A Diazotization—Coupling Reaction: The Preparation of Methyl
Orange
Formation of a diazonium ion
Azote is an old word for nitrogen. Hence, the presence of azo in the name
of a chemical implies that nitrogen is present in the structure. Therefore, diazo
means two nitrogen atoms. When combined with onium, we have diazonium,
which means two nitrogen atoms and a positive charge (i.e., N2+). Diazonium ions
are produced when an aryl amine reacts with cold nitrous acid. Nitrous acid is
unstable and is prepared just prior to its use by a reaction between sodium nitrite
and hydrochloric acid. Figure 1 shows the conversion of aniline into the positive
benzene diazonium ion.
NH2
+
aniline
HNO2
HCl
nitrous acid
N N Cl+
2 H2O
benzene
diazonium chloride
Figure 1. The diazotization of aniline.
Substitution Reactions of Diazonium Ions
A diazonium ion is the cation of a salt, and it is a reactive intermediate that
undergoes substitution or coupling reactions. Table 1 shows some groups that may
substitute for a diazo group bonded to an arene (ArN2+).
Table 1. Groups that substitute for N2+ in ArN2+.
Reagent
Group
CuBr (HBr)
Br
CuCl (HCl)
Cl
CuCN (KCN)
CN
KI
I
Cu2O, Cu(NO3)2, H2O
OH
H3PO2
H
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Product
ArBr
ArCl
ArCN
ArI
ArOH
ArH
1
A generalized equation for a substitution reaction is shown below in which
the reagent can be any reagent in Table 1 and X any group.
N2+
X
reagent
Coupling Reactions of Diazonium Ions
Compounds such as aniline and phenol, which contain strong electron
donating groups (e.g., -OH and –NH2) that activate the ortho and para positions on
a benzene ring, can undergo coupling reactions with a diazonium ion. A coupling
reaction is one in which two aryl rings are joined by an azo group. These
coupling reactions usually occur at the para position of the o,p director. Figure 2
shows the coupling of benzene diazonium ion with phenol at the para position of
phenol.
N N Cl-
+
H
OH
phenol
benzene
diazonium chloride
N N
OH
+
HCl
diazo product
Figure 2. Coupling reaction with phenol.
Aniline may serve as the substrate for the formation of a diazonium ion as
shown in Figure 1, and it may serve as the substrate for a coupling reaction with
the diazonium ion as shown in Figure 3.
N N Clbenzene
diazonium chloride
+
H
NH2
aniline
N N
NH2 + HCl
diazo product
Figure 3. Coupling reaction with aniline.
The mechanism for a coupling reaction is shown in Figure 4. The reaction is
an electrophilic aromatic substitution reaction; therefore, the mechanism is similar
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to that for the nitration of benzene (i.e., it is a two-step reaction in which the first
step is rate determining).
N N+ H
benzene
diazonium ion
OH
H
N N
slow
OH
phenol
fast
N N
OH
diazo product
Figure 4. Mechanism of coupling.
The Experimental Reaction
In this experiment, sulfanilic acid is the primary aryl amine that will be
diazotized, and N,N-dimethylaniline is the ring-activated benzene derivative to
which the diazonium ion will couple in the para position. Figure 5 shows the
structures of aniline, N,N-dimethylaniline and sulfanilic acid. Both reactants are
derivatives of aniline.
NH2
H3C
N
CH3
NH2
SO3H
aniline
N,N-dimethylaniline
sulfanilic acid
Figure 5. Aniline derivatives.
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Sulfanilic acid contains a sulfonic acid group (SO3H) and an amino group
(NH2); therefore, it undergoes an internal acid-base reaction to form a di-ionic
species. Sodium carbonate acts as a base and deprotonates the quaternary
ammonium ion. The ensuing sulfonate salt is diazotized with cold nitrous acid, as
shown in Figure 6.
NH2
NH3
H+ transfer
N2+
NH2
Na2CO3
HNO2
cold
SO3-
SO3H
SO3-
SO3-
diazotized
sulfanilic acid
sulfanilic acid
Figur
e 6. Diazotization of sulfanilic acid.
The diazotized sulfanilic acid couples with N,N-dimethylamine, as shown in
Figure 7. Adjustment of the pH to about 4 by the addition of NaOH yields an
orange product known as methyl orange. Methyl orange is the well-known acid—
base indicator and is an example of an anil, an aniline-based dye. If the pH is not
properly adjusted a dark red compound helianthin might be isolated instead of
methyl orange.
-O S
3
N N
NaO3S
N(CH3)2
+
diazotized
sulfanilic acid
N N
N(CH3)2
NaOH
N N
H
N(CH3)2
H+ transfer
N,N-dimethylaniline
methyl orange
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-O S
3
-O S
3
H
N N
N(CH3)2
helianthin (red)
4
Procedure
Preparation of the Diazonium Ion of Sulfanilic Acid
1. Weigh 0.6-g anhydrous sodium carbonate and transfer it to a 25-mL Erlenmeyer
flask.
2. Add 5-mL water to the Erlenmeyer and dissolve the solid completely.
3. Weigh 0.2-g sulfanilic acid and transfer it to the Erlenmeyer.
4. Fill a 150-mL beaker half full of water and heat to boiling on a hot plate.
5. Immerse the Erlenmeyer flask in the hot-water bath, using a pair of tongs or test
tube holder.
6. After all of the sulfanilic acid dissolves completely, remove the Erlenmeyer
flask and allow it to cool to room temperature on the bench top.
7. Weigh 0.08-g sodium nitrite, NaNO2, and transfer it to the cooled Erlenmeyer
flask; stir the solution until the solid dissolves.
8. Cool the 25-mL Erlenmeyer flask in an ice-water bath for 10 min.
9. Add five drops of concentrated hydrochloric acid to the Erlenmeyer while it
remains in the ice bath.
The diazonium salt of sulfanilic acid precipitates as a finely divided, white
solid. Keep the suspension in the ice bath until needed in Step 2 below.
Preparation of the Diazo Dye Methyl Orange
1. Add four drops of N,N-dimethylaniline and two drops of glacial acetic acid to a
small test tube.
2. Transfer the solution of N,N-dimethylaniline from Step 1 to the 25-mL
Erlenmeyer flask in the ice bath. Keep the Erlenmeyer in the ice bath.
3. Stir the mixture vigorously with a stirring rod.
A red precipitate of helianthin forms.
4. Keep the Erlenmeyer flask in the ice bath for 10 min.
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5. Slowly add 1.5-mL 10% sodium hydroxide to the Erlenmeyer while it remains
in the ice bath.
6. Remove the Erlenmeyer from the ice bath, dry it with a paper towel, place it on
the hot plate and heat the mixture to boiling.
7. Remove the Erlenmeyer from the hot plate and add 0.5-g sodium chloride to the
hot solution.
8. Allow the Erlenmeyer to cool on your bench for two min., and then place the
flask in an ice-water bath. After crystallization is complete, collect the dye on a
pre-weighed filter paper in a Büchner funnel.
9. Rinse the Erlenmeyer flask with 3 mL of an ice-cold, saturated solution of
sodium chloride; swirl the Erlenmeyer and pour this solution into the Büchner
funnel.
10. Allow the dye to dry on its filter paper, and then re-weigh the filter paper to
determine the yield of methyl orange. Calculate the percent yield.
If your next experiment is an unknown sugar, continue with the
following procedure, otherwise skip to Step 7 below.
Preparation of Solution for Specific Rotation
(To be conducted one week before observing the rotation)
1. Obtain an unknown sugar, show the instructor the number of the unknown and
record it in your lab notebook.
2. Keep about 0.3 g of the unknown for next week’s experiments.
3. Tare a 25-mL volumetric flask on an analytical balance that measures to four
decimal places (instrument room).
4. Transfer the remainder of the unknown sugar (about 3 g) to the volumetric flask
and record the mass of the unknown.
5. Add distilled water to the volumetric flask until the water level reaches about 2
cm below the etched mark on the flask.
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Water will be added to the mark at the next lab meeting. The sugar will
undergo mutarotation during this time.
6. Stopper the volumetric flask, mark it with your initials, and save it in the
designated location until next week.
7. Clean the glassware, ensure the balance area is clean and the balances are turned
off. Ensure the sinks are clean with no solid materials left in them.
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Diazotization Questions
Last name___________________________, First name________________________
1. Draw the structure of m-bromo-N,N-diethylaniline.
2. Consider the structure of sulfanilic acid. It has two functional groups, one is an
acid and the other is a base. Write the formula for the partial structure that
constitutes the acid part of sulfanilic acid. _____________ Write the formula for
the partial structure that constitutes the base part of sulfanilic acid. _____________
From what inorganic acid and base are these partial structures derived?
______________________ and ___________________
If you answer these questions correctly, you should quickly realize why these two
groups react.
3. Why is the amino group in aniline an o,p director?
4. Draw resonance structures that support your answer to problem 3.
5. Write an equation for the diazotization of aniline.
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Complete the following equations by inserting the reagent or product.
6.
CN
N N
+
7.
N N
+
8.
N N
+
KI
NH2
9.
N N
+
OH
N N
10.
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+
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