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Experiment 17: Preparation of Methyl Orange, Lab Reports of Chemistry

Structure and Preparation of Methyl Orange from sulfanilic acid in the presence of NaCl, Na2CO3, NaNO2 which work as a catalyst

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2020/2021

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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
pf3
pf4
pf5

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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

CH

3

CH

3

O

O

HO

Figure 17. 1 Structure of Methyl Orange

S N N N

CH

3

CH

3

O

O

X

O

yellow, pH > 4. 4

H

S N N N

CH

3

CH

3

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 NH

2

O

O

HO S N

O

O

HO N

S N N N

CH

3

CH

3

O

O

HO

N

CH

3

CH

3

Cl

Figure 17. 2 Preparation of Methyl Orange

sulfanilic acid

  1. Na 2

CO

3

  1. NaNO 2

  2. 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

acetic acid. This forms the dimethylaniline acetate salt. Neutralize this in situ and the

dimethylaniline becomes a good nucleophile due to the activating effect of the

dimethylamine substituent. Attack is in the para position due to hindrance at the ortho

position by the bulky dimethylamine substituent.

Na

O

C

O

O

  • Na

H

2

O

N O H-Cl

S NH

2

O

O

H O S NH

2

O

O

Na

O S N

O

O

HO

H

H

N O

HCl

H Cl

O

N

O

H

H

O

N

O

H

Na

O

N

O

H 2 O O N

H-Cl

S N

O

O

HO

H

N O

H

Cl

S N

O

O

HO N

S N

O

O

HO N O

H

H

H-Cl

H

2

O

HO

    • O

C

O

CH

3

N

CH

3

H

3

C

CH N

3

H

CH

3

H

2

O

S N N N

CH

3

CH

3

O

O

HO

S N N N

CH 3

CH

3

O

O

HO

H

H

sulfanilic acid

Nitroso ion formation

Figure 17. 3 Mechanism for Methyl Orange Formation

Diazonium salt

nitroso ion

We neutralize the protonated dimethylaniline

to make it into a good nucelophile.

Physical Constants

Compound Mol. Wt (g/mol) Density

(g/mL)

b.p. (°C) m.p. (°C)

Sulfanilic acid 173.19 solid -

N,N-Dimethylaniline 121.18 0.956 193-194 1.5-2.

Sodium carbonate

(anhydrous)

105.99 solid - 851

Sodium carbonate

(monohydrate)

Sodium nitrite 69.00 2.168 - 271

Acetic acid 60.05 1.049 116-117 15-

Methyl orange 306

Filter the crystals by suction filtration, rinse them with 10 – 15 ml cold water and

allow them to dry. Calculate the percent yield and turn in your crystals in a vial along

with your Organic Yield Report Sheet.

Do not attempt to take the melting point of your methyl orange as it decomposes

on heating. In order to observe the beautiful color change that occurs with our orange

indicator dye, dissolve a small amount of the methyl orange in 2-3 mL of 95% ethanol

and add 5% HCl a few drops at a time until you see the color change. Describe your

results in your notebook.