Synthesis of Para Red Dye

Abdul Rafay Kamal

Contribution from Department of Chemistry, LUMS School of Science and Engineering,

Opposite U Block, 54792, DHA
Received: 4th April, 2024 email: 25100299@lums.edu.pk
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ABSTRACT: Para Red dye was formed as a combination of P-nitro aniline, nitrous acid, and 2
naphthol. A diazotization reaction was employed to form a diazonium salt, which was then
reacted with naphthol to form an azo dye. Following that, HCl was added and the solution was
left to be heated for 30 minutes, before vaccum filtration, and drying in the oven. The final
percentage yield obtained was 338%, and the sample was characterized through IR, and the
spectrogram could be contrasted with literature values.

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Introduction
The color spectrum extends far beyond what the human mind can conceive, and through
chemical synthesis, we have the ability to create a diverse array of colors. In this particular
experiment, we focused on synthesizing para red dye. Azo dyes belong to a group of large
synthetic dyes characterized by nitrogen as the primary chromophore in their chemical structure.
Among disperse dyes, disperse azo dyes constitute the largest category, comprising over 50% of
the total disperse dyes. These dyes are highly favored for coloring synthetic fibers due to their
exceptional properties, including comprehensive color range, high exhaustion capacity, and
excellent colorfastness. The diazo components utilized in the synthesis of disperse azo dyes are
typically weakly basic amines. Nitrosyl sulfuric acid, a potent nitrosation agent, is essential for
the diazotization of these weakly basic amines due to their low reactivity, resulting in diazonium
salts that are predominantly sulfates. In pharmaceutical applications, azo linkage has been
employed to shield drugs from undesirable reactions; for instance, prontosil was discovered to
safeguard against and treat streptococcal infections in mice. Notably, while prontosil exhibited
no activity on bacterial cultures and was completely inactive in vitro, it displayed excellent
activity in vivo. Given the diverse nature of biological treatment systems, particularly in textile

Abdul Rafay Kamal, LUMS School of Science and Engineering

effluents, numerous factors may influence the biodegradation rate of azo dyes. Researchers have
extensively discussed various challenges associated with dye biodegradation, some of which may
be anticipated and addressed, while others may arise unexpectedly. Azo dyes have garnered
significant attention for their applications in biological systems and as indicators in
complexometric titration within analytical chemistry. The most significant category of synthetic
colorants, azo dyes are commonly regarded as xenobiotic compounds that exhibit high resistance
to biodegradation processes. Aromatic azo compounds, in particular, serve as acid-base
indicators and find applications in biological strains as well as commercial colorants for textiles,
plastics, cosmetics, and food and beverages. Changes in color result from alterations in the extent
of electron delocalization; increased delocalization shifts the absorption maximum towards
longer wavelengths, causing the absorbed light to appear redder, while decreased delocalization
shifts the absorption maximum towards shorter wavelengths. The development of various classes
of synthetic dyes, including azo dyes, stemmed from continuous efforts to discover specific dyes
suitable for diverse industrial materials, notably textile fabrics, among others.

Abdul Rafay Kamal, LUMS School of Science and Engineering

Experimental_Section:

Chemicals and materials. 2-Naphthol, p-nitro aniline, sodium nitrite, sodium hydroxide, tri-
sodium phosphate, HCl, and ethanol.

Glassware and equipment. Three beakers (a 250 mL and two 100 mL), a pipette, a pipette
sucker, a magnetic bar, a hot plate, hardened filter paper, Buchner funnel, an Erlenmeyer flask
(100 mL), a glass rod, a 97 graduated cylinder, a spatula, an ice bath and a water bath etc.

Procedure:
In a 500 mL beaker dissolve 1 g (0.025 mol) of sodium hydroxide, 9.5 g (0.025 mol) of
commercial tri-sodium phosphate in 200 mL of water as a buffer solution to maintain pH of
reaction mixture and finally add 1.4 g (0.01 mol) of 2-naphthol, in the order given. Stir this
solution until all of the 2-naphthol is completely dissolved.

Chill the solution by allowing it to stand in an ice bath for 5 min while stirring intermittently.
Take a 100 mL beaker and add 3 mL of distilled water in it. Pipette out 3 mL of concentrated
hydrochloric acid, and add into beaker containing water. Dissolve 1.4 g of p-nitro aniline in
dilute acidic solution in the beaker by gentle warming on hot plate. Heating should be continued
until all of the p-nitro aniline is dissolved. Cool down the solution to room temperature and pour

Abdul Rafay Kamal, LUMS School of Science and Engineering

the solution into a 100 mL Erlenmeyer flask containing about 10 g of chopped ice

Swirl the mixture to obtain a fine suspension of the crystals of pnitro aniline hydrochloride,
while maintaining the mixture at 5–10 °C in the ice bath and stirring it vigorously.
Add 0.8 g (0.011 moles) of sodium nitrite in 3–4 mL of water and pour into the cold solution of
p-nitro aniline hydrochloride as quickly as possible. Swirl the mixture until most of the
hydrochloride has dissolved (2-3 min), allow it to stand for a few minutes to complete the
diazotization, and proceed at once to the next step.
NH2
H H H
O N O HO N O O N O N O N O
H -H2O

NO2

H
N N O
H

O
H H
NO2

H
N N O

H
H H
N N O
N N O NO2
O
H H H H
O
H
NO2
NO2 H H
O
H

H
N N O N N
H
H2O
+

NO2 NO2

Abdul Rafay Kamal, LUMS School of Science and Engineering

Figure 3: Diazonium ion formation mechanism

Pour the diazonium salt solution all at once into the chilled alkaline solution of 2– naphthol. Stir
the material vigorously for a few minutes to ensure complete reaction and then, after adding 5
mL of concentrated hydrochloric acid, raise the temperature to about 30 °C on a water bath and
stir for 30 - 40 min with the help of glass rod. Collect the bright red dye after vacuum filtration,
using a hardened filter paper. Allow the solid to dry completely and then re-disperse it in 100 mL
of water. Collect the solid by vacuum filtration and

wash it with distilled water until the filtrate is essentially free of chloride ion (filtrate show
neutral pH) – yellow on the universal indicator spectrum. Wash the crystals of azo-dye finally
with small portions of ethanol. After it is dried, the product weighs about 10.36g Do not try to
determine its melting point, to ensure its purity.

Figure 4:

Abdul Rafay Kamal, LUMS School of Science and Engineering

Figure.

Figure 5: Para Red formation mechanism

Abdul Rafay Kamal, LUMS School of Science and Engineering

RESULTS AND DISCUSSION:

Percentage recovery of the sample:

Experimental mass of 10.36g

the product:
Moles of p-nitroaniline 0.0105
Moles of the product 0.0105
Theoretical mass of 3.06g
the product:
% yield of the product: (10.36/3.06)*100= 338%

It can be noted that the sample was not completely dry, when the product was weighted,
hence such an astronomical value.
Analysis of the sample:

Fig. IR of derived sample.

Abdul Rafay Kamal, LUMS School of Science and Engineering

 Fig Literature Values

Peaks Reference Obtained

Values IR peaks

O-H 3390 cm-1 No broad

and wide
distinct
peak
present.

N=N 1504 cm-1 1585 cm-1

NO2 1328 cm-1 1322 cm-1

C-N Stretching Peak Also have

between several
1200 cm-1 peaks
to 1350 between
cm-1 1100 cm-1-
1300 cm-1

Benzene Ring Aromatic Peaks at

Stretching 1448 cm-
1
peaks and 1585
between cm-1
1450 cm-
1
-1600
cm-1.

overtones Overtones
between are present
1667 cm- between

Abdul Rafay Kamal, LUMS School of Science and Engineering

 1
-2000 1667 cm-1-
cm-1 2000 cm-1

Peaks Peak at
slightly 3065 cm-1
above
3000 cm-1
at 3051
cm-1

Post Lab Questions:

1. Draw the possible products of this reaction.

2. What are the best reactants to prepare this azo dye?

Abdul Rafay Kamal, LUMS School of Science and Engineering

 3. Describe the significance of controlling reaction conditions, such as temperature and
pH, during the synthesis of Para Red dye, and how it affects the yield and purity of
the product.
Since, it is a highly temperature dependent reaction. The synthesis of diazonium ion is
only possible in the 0o -5oC temperature range. Any temperature beyond that will result
in degradation of diazonium ion or initiation of a side reaction instead. Ruling out the
formation of red para red. Similarly with respect to Ph, it will negatively affect the yield.
Increasing the amount of non para red formation within the product, and a smaller or no
yield of para red.

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Abdul Rafay Kamal, LUMS School of Science and Engineering

degradation of Congo red dye. Journal of
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Abdul Rafay Kamal, LUMS School of Science and Engineering