C25 - Aromatic compounds Flashcards
What is benzene
C6H6
What are properties of benzene
Colourless
Sweet smelling
Highly flammable
Liquid
Carcinogen
Aromatic
What are the two models of benzene
Kekule’s model
Delocalised model
What is an arene
Aromatic (containing one or more benzene rings) hydrocarbon
What is Kekule’s model
Hexagonal carbon ring, joined by alternate double and single bonds
Evidence to disprove Kekule’s model
-Lack of reactivity of benzene: it does not undergo electrophilic addition reactions or decolourise bromine suggesting lack of double bonds
-Lengths of C-C bond lengths: bond lengths should be alternating but they are uniform
-Hydrogenation enthalpies: enthalpy should be three times cyclohexane, but isn’t
What is the delocalised model of benzene
-Planar, cyclic hexagonal hydrocarbon (C6H6)
-Each C atom uses its available 4 electrons in bonding to 2 other C atoms and 1 hydrogen atom
-Each C atom has 1 electron in a p-orbital, perpendicular to the plane
-Adjacent p-orbital overlap sideways, above and below plane to form ring of e- density
-Overlapping of p-orbitals creates system of pi-bonds, 6 delocalised e- in system of pi-bonds
Naming aromatic compounds with one substituent group
-Alkyl groups, halogens, nitro groups are prefixes
-Benzene substituent to alkyl chain (phenyl is prefix)
What is benzoic acid
What is phenylamine
Naming aromatic compounds with more than one substituent group
Alkyl group takes priority (alkylbenzene becomes parent chain)
What is benzaldehyde
Benzene electrophilic substitution
H atom on benzene ring is replaced by another atom/group of atoms (electrophile)
Nitration of benzene
C6H6 + HNO3 –> C6H5NO2 + H2O
Conditions- H2SO4 + 50C
What happens if nitration of benzene occurs at a higher temp
Further substitution, forming dinitrobenzene (1,3)
Mechanism of nitration of benzene
What does the halogenation of benzene require
Require halogen carrier catalyst (can be generated in situ)
Eg. AlCl3, FeCl3, AlBr3, FeBr3
Bromination of benzene
C6H6 + Br2 —> C6H5Br + HBr
Conditions- RTP, FeBr3/AlBr3
Bromination of benzene mechanism