Electrophilic Aromatic Substitution (EAS)
Nitration - an example of an electrophilic aromatic substitution
General scheme
HNO3
nitric acid
benzene
very stable
undergoes EAS
NO2
H2SO4
catalyst
nitrobenzene
Caution: Nitric acid-turns skin yellow, corrosive
Step 1: Generation of the Electrophile (NO2+)
O
H O S O H
O
+
H
O
H
O
N
Mechanism
H
O
N
+
O
O
H
protonated
nitric acid
O
O
N
O
O S O H
O
O N O
O
O
H
protonated
nitric acid
H O H
nitronium ion
actual electrophile
Step 2: Reaction between the Electrophile and Benzene
H
O N O
H
NO2
H
H
N-C bond
forms
Step 3: Proton abstraction
H
H
NO2
H
H
O
NO2
H
+
H
H O H
Scheme: dependent upon unknown liquid
Br
Br
HNO3
nitric acid
Br
NO2
+
H2SO4
catalyst
NO2
1-bromo-4-nitrobenzene
(major)
1-bromo-2-nitrobenzene
(minor)
Bromine is said to be an ortho-para directing group
O
O
O
O
HNO3
nitric acid
plus trace amounts of ortho- and para-
H2SO4
catalyst
NO2
~ 90 %
methyl-3-nitrobenzoate
A carbonyl is said to be a meta directing group
Procedure:
TA assigns unknown letter (A or B)
1) In an ice bath, place a beaker containing 4 mL of conc. HNO3 (wear gloves)
2) With swirling, add 6 mL of H2SO4 dropwise (Caution: exothermic)
3) Dropwise (w/ mixing) add 1 mL of unknown over ca. 2 min
4) Allow to sit at room temperature for 20 min with occasional swirling
5) Pour slowly into 30 mL of ice cold water (Caution: exothermic)
6) Collect precipitate on a filter
7) Recrystallize from ethanol ~ 5-10 mL
8) Interpret 1H NMR spectrum for unknown
1
H NMR Spectrum
a) 1-Bromo-4-nitrobenzene
Here, two sets of equivalent protons give rise to two sets of signals.
Br
HA – a doublet of doublets
HA
HA
HB – a doublet of doublets
HB
HB
NO2
b) Methyl-3-benzoate
Here four distinct protons will give rise to four sets of signals
O
O
HA – a doublet with different degrees of splitting.
HA
HD
HB – a triplet with strong splitting
HC – doublet with different degrees of splitting
HB
NO 2
HD – a singlet with some splitting
HC
9) Identify the substitution pattern
10) Identify the unknown liquid