Chem214L -Nitration and para-red synthesis.pdf - April 8th...

Chem214L April 8
th
and 19
th
Nitration (EAS) of Acetanilide and Conversion to Para-red Dye
Due date: April 22
Background reading: Loudon p708-709, 1093-1094 (nitration)
Loudon p1097-1099 (diazotization)
Refluxing (handout)
Chromatography (available in the science library)
Background
Nitration of an aromatic ring is one of several synthetically useful electrophilic aromatic
substitution reactions that you will be learning about in class. In this experiment, you
will prepare p-nitroaniline via the nitration of acetanilide followed by the hydrolysis of the
amide group to an amine. The steps of the reaction are seen below.
1) Create NO
2
+
(the very strong electrophilic nitrating agent)
H
2
SO
4
HNO
3
NO
2
+ H
3
O+ HSO
4
-
+
2
+ +
2
2) Add the acetanilide to the nitrating mixture
NO
2
+
N
H
C
O
CH
3
N
H
C
O
CH
3
O
2
N
acetanilide
p-nitroacetanilide
3) Hydrolyze off the acetyl group in a dilute solution of acid
N
H
C
O
CH
3
O
2
N
H
3
O+
NH
4
OH
NH
2
O
2
N
C
O
CH
3
-O
p-nitroacetanilide
1)
2)
p-nitroaniline
+
Notice that the nitro group in step 2 only adds to the para position (directly across from
the substituent). Electrophilic aromatic substitution reactions are regioselective. We will
ensure that primarily the para isomer is formed by keeping the reaction temperature low
(at 5°C or below). A small amount of the ortho isomer may also form, but this can be
separated from the para isomer by recrystallization.

p-nitroaniline can be converted into a commercially useful azo dye called para-red.
This dye is used in a number of applications – basically whenever you want to dye
something (like a flag) red. Azo refers to the nitrogen-nitrogen double bond in the
molecule. The first step in this reaction is called a diazotization, where the amine of
aniline is converted into N
2
+
(called a diazonium salt). It’s a salt because you have the
strong electrostatic interaction of the positively charged diazo group and the negatively
charged hydrogen sulfate group. In the second step of the reaction, the pi electrons of
the aromatic ring of 2-naphthol attack the terminal (end) nitrogen of the diazonium salt
to form the azo group.