Aromatic

• Benzene's structure was first proposed 150 years ago
• Benzene structure


  A cyclic structure for benzene, C6H6

  
• Benzene molecule did not react as if it had any unsaturation

  
• Aromatic compounds


  Originally named for the pleasant smell of resins from tropical trees (early source)
• Most common group of aromatic compounds is based on the 6-member aromatic ring
• Benzene ring


  • Six carbon atoms
  • Joined in a planar hexagonal arrangement
  • Each carbon is bonded to one hydrogen atom

  
• Aromatic compounds do not undergo addition reactions like the alkenes and alkynes because they do not want to lose their aromatic properties
• Aromatic benzenes will undergo reaction without losing its aromaticity
• Electrophilic substitution reaction


  Reactions are special and occur in the presence of catalysts such as FeBr3
• Conditions for aromaticity


  • Cyclic molecule
  • Planar
  • Total number of π-electron pairs is odd (1,3,5...)
  • Conjugated
  • Huckels rule (4n+2)

  
• Compounds that are not aromatic

  • Cyclobutadiene has an even number of pairs of π electrons
  • Cyclooctatetraene has an even number of pairs of π electrons and it is not planar

  
• Benzene is aromatic because it must have an uninterrupted cloud of π electrons and the π cloud must have an odd number of pairs of π electrons
• Aromatic cyclic ions


  • Cyclopentadiene (2 π electron pairs)
  • Cyclopentadienyl cation (2 π electron pairs)
  • Cyclopentadienyl anion (3 π electron pairs)

  
• Polycyclic aromatic hydrocarbons


  • Naphthalene (planar, 5 pairs)
  • Azulene (planar, 5 pairs)

  
• Polynuclear aromatic hydrocarbons (PAH) are composed of two or more aromatic rings joined together

  
• Heterocyclic aromatic compounds


  Rings with at least one atom other than carbon as part of the structure of the aromatic ring, typically O, N, S

  
• Heterocyclic aromatics are similar to benzene in stability and chemical behavior

  
• Many heterocyclic aromatics are significant biologically, e.g. purines and pyrimidines in DNA/RNA, pyridine in nicotine, pyrrole in hemoglobin and chlorophyll

  
• Nomenclature of mono-substituted benzenes

  Name the substituent group and add "benzene"
• Nomenclature of di-substituted benzenes

  Name the groups in alphabetical order, the first named group is at position 1, if a "special group" is present it must be number 1 on the ring
• Naming di-substituted benzenes


  • p-Nitroanisole or 4-Nitroanisole
  • p-Chlorostyrene
  • m-Nitrotoluene
  • o-Ethylphenol

  
• Benzene acting as a substituent

  The term "phenyl" is used to designate the presence of C6H5- as a substituent

  
• Phenylethene


  The benzene ring is a substituent, the principle functional group is an alkene so the suffix is -ene, the longest continuous chain is C2 so the root is eth-

  
• Benzene typically undergoes electrophilic aromatic substitution reactions
• Electrophilic aromatic substitution mechanism

  Addition of the electrophile to the ring, then elimination of a hydrogen to re-form the pi bond and restore aromaticity
• Lewis acids (AlCl3 or FeCl3) are used as catalysts to speed up electrophilic aromatic substitution reactions

  
• Types of electrophilic aromatic substitution reactions

  • Halogenation
  • Nitration
  • Sulphonation
  • Friedel-Craft Alkylation
  • Friedel-Craft acylation
• Halogenation


  Places a Br or Cl on the benzene ring, using Br2 or Cl2 with iron or iron halide catalysts

  
• Nitration


  Places the nitro group (NO2) on the benzene ring, using sulfuric acid as a catalyst
• Sulfonation


  Places an SO3H group on the benzene ring, using concentrated sulfuric acid as a catalyst
• Friedel-Craft alkylation


  Places an alkyl group (R-) on the benzene ring, using a Lewis acid catalyst

  
• Friedel-Craft acylation


  Places an acyl group (R-C(O)-) on the benzene ring, using a Lewis acid catalyst

  
• Oxidation of alkylbenzenes can occur, converting the alkyl group to an aldehyde or carboxylic acid
• Oxidation of alkylbenzenes


  • CH3 -> CHO

  CH2R -> COOH

  CHR2 -> No reaction (3° carbon, no C-H bond)

  
• Reduction of nitrobenzenes can occur using Zn(Hg) and HCl
• Character, Chatter, Claim, Command, Creatures, Degree, Echo, Gender, Gesture, Lexigram, Mammal, Organ, Personality, Pod, Primate, Species, Subject, Swagger, to Coin, to Echo, to Headback, to Interact, to

  write questions on this vocabulary:

  Pickup, to Reassure, to Vocalize, to Wag, Upright, Actually, Complex, Component, Concept, Enthusiastically, Essence, Findings, Gap, Life Expectancy, polls, Reasonable, Solidarity, Sum Total, to Balance, to Catch on, to Measure, Trend, Universal, Wellbeing.