Aromatic Chemistry: Electrophilic Substitution Flashcards
What type of mechanisms do benzenes undergo?
- Electrophilic substitution
Why do benzenes tend to undergo electrophilic substitution reactions, rather than electrophilic addition reactions?
- Benzene rings is very stable due to delocalised ring of electrons
- Electrophilic addition reactions would destroy delocalised ring of electrons
- Instead, one of hydrogen atoms or other group bonded to ring is substituted by electrophile
Draw a general mechanism for the reactions of aromatics using E to represent electrophile
Explain what occurs at each step of the general mechanism for the reactions of aromatics
- Electron-dense delocalised ring of electrons attracts an electrophile
- Electrophile steals pair of electrons from centre of benzene ring and forms bond with one of carbons
- This partially delocalised ring of electrons and gives molecule positive charge
- To regain stability of benzene ring, carbon bound to electrophile loses a hydrogen to the ring
- Get substitution of H+ with electrophile
What conditions are needed for the nitration of benzene?
- Concentrated nitric acid
- Concentrated sulfuric acid catalyst
- Maximum temperature of 50°C
What is nitration of benzene?
- Addition of NO2+ (nitronium ion) to benzene
Why is sulfuric acid needed for the nitration of benzene?
- Acts as catalyst
- Generates electrophile (NO2+)
Give the overall word equation for the nitration of benzene
- Nitric acid + benzene → nitrobenzene + water
- Sulfuric acid above →
Give the overall symbol equation for the nitration of benzene
- HNO3 + benzene / C6H6 → nitrobenzene / C6H5NO2 + H2O
Give the symbol equation for the formation of the electrophile, the nitronium ion, during the nitration of benzene
- HNO3 + H2SO4 → HSO4- + NO2+ + H2O
Draw the mechanism for the nitration of benzene
What happens to the produced H+ ion made from the nitration of benzene?
- Reacts with HSO4- to reform H2SO4
In which cases must we keep the temperature below 55°C during the nitration of benzene and why?
- Applies if only want one NO2 group added (mono nitration)
- Above this temperature you’ll get multiple substitutions
What are the uses of nitration reactions?
- Nitro compounds can be reduced to form aromatic amines, used to manufacture dyes and pharmaceuticals
- Explosives e.g TNT
What are the reactions between benzenes and acyl chlorides known as?
- Friedel-Crafts acylation