Arenes

• Kekule model of benzene


  • 3 localised π bonds
  • π bonds formed by sideways overlap of 2 neighbouring p-orbitals above and below the plane of the ring
  • the benzene ring contains 3 localised π bonds
  • each localised π bond contains 2 shared electrons between 2 carbon atoms
  • higher electron density than delocalised model

  
• Delocalised model of benzene

  • one delocalised π bonds
  • π bond is formed by sideways overlap of 6 p-orbitals above and below the plane of the ring
  • the benzene ring contains one delocalised π bond
  • the delocalised π bond contains 6 shared electrons between carbon atoms
  • lower electron density than Kekule model

  
• Evidence for the structure of benzene

  1. Reactivity with Bromine- if true to Kekule model, benzene would react with bromine (localised π bond -> high e density -> induce dipole in Br). BUT benzene does not react with bromine at RTP
  2. Enthalpy change of hydrogenation- if Kekule, enthalpy change would be 3× cyclohexene (-120). BUT the enthalpy change of benzene was less exothermic (-208)
  3. X-ray diffraction- if Kekule, alternating double and single bonds BUT regular hexagonal shape with each same C-C bond length (between single & double bond length)
• Summary of Kekule vs Delocalised evidence

  1. Enthalpy change of hydrogenation of benzene is less exothermic than expected
  2. Benzene is less reactive than alkenes, benzene will not decolourise bromine water (benzene onreacts with bromine at high temp or w halogen carrier catalyst)
  3. X-ray diffraction experiments show all 6 C-C bonds have same length (intermediate between short C=C and long C-C)
• Benzene vs alkene

  • Benzene is less reactive than an alkene
  • Benzene is more stable than an alkene
  • π ring (benzene) is less e dense than π bond (alkene)
  • Cannot repel e in Br & induce dipole/ generate electrophile
• Describe Kekule model (compared to delocalised)

  • π bonds formed by sideways overlap of 2 neighbouring p-orbitals above & below plane of the ring
  • Benzene ring contains 3 localised π bonds
  • each localised π bond contains 2 shared electrons between 2 carbon atoms, higher e density

  
• Describe Delocalised model (compared to Kekule)

  • π bond is formed by sideways overlap of 6 p-orbitals above & below the plane of the ring
  • the benzene ring contains one delocalised π bond
  • the delocalised π bond contains 6 shared electrons between 6 carbon atoms, lower e density

  
• Evidence Kekules structure is not fully correct

  1. X-ray diffraction/bond length - Kekulé’s structure would suggest that all carbon-carbon bond lengths in the ring would not be equal whereas x-ray diffraction proved that they were
  2. Enthalpy of hydrogenation - Kekulé’s structure suggests enthalpy of hydrogenation of benzene would be 3 times cyclohexene. Experimental data showed that this reaction was less exothermic / benzene was more stable
  3. Reactivity w bromine - Kekulé’s structure suggests the presence of C=C bonds. Other alkenes react at rtp to decolourise bromine. Benzene does not.
• Compare & explain reactivity of benzene & cyclohexene towards bromine

  • Cyclohexene decolourises bromine water (orange to colourless)
  • π bond is localised between 2 carbon atoms
  • higher π e density than in benzene
  • better able to attract Br2 electrophile (better ble to induce dipole/polarise) Br2 molecule
  • Benzene only reacts w Br in presence of halogen carrier catalyst
  • π bond delocalised over 6 C (benzene)
  • lower π density in ring
  • less able to attract Br2

  
• Electrophilic substitution of benzene in the presence of halogen carrier catalyst

  iron(III) bromide or aluminium chloride

  overall eq: C6H6 + X2 → C6H5X + HX

  Conditions: Heat/ reflux and Halogen carrier e.g. AlX3/ FeX3

  
• Nitration of benzene

  Reagents and conditions: conc. HNO3 / conc. H2SO4 / reflux 55 °C

  If reaction temp rises above 95°C, further substitution could occur so temp is controlled using water bath

  
• Acylation of benzene

  Acylation of benzene reagents and conditions: acyl chloride / AlCl3 / anhydrous

  
• Alkylation of benzene

  Reagents and conditions: haloalkane / AlCl3 / anhydrous

  
• Phenol solubility in water

  • Low solubility in water
  • Hydrophilic OH can form hydrogen bonds with water
  • benzene ring is non-polar and hydrophobic
• Bonding in phenol

  In phenol one of the lone pairs of e in a p-orbital is delocalised into the ring

  
• Effect of delocalisation on acidity in phenol

  • Phenol can lose a H ion since phenoxide ion (C6H5O-) is fairly stable -> so more likely to form
  • 1 lone e pair from oxygen delocalised with e cloud of whole phenoxide ion spreading its charge makes the ion more stable
  • C6H5OH ⇌ C6H5O- + H+
  • O is most electronegative element in ion so delocalised e drawn to it
  • Lot of charge around O so H ion is attarcted again
  • So phenol behaves like a weak acid
• Effect of delocalisation on electrophilic attack in phenol

  • OH- group is electron donating
  • There is increased e density in the π delocalised system
  • The π ring system is activated towards electrophilic attack
  • Electrophilic substitution is directed towards positions 2 and 4 (or 6)
  • Phenol is more able to attract an electrophile than benzene & undergoes electrophilic substitution reactions much more readily than benzene does
• Acid reactions of phenol

  (acid + metal → phenoxide salt + hydrogen) – Phenols react vigorously with reactive metals such as sodium (Na). A soluble salt is formed and hydrogen gas is given off

  Observation: effervescence

  Reaction with sodium hydroxide solution (acid + base → phenoxide salt + water). Phenols dissolve in alkaline solutions and undergo acid-base reactions with bases to form a soluble salt and water

  Observation: phenol dissolves

  
• Acid reaction of alcohol

  Alcohol + sodium → alkoxide salt + hydroge

  
• Acid reactions of carboxylic acid

  Carboxylic acid + sodium → carboxylate salt + hydrogen

  Carboxylic acid + sodium hydroxide → carboxylate salt + water

  Carboxylic acid + sodium carbonate → carboxylate salt + water + carbon dioxide
• Strength of weak acids

  Strongest

  1. Carboxylic acid
  2. Phenol
  3. (Aliphatic) Alcohol

  Weakest
• Reactions of weak acids

  • CA reacts w metal & metal hydroxide & metal carbonate
  • Phenol reacts w metal & metal hydroxide
  • Alcohol reacts w metal

  
• Esterification of phenol

  Phenol is a weak nucleophile because the lone pairs on the oxygen atom are delocalised into the ring. They do not react readily with carboxylic acids; instead, a more reactive acyl chloride or acid anhydride is used

  
• Benzene vs phenol

  • lone pair of e from oxygen p-orbital is delocalised in to π ring
  • π region in phenol has higher e density
  • so can polarise/ generate an electrophile more readily than benzene
  • so phenol is more reactive than benzene

  
• Halogenation of phenol

  • Phenols also undergo electrophilic substitution reactions when reacted with bromine water at room temperature
  • Phenol decolourises the orange bromine solution to form a white precipitate of 2,4,6-tribromophenol

  Observation: bromine decolourises (orange to colourless) / white solid formed

  Phenolic compounds are used as antiseptics

  
• Nitration of phenol

  Phenols can undergo electrophilic substitution reactions when reacted with dilute nitric acid (HNO3) at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol. When concentrated HNO3 is used, the product will be 2,4,6-trinitrophenol instead. A hydrogen atom in the benzene ring is substituted by a nitro (-NO2) group

  
• Directing effect of substituted aromatic compounds

  Substituted aromatic compounds, C6H5X can be nitrated, chlorinated, alkylated or acylated in a similar way to benzene. The side chain, X, influences the rate of reaction and the position of the substitution
• Electron donating and electron withdrawing sides groups

  Side groups on a benzene ring can affect the position on the ring of where substitution reactions will occur. The side groups on a benzene ring can either be electron-withdrawing or electron-donating groups
• Electron donating groups (activating 2,4 directing)

  Electron-donating groups, such as -CH3, -OH and -NH2, donate electron density into the π system of the benzene ring making it more reactive.

  These groups activate attack by electrophiles and direct the incoming electrophile to attack the 2 and/or 4 positions
• Electron withdrawing groups (all are –X=O) (deactivating 3,5 directing)

  Electron-withdrawing substituents, such as -NO2, remove electron density from the π system in the benzene ring making it less reactive

  These groups deactivate attack by electrophiles and direct the incoming electrophile to attack the 3 position
• Reduction of nitrobenzene

  Nitrobenzene is used in the synthesis of many organic compounds such as phenylamine, an intermediate in the production of dyes

  Reagents and conditions: Sn / conc. HCl / reflux