Experimental
In a 50-mL beaker, 0.9 mL of aniline, 1.4 mL of water and 2.1 mL of concentrated hydrochloric acid were combined. It was then cooled to 0-5 °C, with 1 g of ice and
Texture: fine Odor: paint-like smell Melting Point: 131-133 °C Chemical test
stirred constantly. Sodium nitrate (0.75 in 3 mL H2O) was then dropped until the solution contains excess acid. A drop of the solution was placed to test reaction for excess nitrous acid, indicated by a blue color. Note that the temperature must be below 5 °C. 1.4 g of β-naphthol was dissolved in
3 mL warm sodium hydroxide. This was then cooled to 5°C. Afterwards, the phenyldiazonium chloride was added dropwise, while stirring. For 15 minutes, the solution was kept standing in 0-5 °C. The product was collected through filtration on fluted filter paper. Cold water was used to wash several portions of it. It was recrystallized from ethanol. It was then dried and weighed. A pinch of the product was dissolved
0.1 M NaOH
least intense
1 M HCl
most intense
NH4OH
moderately intense
CH3COOH
intense
Discussion
The synthesis of 1-phenylazo-2naphthol was possible through diazotization, followed by a diazoniumcoupling reaction. From the reaction of aniline to sodium nitrate in hydrochloric acid, a diazonium salt, phenyldiazonium chloride, was produced. The solution was cooled, to ensure that the intermediate does not decompose, as it is very unstable. It goes back to aniline when it warms up to a temperature slightly higher than the boiling
in 5 mL distilled water. Three drops of the
point. From here, the salt can either
solution was then added to 2 mL 0.1 M
undergo coupling reaction or nucleophilic
sodium hydroxide. This test was repeated
aromatic substitution (NAS). NAS does not
using 0.1 M hydrochloric acid, ammonium
occur in this experiment, because it was
hydroxide and acetic acid.
made sure that the solution was cold.
Results
Physical test Appearance: powder
The coupling reaction starts off when the diazonium salt was then reacted with β-naphthol. Phenyldiazonium chloride
attacks the ortho and para positions with respect to the activator, -OH, forming a
Color: red-orange
resonance-stabilized intermediate. The
meta position was not attacked, because
through diazotization, followed by a
activating groups are not meta-directors.
diazonium-coupling reaction.
The ortho position was favored here,
2. Write the reaction pathway and
because more resonance represantion
mechanism for the synthesis of 1-
products are present in this state. The
phenylazo-2-naphthol.
product is now the dye, 1-phenylazo-2naphthol.
Formation of the diazonium salt:
When it was tested with 0.1 M sodium hydroxide, 1 M hydrochloric acid, ammonium hydroxide and acetic acid, it was found the the intensity of the color
Mechanism of the formation of
varied. The color intensity decreases from
diazonium salt:
the hydrochloric acid solution, followed by acetic acid, ammonium hydroxide, to sodium hydroxide. The phenyldiazonium intermediate was to be kept acidic, to prevent nucleophilic attack of –OH. When that happens, the solution turns into pink. This was evident in the addition of the bases, ammonium hydroxide and sodium hydroxide. On the other hand, the β -
naphthol was to be kept basic, to prevent protonation. It has the auxochrome of the dye, -OH, which is the color deepener. In the industrial setting, dyeing baths are acidified to protonate the auxochrome. In
Attack of diazonium salt to β -
this experiment, the addition of acids did
naphthol:
protonate the auxochrome, which resulted in the intensification of the color of the dye. Questions:
1. What are the reactions involved in the synthesis of 1-phenylazo-2naphthol?
As stated above, the synthesis of 1phenylazo-2-naphthol was possible
As stated above, the unstability of the diazonium salt requires low temperature for it not to go decompose into aniline. 4. Explain the principle behind the use of starch iodide paste in detecting excess nitrous acid.
Iodine is formed by the oxidation of the iodide ion by nitrous acid. When the ion reacts with the acid, it forms a blue product. In this experiment, the presence of the blue color indicates the presence of excess nitrous acid. 5. Why is the rate of coupling of phenyldiazonium ion with β-
naphthol dependent upon the pH of the reaction medium? When the reaction medium’s pH is
Resonance stabilized
high, the coupling reaction is slowed
intermediate:
down by the conversion of amines into ions. Applications (Congo red)
Structure of congo red:
Synthesis of Congo red (diazotization and coupling steps) 3. Why should the temperature of the reaction mixture be kept below 5 °C?
Conclusion and recommendation
The formation of the diazonium salt is very useful in synthesizing different compounds, because it allows the benzene ring to link with other aromatic compounds. In this experiment, phenyldiazonium chloride was formed from the nitration of aniline. The only limitation that this Site of protonation largely
experiment has is the availability of the
responsible for the blue color of
reagents. This experiment was not done in
Congo red: N=N
the laboratory, because of that. References:
Two resonance structures for the
http://www.freewebs.com/mpniistal/fr3_sy
second protonated substance that
nthesis_of_1_phenylazo_2_naphthol.pdf
shows the nature of the chromophore responsible for the blue color: Structures in (b) is when it is acidic (blue in color)
http://www.chemistry.sc.chula.ac.th/bsac/ Org%20Chem%20Lab_2012/Exp.11[1].pdf http://devchandcollege.org/chemistry%20n otes/B.Sc.I%20Chemistry%20Diazonium%20 salts.pdf http://www.sciencedirect.com/science/arti cle/pii/S0304389406013860 http://en.wikipedia.org/wiki/Acid_dye http://www.intechopen.com/books/ecofriendly-textile-dyeing-andfinishing/decolorization-of-dyeingwastewater-using-polymeric-absorbentsan-overview http://www.scribd.com/doc/85814501/Last -Fr-Chem-31 Sir Quiming’s lecture
