Topic 27: Aromatic Chemistry Flashcards
Aliphatic
Contain carbon and hydrogen atoms joined together in straight chains, branched chains or non aromatic rings
Reasons for benzene structure
All bonds same length
Hydrogenation of benzene enthalpy lower than expected
Delocalised pi system is highly saturated but substitution reaction rather than addition occured
Hydrogenation of benzene
Adding hydrogen to the benzene ring.
If alternate double bond structure was used would only be able to happen thrice so 3x enthalpy, benzenes actual hydrogenation enthalpy is less exothermic than that. Both add 3 H so bond making enthalpy is the same. Bond breaking is more for benzene ring as enthalpy is less negative so benzene is more stable.
Benzene structure
Delocalised pi system
Delocalised pi system
Unpaired electrons in the p orbital of each carbon overlap and are delocalised.
Benzene mechanism name
Electrophilic substitution
Equation to create nitration electrophile
conc. HNO3 + conc. H2SO4 —> NO2+ (nitronium ion electrophile) HSO4^- + H2O
Electrophilic substitution of benzene mechanism
Benzene ring arrow from ring onto N on NO2+ (+ on N)
New molecule
Horseshoe ring centred around carbon with H and NO2 attached
Positive inside ring
Arrow from H bond to inside ring
Creates NO2 on benzene, nitrobenzene
Types of benzene electrophilic substitution
Nitration
Friedel-Crafts Acylation
Reaction to form electrophile for Friedel-Crafts Acylation
RCOCl + AlCl3 —> RCO+ + AlCl4-
Friedel-Crafts Acylation mechanism
Benzene ring arrow from ring onto C on RCO+ (+ on C)
New molecule
Horseshoe ring centred around carbon with H and RCO attached
Positive inside ring
Arrow from H bond to inside ring
Creates RCO on benzene, phenylketone
Inductive effect on benzene
Benzene rings draw electron density towards themselves, away from any substituent group such as a halogen atom, negative inductive effect. Lone pair on branched group becomes delocalised onto benzene ring so less available.
Conditions for nitration of benzene
Reflux at 55*C
Benzene substituent name
Phenyl group
Why does the reaction to form the nitronium ion work?
Sulphuric acid is a stronger acid so donates a proton to nitric acid, acts as bronsted-lowry acid
Reagents for Friedel-Crafts Acylation
Acyl chloride, anhydrous aliminium chloride, AlCl3
Conditions for Friedel-Crafts Acylation
Reflux 50*C, dry inert solution (ether)
Electron donating groups on benzene
OH, CH3, NH2 increased e- density in ring, add to 2,4,6 position
Electron withdrawing groups on benzene
NO2, e- density in ring decreases, add to 3,5 position
