aromatic compounds

• Aromatic compounds


  Organic chemicals that include one or more ring of six carbon atoms with delocalised bonding
• Aliphatic compounds


  Straight or branched chain organic substances
• Benzene


  The simplest aromatic compound, with the molecular formula C6H6
• Benzene's structure


  • Six C atoms in a hexagonal ring, with one H atom bonded to each C atom
  • Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds, leaving one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring
  • The six p electrons are delocalised in a ring structure above and below the plane of carbon atoms
• Kekule's proposed structure for benzene, with alternate single and double covalent bonds, is not correct
• Evidence suggests all the C-C bonds in benzene are the same length and have a bond energy between a C-C single and C=C double bond</b>
• Delocalisation


  The six electrons in the pi bonds are delocalised and spread out over the whole ring, not attached to a particular atom
• The H-C-C bond angle in benzene is 120°
• Delocalisation of electrons in benzene

  Benzene is more thermodynamically stable than a non-delocalised structure
• Delocalisation energy


  The increase in stability connected to delocalisation
• Naming aromatic compounds


  • The simplest molecules are derivatives of benzene and have benzene at the root of the name
  • If two or more substituents are present, their positions must be indicated by numbers to give the lowest possible numbers to the substituents
  • When two or more different substituents are present, they are listed in alphabetical order and di, tri prefixes should be used
• Phenyl group


  The C6H5- group, which can be regarded as a substituent side group on another molecule
• Benzene does not generally undergo addition reactions as this would involve breaking up the delocalised system
• Most of benzene's reactions involve substituting one H for another atom or group of atoms, usually electrophilic substitutions
• Benzene vs alkenes in reaction with bromine

  Benzene does not react with bromine without additional halogen carrier chemicals, while alkenes react easily with bromine at room temperature
• Toxicity of benzene


  Benzene is a carcinogen and is banned for use in schools, while methylbenzene is less toxic and reacts more readily
• Nitration of benzene


  1. Reaction with conc. nitric acid in the presence of conc. sulfuric acid catalyst

  2. Forms nitrobenzene via electrophilic substitution, with the NO2+ electrophile
• Halogenation of benzene


  1. Reaction with bromine or chlorine, using FeBr3 or AlCl3/FeCl3 catalyst

  2. Forms bromobenzene or chlorobenzene via electrophilic substitution, with the Br+ or Cl+ electrophile
• Friedel-Crafts alkylation


  1. Reaction of benzene with a chloroalkane, using anhydrous AlCl3 catalyst

  2. Forms an alkylbenzene via electrophilic substitution, with the R+ electrophile
• Friedel-Crafts acylation


  1. Reaction of benzene with an acyl chloride, using anhydrous AlCl3 catalyst

  2. Forms a phenyl ketone via electrophilic substitution, with the RCO+ electrophile
• Phenols


  Compounds where the OH group is directly attached to the benzene ring
• Properties of phenols


  • Phenols are very weakly acidic, weaker than carboxylic acids
  • Phenols react with Na and NaOH, but not Na2CO3 like carboxylic acids
• Reaction of phenol with bromine

  Phenol undergoes multiple substitution with bromine at room temperature, forming 2,4,6-tribromophenol
• Reaction of phenol with nitric acid

  Phenol reacts with 4M HNO3 at room temperature, forming 2-nitrophenol or 4-nitrophenol via electrophilic substitution
• Effect of side groups on substitution

  Electron-donating groups like OH and NH2 direct further substitution to the 2- and 4-positions, while electron-withdrawing groups like NO2 have a 3-directing effect
• Effect of delocalisation on side groups with lone pairs


  Delocalisation extends to include the lone pairs on N, O and Cl, changing the properties and reactions of the side group