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Week 6:Aromatic Compounds ((Have a parent) Benzenes: six-membered rings…
Week 6:Aromatic Compounds
(Have a parent) Benzenes: six-membered rings with 3 alternate double/ single bonds
Description
molecular formula = C6H6
all 6 carbon atoms are equivalent
double bonds --> sp2 hybridization (sp2 orbital & p orbital)
all 6 hydrogen atoms are equivalent
All 6 bonds are the same
All 6 p orbitals overlap continuously
All p electrons in the p orbitals are delocalised
(decalised) For stable benzene
does not undergo the addition reactions unlike alkenes.
disrupts delocalised electrons
but undergoes
Nomenclature
Disubstituted
has 3 possible isomers
Meta- (1,3)
Para- (1,4)
Ortho- (1,2)
When substituents (X) are different, name according to alphabetical order
More than two substituents: their positions are designated by numbering the ring
DO NOT USE
Monosubstituted
Some common names
can be included in
Electrophilic Aromatic Substitution: driven by catalyst
General chemical equation
have 5 examples
Sulphonation: One of the H in benzene replace with SO3H
The catalyst, H2SO4 will be regenerated
Friedel-Crafts Alkylation:One of the H in benzene replace with “alkyl group, R”
The catalyst, AlCl3 will be regenerated
Nitration: One of the H in benzene replace with NO2
catalyst, H2SO4 will be regenerated
Friedel-Crafts Acylation: One of the H in benzene replace with “acyl group, R- C = O”
Halogenation: One of the H in benzene replace with halogen (Cl, Br)
catalyst, FeX3 (FeCl3, FeBr3) will be regenerated
Reaction mechanism
Part I: Electrophile generation
Benzene is rich in electrons and is stable (delocalization of p electrons), thus, need the reactive electrophile
Part II: Two-step substitution
2 p electrons flow to E+ and E attach to benzene
results in formation of carbocation intermediate, benzenonium ion
1 H atom from carbon with E leaves benzene to reconstitute the more stable aromatic structure.
Directing effects of substituents
Electrophilic substitution of benzene gives one product
Electrophilic substitution of substituted benzene gives 3
The directing effect affects the position of the reactant added to the benzene
Ortho- and Para- Directing: Y = electron donating groups
Examples
halogens (Cl, Br),
alkyl (CH3, CH3CH2),
OH,
NH2
Meta Directing: Y = electron withdrawing groups
Examples
carboxylic acid (COOH),
aldehyde (CHO),
ketone (COR),
NO2,
SO3H
