2. reactions of aromatics

• reactions of aromatic compounds

  stability of the delocalised ring structure - do not readily take part in the typical reactions of the alkenes,  e.g. addition reactions.
• aromatics reactions


  usually take part in substitution > addition reactions - allow to keep the stability of the benzene ring
• aromatics reactions - role of benzene

  benzene ring reacts with an electrophile - electrophilic substitution
• aromatic reactions - electrophile

  attracted to high regions of electron density about and below the plane of the ring
• Why do nucleophiles not tend to react with benzene rings?


  The lone pair of electrons on the nucleophile is repelled by the region of high electron density above and below the plane of the ring.
• general mechanism for reactions of aromatics

  
• mechanism of aromatics

  
• what does benzene being fairly unreactive mean?

  that extreme conditions are needed for electrophilic substitutions to occur - to generate a very strong electrophile (with a full + charge)
• nitration 


  the addition of an NO2+ (electrophile) to a compound
• nitration conditions 


  a mixture of concentrated nitric and concentrated sulphuric acids at 50°C
• what does the sulphuric acid act as in nitration?

  catalyst - needed to generate the electrophile
• formation of the nitration catalyst

  H2SO4 + HNO3 -> HSO4- + H2NO3+

  H2SO4 + HNO3 -> HSO4- + NO2+ + H2O
• nitration

  
• nitration mechanism

  
• During the nitration of methylbenzene the ring is substituted with 3 nitro groups rather than one, give a possible reason for this.


  • The presence of the methyl group, which is electron releasing, makes the benzene ring more susceptible to attack by electrophiles. ​
  • So rather than the substitution of one nitro group, three nitro groups are present in the product
• Friedel-Crafts Acylation 


  Benzene can react with acyl chlorides in the presence of an aluminium chloride catalyst to form an aromatic ketone.
• friedel-crafts acylation


  CH3COCl + AlCl3 -> CH3CO+ + AlCl4–
• ethanoyl chloride and aluminium chloride reaction

  CH3COCl + AlCl3 -> CH3CO+ + AlCl4–
• The aluminium chloride attracts a pair of electrons from the C–Cl bond in the acyl chloride to produce the electrophile (an acylium ion).
• Why must we use anhydrous conditions in Friedel-Crafts reactions?

  The aluminium chloride and the acyl chloride both react readily with water.
• ethanoyl chloride + benzene -> phenylethanone + HCl

  
• electrophilic substitution