A2- Benzene
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Draw benzene and give its formula
regular hexagon, with ring/circle in the middle C6H6
Why is benzene stable?
because it has delocalised pi orbital system of electrons
Based on shape only, why can we say cyclohexatriene is only a hypothetical structure?
as the bonds lengths would not be possible if it was in this shape, therefore benzene has an intermediate bond length making it a regular hexagon
How does hydrogenation show the hypothetical cyclohexatriene/kekule is not the real structure of benzene?
as the expected enthalpy change would be 152kJ mol-1 more negative than the value obtained.This means benzene is 152kJ mol-1 more stable, which is due to the delocalised rings
Why might the reactions of benzene suggest it is not the kelule structure?
does not undergo addition reactions, as would be expected
How does adding other atoms impact the stability of benzene?
it makes is less stable, as it uses some of the delocalised electrons
Give the equation and reaction conditions for the nitration of benzene
Benzene + HNO3 –> Nitrobenzene + H2O - 50C - conc. H2SO4 as a catalyst
What 2 equations show the formation of NO2+ ions?
HNO3 + H2SO4 –> HSO4- + [N2NO3]+ [H2NO3]+ –> H2O + NO2+
What mechanism is the nitration of benzene?
electrophilic substitution
Describe the bonding in Benzene
• 6 C-C sigma bonds • 6 C-H sigma bonds • P orbitals of C atoms overlap • forms delocalised Pi system
What is needed for benzene to react with chlorine?
halogen carrier, AlCl3
What are the reaction conditions for the chlorination of benzene?
25C, AlCl3 and Cl2
How is a Cl+ generated for the chlorination of benzene?
in situ, by mixing AlCl3 with Cl2 and Benzene AlCl3 + Cl2 –> AlCl4- + Cl+
How is AlCl3 regenerated?
AlCl4- + H+ –> AlCl3 + HCl
What are Friedel crafts reactions?
substitution reactions where an alkyl or acyl chain replaces a hydrogen on an aromatic compound
What are the reaction conditions for alkylation?
room temp, AlCl3 and chloroalkane
What are the reaction conditions for acylation?
AlCl3, 60C, halogen carrier and acyl chloride, anhydrous conditions
What would happen if water was present in an acylation reaction?
acyl chloride would react to form COOH and HCl(g)
How could a reaction mixture after alkylation be separated?
fractional distillation
What will an excess of benzene lead to in electrophilic substitutions?
single substitution
Describe the structure/shape of benzene
• planar • regular hexagon • same bond angles and lengths