3.3.10 Aromatic Chemistry Flashcards
What is the structure of a benzene ring? (4)
Planar structure - six carbon atoms joined in flat ring
Each carbon is covalently bonded to two others and a hydrogen atom
The final unpaired electron is located in a p-orbital, sticking out above and below the ring, combining to form a delocalised ring of electrons
All bonds are 140pm, between a single C-C bond and a double C=C bond
How does enthalpy of hydrogenation indicate the stability of benzene?
If benzene had three double C=C bonds (theoretically called cyclohexa-1,3,5-triene, it’s enthalpy of hydrogenation would be three times that of cyclohexene; 3 x -120 = -360 kJ mol^-1
The actual enthalpy of hydrogenation is -208 kJ mol^-1, less exothermic than expected
This means more energy must have been put in to break the bongs in benzene so it is more stable, possibly due to the delocalised ring of electrons
Why does benzene undergo substitution reactions instead of elimination reactions?
The benzene ring is too stable so electrophilic addition reactions would destroy the delocalised ring
What is nitration? (3)
Benzene + warm conc. HNO3 > nitrobenzene + H2O
Sulphuric acid acts as a catalyst, forming the electrophilic nitronium ion; HNO3 + H2SO4 > HSO4- + NO2+ + H2O
After forming the nitrobenzene, the remaining H+ reforms the catalyst with HSO4-
What are the products of nitration reactions used for? (3)
Reduced to form aromatic amines
Used to manufacture dyes and pharmaceuticals
Decompose violently when heated so used as explosives
What is Friedel-Crafts acylation? (5)
Heated under reflux in dry ether
Acyl chloride + benzene > phenylketone + HCl
The partial polar charge on the acyl chloride is insufficient to act as an electrophile on the benzene ring
AlCl3 is used to remove the chlorine from the acyl chloride to make it a stronger electrophile; RCOCl + AlCl3 > RCO+ + AlCl4-
The H+ ion reacts with AlCl4- after the reaction, reforming AlCl3 and HCl
