3.10: Aromatic Chemistry

What is the structure of benzene?

• Planar molecule

• Each carbon has 3 bonds

• One unhybridized 2p orbital with 1 electron

• Overlap of 6 parallel 2p orbitals form continuous pi bond

• Electron density lies in 1 cloud above and below the plane

• Equal bond lengths

Compare the structures of benzene and cyclohexa-1,3,5-triene

• Benzene is more stable than cyclohexa-1,3,5-triene

• The enthalpy of hydrogenation of benzene is less exothermic

• Due to the delocalization of e- in benzene 

• Both are planar molecules 

• Benzene has equal C-C bond lengths, whereas cyclohexa-1,3,5-triene as varied bond lengths as the hexagon is distorted 

Why is benzene more stable than cyclohexa-1,3,5-triene

Delocalisation of p e- in benzene means they form continuous pi bond

Why does electrophilic substitution of benzene take place instead of addition?

• The continuous pi bond of delocalised e- would have to be broken

• So stability would be significantly reduced 

What is the structure of TNT? 

What is the structure of aniline?

What is the name of the mechanism for the nitration of benzene/friedel-crafts acylation?

Electrophilic substitution

What are the conditions required for electrophilic substitution?

• 50⁰C

• Conc HNO₃+ conc H₂SO₄

What is the equation for the formation of reactive catalyst in Friedel-Crafs Acylation?

R-COCl + AlCl₃ → R-CO⁺ + AlCl₄^-