Benzene

Benzene

1. Structure

   • The structure of benzene has been theorized many times
   • Benzene consists of a ring of delocalized electrons in the p orbitals of the carbons
   • The p orbitals of all 6 carbon atoms overlap creating a ring of delocalized electrons with a high e- density
   • Naming of aromatic compounds
     • Can use benzene - eg 1,3-dichloro benzene
     • Cam use phenyl group - eg phenol or phenylamine
   • The old "Keluke model" is that benzene consists of alternating double bonds in the ring, but this was disproved by bond lengths and enthalpies of hydrogenation

2. Electrophillic substitution

   • Electrophillic substitution in benzene results in a hydrogen atom being substituted by an electrophile
   • Halogen carrying compounds such as AlCl4 make good catalysts for this reaction
   • A halogen carrier accepts a lone pair of e- from a halogen atom on an electrophile - as the lone pair of e- is pulled away the polarisation in the molecule increases and a carbocation forms (C+) - this makes the electrophile stronger
   • Regeneration of catalyst
     • The catalyst polarises the halogen making an electrohile
     • During the reaction a halogen replaces a H
     • This H reacts with the positively charged catalyst to regenerate the catalyst and form a HX (eg HCl, HBr)

3. Reactions

   • Benzene will not undergo simple addition reactions like alkenes or cycloalkenes because benzene is more stable - this is due to the e- density being more spread out in the molecule
   • Friedel-crafts reaction
     • This reaction forms C-C bonds in organic synthesis
     • Alkylation
       • Adding a CH3 or larger alkyl group to benzene using a haloalkane (halogen part of haloalkane acting as electrophile) and a halogen carrying catalyst
     • Acylation
       • Substitutes an aldehyde or ketone for a H atom on benzene
   • Nitric acid electrophile and H2SO4 catalyst
     • Benzene needs to be warmed with conc nitric acid and conc sulfuric acid
     • The product is nitrobenzene

4. Phenol

   • Phenol is a benzene ring with an -OH group
   • Reactivity
     • It is more reactive than benzene - so is more likely to undergo electrophillic substitution
     • One of the lone pairs on the oxygen p orbital overlaps with the delocalized ring of e-
     • This increases e- density in the ring making it more likely to be attacked by electrophiles
     • The lone e- pair in the p orbital partially delocalises into the benzene ring
   • Positioning of substitution
     • In an unsubstituted benzene ring all the carbons are the same and have the same chance of being attacked by an electrophile
     • Functional groups change the e- density at certain carbons making them more or less likely to react
     • Electron donating groups
       • eg -OH and -NH2
       • They have e- in orbitals that overlap with the delocalised ring and increases its e- density
       • They increase the e- density at carbons 2,4 and 6 relative to the functional group
       • So electrophiles are more likely to attack carbons 2,4, and 6
     • Electron withdrawing groups
       • eg -NO2
       • NO2 doesnt have any orbitals it can overlap with the delocalised ring and its electronegative, so withdraws e- density from the ring
       • NO2 decreases e- density around carbons 2,4 and 6 - so electrophiles are unlikely to attack there
       • This makes carbons 3 and 5 more likely to be attacked by electrophiles
   • Reactions
     • Phenol + bromine water
       • If you shake phenol with orange bromine water it will react decolourising it
       • As the OH in phenol is a e- donating group the product is 2,4,6-tribromophenol
     • Phenol + dilute nitric acid
       • Phenol + HNO3 -> nitrophenol + H2O
       • It produces two different isomers of nitrophenol - 2-nitrophenol and 4-nitrophenol
       • This reaction occurs because the OH has an activating effect on carbons 2 and 4
     • Phenol + bases or Sodium -> salts
       • Phenol is weakly acidic
       • Phenol reacts with NaOH at RTP in a neutralisation reaction to form sodium phenoxide and water
       • It doesnt react with NaCO3 though as it is not a strong enough acid

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