primary amines such as aniline with nitrous acid (HNO2) prepared from ...
Primary aliphatic amines react with nitrous acid to give highly unstable diazonium
salts.
Experiment
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Synthesis of Sudan-‐I
Introduction
Dyes were in use before recorded history. Indigo, a pigment extracted from a plant, was
used to dye burial cloths for Egyptian mummies over 4,000 years ago. Indigo is used today to dye blue jeans. Tyrian purple was derived from Mediterranean mollusks. Approximately 9,000 mollusks were needed to give one gram of the dye.
American Indians used cochineal, a scarlet dye extracted from the dried bodies of
insects, to color their baskets and clothing. The red dye alizarin, extracted from madder root, was known to the ancient Egyptians and Persians. Alizarin was used to dye the red coats of British soldiers in the American Revolution, the red caps and trousers of French soldiers in the French Revolution, and the violins of Antonio Stradivari. Structures for these dyes are shown in Figure 1. Figure 1 Examples of dyes
Colored compounds absorb some, but not all, wavelengths of visible light. When one
looks at the light reflected from a colored object, the eye sees white light minus the
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wavelengths absorbed by the object. Therefore, a colored object appears as the color complementary to the wavelengths it absorbs.
Functional groups that absorb light are called chromophores. They are typically highly
conjugated systems containing several double bonds and one or more characteristic groups such as azo (-‐N=N-‐), nitro (-‐NO2), and carbonyl (>C=O).
An azo dye is defined by having an azo linkage (-‐N=N-‐) as part of its chromophore. Azo
dyes are made in two steps. First, a primary aromatic amine is reacted to give a diazonium salt, as shown in Equation 1. Second, the diazonium salt is reacted or coupled with a strongly activated aromatic system, such as phenoxide, as shown in Equation 2.
The first step, the formation of diazonium salts, is called diazotization. The reaction was discovered in 1858 by Peter Griess, who subsequently discovered several reactions of this new compound. The most important method for the preparation of diazonium salts is treatment of primary amines such as aniline with nitrous acid (HNO2) prepared from sodium nitrite (NaNO2) and a mineral acid (e.g. HCl) (Scheme 1). A detailed diazotization reaction mechanism is shown in the Scheme 2.
Scheme 1 Generation of nitrous acid and nitric oxide cation (NO+)
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Scheme 2 Mechanism for diazotization of primary amine Primary aliphatic amines react with nitrous acid to give highly unstable diazonium salts which spontaneously decompose to N2 gas and carbocations (Scheme 3). The carbocations react further to produce a mixture of alkenes, alcohols or alkyl halides, with alcohols as the major product. This reaction is of little synthetic importance because the diazonium salt is too unstable, even under cold conditions.
Scheme 3 Degradation of aliphatic diazonium salt
Primary aromatic amines can form diazonium salts which are stable at low temperature.
In aqueous solution these salts are unstable at temperatures higher than +5 °C; the -‐N+≡N group tends to leave as N2, i.e. nitrogen gas. One can isolate diazonium compounds as tetrafluoroborate (BF4-‐) salts, which are relatively stable at room temperature. In general, diazonium compounds are not isolated and once prepared, used immediately in further reactions.
An azo coupling is a reaction between a diazonium compound and an aniline, phenol or
other aromatic compound which produces an azo compound. In this reaction the diazonium salt is an electrophile and the activated arene is a nucleophile in an electrophilic aromatic substitution. In most cases, including the example in Equation 2, the diazonium compound is also aromatic. The product absorbs longer wavelengths of light than the reactants because of increased conjugation.
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NH 3 +Cl-
NH 2
Cl-
+N N
diazotization
HCl
(NaNO2 + HCl) aniline
Cl-
benzene diazonium chloride
anilinium chloride
+N N OH +
benzene diazonium chloride
N
NaOH
N OH
coupling β-naphthol
Sudan-1 (1-phenyl-azo-2-naphthol)
Scheme 4 Synthesis of Sudan-‐I
In this experiment, an azo dye called Sudan-‐I will be synthesized from aniline and β-‐
naphthol (Scheme 4). Sudan-‐I is also commonly known as CI Solvent Yellow 14 and Solvent Orange R. It is a lysochrome, a diazo-‐conjugate dye systematically named 1-‐phenylazo-‐2-‐ naphthol. Sudan-‐I is a powdered substance with an orange-‐red appearance. It is mainly used to color waxes, oils, petrol, solvents and polishes. Sudan I has also been adopted for coloring various foodstuffs, including particular brands of curry powder and chili powder, although the use of Sudan-‐I in foods is now banned in many countries because it has been classified as a category 3 carcinogen by the International Agency for Research on Cancer.
Experimental Procedure 1)
Place about 0.2 g of aniline in a pre-‐weighed 10 mL round-‐bottom flask. Record the actual weight of aniline.
2)
Add 1 mL of distilled water and 10 drops of concentrated HCl. Swirl the flask in an ice-‐ water bath.
3)
To a clean and dry test tube, place 1 mL of 10% NaNO2 (from burette). Chill this solution in the ice-‐water bath.
4)
Weigh 0.15 g of β-‐naphthol in a 50 mL beaker. Add 1 mL of 10% NaOH and 2 mL of distilled water. Stir the mixture with a glass rod until a homogeneous solution occurs. Chill this solution in the ice-‐water bath.
5)
When all three mixtures are cooled to about 0 °C, use a dropper to transfer the NaNO2 solution into the round-‐bottom flask containing the aniline solution. Stir the mixture 74
thoroughly. Do not add the NaNO2 solution too fast because the internal temperature should be below 10 °C. 6)
Transfer the reaction mixture from the round-‐bottom flask into the beaker containing β-‐ naphthol. Stir the mixture to avoid aggregation of red precipitates.
7)
Stir the mixture in the ice-‐water bath for 3-‐5 minutes.
8)
Vacuum filter the precipitates and wash the filtrate with cold water.
9)
Allow the precipitates to dry on the vacuum filtration set for a few minutes. Transfer this product into a 50 mL Erlenmeyer flask.
10) Recrystallize the crude product in ethanol. 11) Vacuum filter the crystals and wash them with cold ethanol. 12) Dry the crystals on a pre-‐weighed watch glass. 13) Weigh and record the yield of the recrystallized product. 14) Determine the melting point of the product. 15) Submit the product in a plastic bag to your instructor.
Laboratory Safety Precaution 1)
Wear safety goggles and lab coat at all times while working in the laboratory.
2)
Aniline is toxic and irritating.
3)
Sodium nitrite is a toxic oxidizer.
4)
Concentrated hydrochloric acid is toxic. β-‐Naphthol is irritating.
5)
Sodium hydroxide is toxic and corrosive.
6)
Ethanol is flammable and irritating. Keep away from flames and other heat sources.
7)
Wash your hands thoroughly with soap or detergent before leaving the laboratory.
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