Experiment 17
Preparation of Methyl Orange
In this experiment you will prepare methyl orange, an azo dye that forms beautiful
orange crystals and is used as an acid-base indicator (Figure 17.1). The anion form is
yellow and the acid form is red.
S N N N CH3
CH3
O
O
HO
Figure 17.1 Structure of Methyl Orange
S N N N CH3
CH3
O
O
X + -O
yellow, pH > 4.4
H+
S N N N CH3
CH3
O
O
HO
H
methyl orange
red, pH < 3.1
(inner salt form)
You will synthesize methyl orange from sulfanilic acid and N,N-dimethylaniline
using a diazonium coupling reaction just like the one you saw in the previous experiment
in the nitrous acid test for primary aromatic amines. The overall reaction is shown in
Figure 17.2 and the mechanism is shown in figure 17.3. All of this chemistry is in your
text in the chapter on amines. You need to understand it.
S NH2
O
O
HO S N
O
O
HO N
S N N N CH3
CH3
O
O
HO
N
CH3
CH3
Cl-
Figure 17.2 Preparation of Methyl Orange
sulfanilic acid
1. Na2CO3
2. NaNO2
3. HCl
methyl orange
The first step is simply an acid base reaction. In order to dissolve the sulfanilic
acid in the aqueous solution we add sodium carbonate. Then we form the diazonium salt
by the same mechanism that was presented in Figure 16.4 of the previous experiment.
When we add the HCl, the nitroso ion is formed from sodium nitrite and this reacts with
the amine to form a nitrosoammonium adduct that loses water under the acidic conditions
after proton transfer. This gives the diazonium salt. Aromatic diazonium salts are stable
at low temperature. The terminal nitrogen of the diazonium salt is very electron
deficient. It can be attacked by good nucleophiles. We dissolve the dimethylaniline in
