Aromatics

    Cards (35)

    • Benzene is a planar cyclic molecule
    • Benzene has the formula C6H6C_6H_6
    • In benzene, each carbon is bonded to two other carbons and one hydrogen atom and the final lone pair is in the p-orbital which sticks out above and below the planar ring
    • The lone electrons in the p-orbitals combine to form a delocalised ring of electrons which means that all bonds have the same length
    • This is kekules structure:
    • Kekules structure shows benzene with double bonds alternating around the benzene
    • This is the more common structure:
    • This structure has a circle to show the ring of delocalised electron system
    • Benzene is more stable than kekules structure (cyclohexa-1,3,5-triene)
    • We measure the stability of benzene by comparing the enthalpy of hydrogenation in benzene and cyclohexa-1,3,5-triene
    • If you hydrogenate cyclohexene has enthalpy change of -120 so cyclohexa-1,3,5-triene has enthalpy change of -120x3=-360
    • The hydrogenation of benzene is far lower than that of cyclohexa-1,3,5-triene at -208
    • More energy is required to break bonds in benzene than cyclohexa-1,3,5-triene because it is less exothermic and suggests that it is more stable because of its delocalised electron structure
    • Arenes are an aromatic compounds that contain a benzene ring and are named in 2 ways
    • The priority group ranking is
      1. Carboxylics (Can)
      2. Esters (Elephants)
      3. Amides (Ask)
      4. Nitriles (Narwahls)
      5. Aldehyde (About)
      6. Ketone (Keeping)
      7. Alcohol (Anklets)
      8. Arenes (As)
      9. Alkenes (An)
      10. Alkanes (Accessory)
      11. Halogens (Huh?)
      12. Nitros (No!)
    • Anything above arenes in priority is a phenyl prefix, where as anything below arenes in priority have the benzene suffix
    • Arenes undergo electrophilic substitution
    • Arenes undergo electrophilic substitution because it has a high electron density due to its delocalised ring of electrons which attracts electrophiles
    • Benzene is very stable so do not undergo electrophilic additiobn
    • Electrophilic substitution with arenes is when a hydrogen group is substituted for the electrophile
    • Two mechanisms for Arenes is:
      1. Friedel-crafts Acylation
      2. Nitration
    • Benzene is a very important chemical in pharmaceuticals and dyes
    • Benzenes stability is an issue because it makes it difficult to react, Friedel-crafts acylation solved this problem
    • Friedel-Crafts acylation is when an Acyl group (RCO-) is added onto a benzene molecule to weaken the benzene structure and make it easier to modify
    • In Friedel-Crafts acylation, we react an Acyl Chloride with AlCl3AlCl_3to create a strongly positive electrophile to attack the benzene
    • The electrophile in Friedel-Crafts Acylation is made by this reaction where the AlCl3AlCl_3​accepts a pair of electrons from the acyl group forming a carbonation which then goes on to react with benzene
    • The electrophile now reacts with benzene to make a phenylketone under reflux and a dry ether solvent
    • The electrophile is attracted by the delocalised electrons and forms a bond which breaks the ring leaving a positive charge. The hydrogen then breaks off feeding electrons back into the benzene and reforming its delocalised ring.
    • Nitration of benzene is useful because it allows for the production of dyes and explosives
    • To form a positive electrophile for the nitration of benzene there is a 2 step process:
      1. React sulphuric acid with nitric acid
      2. H2NO3+H_2NO_3^+ion decomposes in nitronium ion
    • The reaction for the first step of forming an electrophile for nitration of benzene is:
      HNO3+HNO_3+H2SO4H2NO3+H_2SO_4\rightarrow H_2NO_3^+++HSO4HSO_4^-
    • The second step of forming an electrophile for nitration of benzene is:
      H2NO3+H_2NO_3^+NO2+\rightarrow NO_2^+++H2OH_2O
    • The Nitronium ion (electrophile made) is attracted by the delocalised electrons and forms a bond leaving a positive charge, the hydrogen breaks off and feeds electrons back into benzene and reforms the delocalised ring
    • When doing the nitration of benzene, the temperature must be below 55 degrees because it can undergo multiple substitutions above this temperature and become explosive
    • Nitrobenzene compounds can be used to make:
      1. Dyes and pharmaceuticals by reduction to aromatic amines
      2. Explosives like TNT
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