydrogenation and combustion is lower than expected
• Nomenclature of aromatic compounds


  • Benzene derivatives with one substituent
  • Benzene derivatives with two substituents
  • Benzene derivatives with three or more substituents

  
• Solubility of aromatic compounds


  Nonpolar, immiscible/insoluble in water, solvent for nonpolar compounds
• Density of aromatic compounds


  Less dense than water
• Alkylation of benzene


  1. Friedel-Crafts alkylation

  2. Electrophilic attack on the aromatic ring with the aid of a carbocation

  3. Replacement of hydrogen in the aromatic compound with an alkyl group
• Mechanism of Friedel-Crafts alkylation

  1. Lewis acid catalyst undergoes reaction with alkyl halide → formation of an electrophilic carbocation

  2. Carbocation attacks the aromatic ring → formation of a cyclohexadienyl cation

  3. Deprotonation of the cyclohexadienyl → reformation of C=C bond

  4. Proton goes on to form HCl, regenerating the catalyst
• Halogenation of benzene


  Benzene + Halogen → Aryl Halide (Ar-X)

  
• Reactivity of aromatic compounds (electrophilic aromatic compounds)


  Electrophile (E+) reacts with aromatic ring and substitutes for 1 of the hydrogen

  
• Mechanism of electrophilic aromatic substitution


  1. E+ accepts an electron pair and forms a bond with a carbon atom in the benzene ring

  2. A proton is removed from the carbocation restoring the aromaticity of the ring
• Addition reaction vs substitution reaction

  Addition reaction leads to the resonance stabilization of the aromatic ring would be lost and the overall reaction would be energetically favorable
• Position of substitution


  • Ortho-para directors (2-4 directors)
  • Meta directors (3 directors)
• Electron donating group (EDG)


  Activating group, increase electron density of benzene, increase benzene reactivity
• Electron withdrawing group (EWG)


  Deactivating group, decrease electron density of benzene, decrease benzene reactivity
• Resonance effect


  Conjugation between the ring and substituent, delocalization of π electrons between the ring and substituent
• Inductive effect


  Withdraw of sigma electrons away from the ring toward the substituent, due to higher electronegativity of substituent compared to the carbon of the ring
• Classification of aromatic substituents


  • Ortho-para directors
  • Meta directors
• Halogens (Group VIIA) deactivate the ring by inductive effect, not by resonance effect
• Moderate Activators


  • -OR; -NH-CO-R; -O-CO-R
• Weak Activators


  • -R; -C6H5
• Mild Deactivators


  • -F; -Cl; -Br; -I
• Very Strong Deactivators


  • -N+R3; -NO2; -CN; -CCl3; -CF3
• Moderate to Mild Deactivators


  • -CN; -SO3H; -CO-R; -COOH; -COOR; -CONH2; -N+H3
• Halogens (Group VIIA)


  • Even though they have an unpaired pair of electrons, halogens deactivate the ring by inductive effect, not by resonance effect
  • The pair of electrons are donated to the ring, but the inductive effect pulls away the electrons from the ring by EN of the halogens
• Limitations


  • Rearrangement of Friedel-Crafts reaction
  • Strong deactivating groups
  • Aryl & vinyl halides
  • Aniline (-NH2) & amine (-NH2; -NHR; -NR2)
  • Phenol (-OH)
  • Polyalkylation
• Carbocation rearrangement due to carbon chain having more than 2 carbons
• Carbocation can undergo rearrangement by hydride shift or methyl shift
• Rearrangement may occur when reacting with primary alkyl halides even in the absence of carbocation
• Alkylation and acylation are the slowest type of electrophilic substitution
• No reaction when strong deactivating group is present
• Friedel-Crafts reactions are not possible due to instability of corresponding carbocations
• Lone pairs of nitrogen

  Lewis base
• The Lewis acid catalyst binds to nitrogen


  Species where nitrogen is (+) charged