University of San Carlos
Department of Chemistry
John Darwin O. Namata Mrs. Lora Mae Villegas
BS Chemistry II Chem 23L
Preparation of Azo Dyes
This experiment synthesizes p-nitrobenzene azoresorcinol and helianthin dyes. P-
nitrobenzene Azoresorcinol was synthesized by first preparing the diazonium salt by adding
sodium nitrite in a hychloric acid-p-nitroaniline solution. The prepared diazonium salt was added
to a cold resorcinol-sodium hydroxide mixture with constant stirring. The solution was then
added with slight excess of hydrochloric acid. The solid was collected through suction filtration
and was washed.
Helianthin was prepared by first preparation of the diazonium salt. The diazonium salt
was prepared by the addition sodium nitrite to a sulfanilic acid-sodium hydroxide solution. The
solution was then added to a solution of dimethylaniline and hydrochloric acid. The resulting
mixture was then made alkalin by the addition of sodium hydroxide. The solids are then
Azo compounds are chemical compounds with the general formula R-N=N-R', where R
and R' can be either aryl (aromatic) or alkyl (aliphatic) functional groups. The N=N group is
called an azo group, although the parent compound, HNNH, is called diimide. The more stable
azo compounds contain two aryl groups. Azo compounds with alkyl functional groups are
particularly unstable and should be handled with care, to prevent them from exploding.
Aryl azo compounds have vivid colors, especially reds, oranges, and yellows. They are
therefore used as dyes and are classified as azo dyes. Examples include methyl red and Disperse
Orange 1. The ability to manufacture azo dyes was an important step in the development of the
chemical industry.( http://www.newworldencyclopedia.org/entry/Azo_compound, March 2011)
Diazonium compounds or diazonium salts are a group of organic compounds sharing a
common functional group R-N2+ X- where R can be any organic residue such alkyl or aryl and X
is an inorganic or organic anion such as a halogen. Diazonium salts, especially those where R is
an aryl group, are important intermediates in the organic synthesis of azo dyes.
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An azo coupling is an organic reaction between a diazonium compound and a
dialkylaniline (C6H5NR2), phenol or other aromatic compound which produces an azo compound
The coupling reaction between diazonium salts and a phenol (and some other activated
arenes) will result in colored compounds called ”azo dyes”. The color of these compounds is due
to the high levels of conjugation which extends through the –N=N- group to the aryl system.
The basic structure (Ar-N=N-Ar) may be substituted with different functional groups
(e.g. –NO2, -SO3Na, -NH2, -CH3, -OH, -OCH3) that affect the color of the dye due to their
conjugation with the azo system.
P-nitrobenzene azoresorcinol Dye
A solution of concentrated hydrochloric acid in 25 mL of water was added to finely
powdered p-nitroaniline. The solution was cooled to 2˚C and was stirred well.
A 0.6g of sodium nitrite in 15 mL water was added slowly to the p-nitroaniline solution.
The resulting solution was poured slowly to a cold solution of 1.0 g resorcinol, 25mL water, and
4mL of 8N sodium hyrdroxide. The resulting mixture was chilled and was added with slight
excess of hydrochloric acid.
The solid was collected on a Büchner funnel, using a very light suction. It was then
washed several times with water. The compound was then dried.
Four grams of Sulfanilic acid was dissolved in 10 mL of 2N sodium hydroxide solution.
The sulfanilic acid-NaOH mixture was then added with a solution of 2 g of sodium nitrite in 20
mL of water. The mixture was cooled in an ice bath and was poured into 10mL of cold 2N
The resulting mixture was then added with a solution of 2.5 g of dimethylamine in 20 mL
of 1N hydrochloric acid. The mixture was made alkaline by the addition of sodium hydroxide.
The solution was left for few hours. After an hour, the solution was filtered through suction.
Results and Discussion
The addition of sodium nitrite to the solution of p-nitroaniline, formed a clear and
colorless liquid. The addition of the sodium nitrite initiates the formation of the diazonium salt
with a temperature approximately 2˚C.
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Formation of the nitrosonium ion
NaNO2 + HCl NaCl + HNO2
Nitrosation of the amine
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Formation of the diazonium salt
The p-nitroaniline underwent a process of diazotization in the presence of hydrochloric
acid and nitric acid. The process formed the diazonium salt.
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The diazonium salt undergoes a coupling reaction with resorcinol in basic medium.
The so produced azo dye was colored red brown.
The preparation of the helianthin dye underwent the same process with 4-
(phenyldiazenyl)benzene-1,3-diol. It first had the formation of the nitrosonium ion which then reacted
with the amine and formed the diazonium salt. The diazonium ion then couples with dimethylaniline and
formed methyl orange.
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Formation of Diazotized Sulfanilic Salt
Addition of Dimethylamine
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One process of synthesizing azo dyes is through Diazo Coupling of a diazonium salt
and an amine. There are two reaction that diazonium compounds undergoes, namely:
coupling and substitution(displacement). Coupling is the reaction undergone by diazonium
compounds to form the azo dye. Substitution is the reaction of a diazonium compound to form your aryl
halid and nitrogen gas.
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