E X P E R I M E N T
2 9
DIAZOTIZATION/DIAZOCOUPLING: PREPARATION OF METHYL ORANGE
Amines have numerous reactions with nitrous acid, HONO. These reactions
involve attack by the nitrosonium ion, :N=0:+, as an electrophile, at the nitrogen atom's
electon pair. They may be summarized as follows:
N2 and alcohols
1 oaliphatic
Ar-N=N:+
1oaromatic
2 oaromatic
N-nitrosoamines
3 oaliphatic
no reaction
3 oaromatic
p-nitrosoamines
One of the more important synthetic tools in organic chemistry is the diazonium salt,
ArN=N:+, which acts as an intermediate in the production of Ar-X, Ar-C=N, and Ar-OH. The
diazonium salt reacts with various nucleophiles to form the substituted product and N 2 gas.
This nucleophilic substitution reaction is thought to occur by a radical mechanism. Some
examples of these reactions are shown below.
Ar-N=N:+ reacts with:
(1) KCN, CuCN
→ Ar-C=N (nitrile) + N2
(2) H+ , H2O
→ Ar-OH + N2
(3) H3PO2
→ Ar-H + N2
(4) KI
→ Ar-I + N2
(5) CuBr (or CuCl) → Ar-Br (or Ar-Cl) + N2
(6) HBF4, Heat
→ Ar-F + N2
A diazonium salt is capable of reacting with highly activated aromatic compounds to form
coupling products which have the formula Ar-N=N-Ar-G. These highly conjugated compounds
are excellent dyes.
Ar-N=N:+ + H-Ar’ –G Ar-N=N-Ar’-G + H+
Coupling Product
In this experiment, a diazonium salt will be synthesized and then reacted with N,Ndimethylaniline in a coupling reaction.
The process of diazotization is carried out by reacting a primal aromatic amine with the
nitrosonium ion under slightly acidic conditions. In this case the compound to be diazotized
is sulfanilic acid, p- SO 3 - -C 6 H 4 -NH 3 + . This difunctional compound normally exists as a
"zwitter ion" which is indicated in the latter formula. This process is summarized as follows:
(1) Reaction of sulfanilic acid with sodium carbonate. This creates the free -NH2 group.
NH3+
2
NH2
+ Na2CO3
2
+ H2O + CO2
-
SO3
SO3Na
1 mole = 173.19 g
(2) Diazotization: Reaction of sodium sulfanilate with sodium nitrite (NaNO2) and HCl. This
reaction involves attack of NO+ at the aromatic nitrogen, followed by elimination of water.
NH2
N N
+ NaNO2 + 2HCl
+
+2 H2O + 2 NaCl
0-5oC
SO3Na
SO3
-
diazonium salt
(3) Diazocoupling: The diazonium salt produced by this process is not purified but is
reacted with the coupling reagent. In this case, the diazonium salt is reacted with N.Ndimethylaniline (1 mole = 121.18g) to yield methyl orange, an azo dye. This reaction is an
example of electrophilic aromatic substitution by the very weak electrophile, the diazonium
ion.
N N+
NH(CH3)2Cl
+
SO3
+ 2NaOH
SO3Na
N N
N(CH3)2 +NaCl + 2H2O
-
diazonium salt
Methyl Orange
Thus, amines react with HONO to yield various products. A primary aromatic amine
is converted to a diazonium salt which in turn leads to numerous substitution and
coupling products. For this reason, diazonium salts are often used in many synthetic processes.
PROCEDURE
I.Diazotization
1. Place 200 mg of sulfanilic acid in a 50-mL Erlenmeyer flask. Add 60 mg of anhydrous
Na2CO3 and 2-mLs of water. Warm this mixture gently on the sand bath to dissolve the
solids. Then, cool the solution to room temperature.
2. Add 85 mg of NaONO (sodium nitrite). Stir to dissolve.
3. Place in an ice-water bath to cool.
4. While this solution is cooling, mix about 2 grams of ice with six drops of concentrated HC1
in a reaction tube. Pour this mixture into the Erlenmeyer flask. Stir thoroughly.
5. The diazonium salt should begin to form within 2-5 minutes. It will appear as a white
suspended precipitate. Set this suspension aside in the ice-water bath in preparation for
the coupling reaction.
II.. Oiazocoupling: Preparation of Methyl Orange.
1. Tare a reaction tube in a small beaker. Add 130 mg dimethyl-aniline and 110 mg of glacial
acetic acid to "this tared reaction tube. Mix well. [Note: dimethylaniline is the
limiting reagent.]
2. Pour the dimethylaniline/acetic acid solution into the Erlenmeyer flask containing the
suspended diazonium salt. Rinse the reaction tube with a little water to transfer the last
traces of the dimethylaniline/acetic acid solution.
3. Stir the contents of the Erienmeyer flask for several minutes. A red, pasty mass of
methyl orange should separate.
4. Add 2-mLs of 10% NaOH and mix. Check with Litmus paper to determine if the solution is
alkaline. Add more NaOH if needed.
5. Add 1.2 grains of NaCI. The color of the crystals should become yellow as the acid form of
methyl orange is converted to its sodium salt in its base form. NaCI is added to decrease the
solubility of this sodium salt in water.
6. Warm the solution to nearly boiling in the sand bath. Allow the solution to cool to room
temperature and then in an ice-water bath. The dye should separate as orangecolored crystals.
7. Suction filter the crystals. Press dry. Obtain the weight of the crystals and turn in the product
in a properly labeled vial.
EXPERIMENT 29: Report and Worksheet
DIAZOTIZATION/ DIAZOCOOPLING: PREPARATION OF METHYL ORANGE
Student Name:__________________ _______
Student Number: ____________________________
Day:_______________
Date:_________________
DATA
Mass of sulfanilic acid used. .... _______ grams _____ moles
Mass of dimethylaniline used. .... ________ grams ____ moles
Theoretical yield of methyl orange: _________ grams
(Show calculation.)
Mass of methyl orange obtained:_____________
Percent Yield:_________
(Show calculation.)
grams
QUESTIONS
1. Draw TWO RESONANCE STRUCTURES for the nitrosonium ion, NO+.
2. What is the equation for generating NO+ in a dilute solution of HCl and NaNO2?
3. What is the BALANCED EQUATION for the reaction between sulfanilic acid and sodium
carbonate ?
4.What is the BALANCED EQUATION for the reaction sodium sulfanilate (product of
previous reaction) and NaNO2 and HCl ?
5. What is the BALANCED EQUATION for the reaction between the diazonium salt; (product of
6. What is the purpose of adding NaOH to the red crystals of methyl orange?
7. WHY is NaC1 added to the reaction mixture before final crystallization?
8. What is the general reaction that occurs between a diazonium salt and a nucleophile, Nu- ?
9. What product is formed when p-CH3-C6H4-N2+Cl- is reacted with each of the following
compounds?
(1) KI
(2) HBF4, heat
(3) CuCN, then, H2 catalyst
(4) phenol
(5) o-methylphenol (o-cresol)
(6) H+, water