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Aromatic Chemistry (Mechanisms) - Coggle Diagram
Aromatic Chemistry
(Mechanisms)
Electrophilic Substitution
arenes v stable
compounds
region of
high electron density
above + below plane of molecule
substitution initiated by electrophile
electrophile
ion or molecule
with lone pair of electrons
that attack regions of high electron density
substitution reaction
where 1 atom/ group of atoms replaced by different atom/group of atoms
general
mechanism
steps
HED of delocalised ring
attracts E+
pair of e- from ring of delocalised pi e-
form bond w/ E+
breaking ring
highly unstable intermediate produced
only partially delocalised e- system
w/ 4 delocalised e-
in unstable intermediate
C-H bond breaks
2 e- in bond move back to pi e- system
reforms stable delocalised ring
Hydrogen lost as H
+
Bromination
monobromination
D: reaction in which 1 or more
Br atoms are added to compound
why not added across C=C bond?
partial C=C π bond requires additional energy
to overcome stability of delocalisation of π electrons
conditions
catalyst
polarises Br molecule
& Br+ acts as electrophile
iron (III) bromide
or iron (reacts w/ Br2 to form^^)
2Fe + 3Br2 --> 2FeBr3
called
'halogen carrier'
room temperature
equations
step 1
formation of electrophile
Br-Br undergoes heterolytic fission
forms bromine cation - Br+
Br2 + FeBr3 ----> Br+ + FeBr4 -
step 2
Br+ forms
coordinate bond
w C atom
by accepting pair of delocalised π e- from benzene ring
benzene ring =
POSITIVE CHARGE
break in delocalisation of π bonding
elimination of H+
= restores delocalisation
mechanism
step 3
reforming the catalyst
H+ eliminated from intermediate
H+ + FeBr4- ----> FeBr3 + HBr
mechanism
Br+ ion
good electrophile
readily attacks electron rich regions
formed by delocalised π-electrons
above & below ring
Nitration
D: addition of 1 or more nitro (-NO2)
groups to a compound
benzene reacts with
NITRATING MIXTURE
conc nitric acid
conc sulfuric acid (acts as catalyst)
nitric + sulfuric acid react = form
Nitronium Ion (NO2+)
active nitrating species
acts as
electrophile
equation
mononitration of benzene
mechanism
step 1
formation of nitronium ion (electrophile)
HNO3 + 2H2SO4 ---> NO2+ + 2H2O4- + H3O+ (hydroxonium ion)
or
HNO3 + H2SO4 ---> NO2+ + H2O4- + H2O
step 2
same as STEP 2 in Bromination
step 3
catalyst (sulfuric acid) reformed
when H+ eliminated from intermediate
H+ + HSO4- ---> H2SO4
conditions
conc sulfuric acid +
conc nitric acid
H2SO4 = catalyst
increases rate of reaction
w/out being used up
@ temp not exceeding 50°C
controls substitutions
Acylation
D: process of
replacing H atom
by an acyl group
benzene reacts with
acyl chloride
in presence of AlCl3 catalyst
form
aromatic ketone
conditions
catalyst =
Aluminium Chloride
- AlCl3
anhydrous
prevents hydrolysis
electrophile
acylium ion
H3C------C+ ===== O
formed from
ethanoyl chloride
aluminium chloride catalyst
mechanism
step 1
formation of electrophile
CH3COCl + AlCl3 ---> H3C----C+====O + AlCl4-
step 2
acylium ion
attacks
delocalised π e- system in benzene ring
forms
intermediate cation
which breaks down
forming
phenylethanone + H+
step 3
catalyst regenerated
H+ + AlCl4- ---> AlCl3 + HCl
Alkylation
D: process of replacing H atom by alkyl group
mechanism
step 1
formation of electrophile
CH3Cl + AlCl3 ---> C+H3 + AlCl4-
step 2
same as acylation
step 3
catalysed regenerated
H+ + AlCl4- ----> AlCl3 + HCl
benzene reacts with
halogenoalkane
in presence of AlCl3 catalyst
form
alkyl benzene
conditions
catalyst - AlCl3
anhydrous
prevent hydrolysis
electrophile
carbocation ion
formed by reaction between
halogenoalkane
AlCl3 catalyst
equation