Aromatic Chemistry ORGANIC Flashcards
(24 cards)
How is “aromatic” used in chemistry?
To describe molecules containing one or more benzene ring
What structure does benzene have?
6 carbon atoms in a hexagonal ring with alternating single and double carbon-carbon bonds
What is the main reaction benzene will undergo?
Replacement of one of the 6 hydrogen atoms
What type of reaction is the replacement of H in benzene ring? Why?
Electrophilic substitution - hydrogen atom substituted by electrophile
Describe the general mechanism for electrophilic substitution in benzene ring.
Positive electrophile attracted to electron density of delocalised π ring in benzene
Intermediate formed - both H and electrophile bonded to ring
Delocalised π system temporarily broken to leave partially delocalised electron system and positive charge
Intermediate loses proton and electrons from C-H bond restore delocalised π system
What three steps are seen in electrophilic substitution reaction in arenes?
Generation of electrophile
Electrophilic attack
Regenerating aromaticity
Describe the nitration of benzene.
Electrophile NO₂⁺ generated by reacting concentrated nitric acid and concentrated sulfuric acid
Once generated, electrophilic attack on benzene ring
How stable are arenes? Why?
Very stable
Due to delocalisation of π electrons in ring
What happens to delocalised ring in substitution reactions?
Delocalised ring maintained
What happens to delocalised ring in addition reactions?
Delocalised ring disrupted
What types of reactions do arenes undergo?
Substitution
Nitration
Friedel-Crafts alkylation
Friedel-Crafts acylation
Hydrogenation
What type of reaction are Friedel-Crafts reactions?
Electrophilic substitution
What needs to be done to use arenes as starting materials for organic synthesis?
Structure changed to be more reactive than usually unstable due to aromatic stabilisation
What is the difference between Friedel-Crafts reactions?
Friedel-Crafts reaction alkylation substitutes H atom in benzene ring for alkyl group
Friedel-Crafts reaction acylation substitutes H atom in benzene ring for acyl group
What three steps are seen in Friedel-Crafts reactions?
Generating electrophile
Electrophile attack on benzene ring
Regenerating aromaticity of benzene ring
What happens in Friedel-Crafts acylation reactions?
Benzene ring reacts with acyl chloride in presence of AlCl₃ catalyst
What type of reaction is the hydrogenation of benzene?
Addition reaction
What happens in hydrogenation of benzene?
Benzene heated with hydrogen gas and nickel/platinum catalyst to form cyclohexane
What reaction conditions is there for halogenation of arenes?
Cl₂/Br₂ with AlCl₃/AlBr₃ catalyst
What reaction conditions is there for nitration of arenes?
Mixture of conc HNO₃ and conc H₂SO₄ at temp 25-60ᵒC
What reaction conditions is there for Friedel-Crafts alkylation of arenes?
Halogenoalkane and anhydrous AlCl₃ catalyst
What reaction conditions is there for Friedel-Crafts acylation of arenes?
Acyl chloride and anhydrous AlCl₃ catalyst
What reaction conditions is there for complete oxidation of arenes?
Hot alkaline KMnO₄ and dilute acid
What reaction conditions is there for hydrogenation of arenes?
Hydrogen and Pt/Ni catalyst and heat
