Experiment: preparation of aniline yellow

How is aniline yellow prepared?

The NCERT Class 12 textbook explains the chemistry: diazotisation (NCERT Section 9.7, page 274) and coupling with aniline (Section 9.9, page 276). It does not describe the method. The method below follows the NCERT Laboratory Manual (Chemistry, Class 12), Unit 10, Experiment 10.5 (pages 111–113), with the diazotisation of Experiment 10.4 (pages 109–111).

These practical descriptions explain the NCERT manual. Carry out laboratory work only under the institution’s approved supervised procedure, including reagent handling, cooling and waste collection.

In plain words: make the diazonium ion from aniline in ice, then let it attack another aniline molecule at the far (para) carbon of the ring. The two rings, joined by –N=N–, give the yellow colour.

Diazotisation of aniline

• Procedure (Experiment 10.4, used for the direct route).

  1. Dilute 6.5 mL of concentrated hydrochloric acid with 6.5 mL of water and dissolve 2 mL of aniline in it (Lab Manual, page 110). Our safety order: add the acid slowly to the water, with cooling, under the approved protocol.
  2. Cool the beaker in an ice bath at 0–5 °C.
  3. Diazotise by adding a solution of 1.6 g of sodium nitrite in 8 mL of water.

  (Lab Manual, page 110.)

• Keep the temperature at 0–5 °C during diazotisation (Lab Manual, page 111). That is 273–278 K, the range in which arenediazonium salts are stable for a short time, so the salt is used at once (NCERT page 274).

• The Lab Manual notes that azo dye syntheses are mostly so nearly quantitative that the amounts used should closely agree with the equations; an excess of some reactants may decompose unused material, and tar may appear (page 111).

Coupling with aniline, or rearrangement of diazoaminobenzene

• Route 1, rearrangement (Experiment 10.5).

  1. Dissolve 3 g of finely powdered diazoaminobenzene in 7 mL of aniline in a 100 mL conical flask, and add 1.5 g of finely powdered aniline hydrochloride.
  2. Warm on a water bath at about 40–45 °C for about one hour, shaking now and then.
  3. Leave the flask at room temperature for about 30 minutes.
  4. Add 9 mL of glacial acetic acid diluted with an equal volume of water and shake well. This turns the excess aniline into its acetate, which dissolves in water.
  5. After 15 minutes, filter the p-aminoazobenzene, wash it with a little cold water and dry it between folds of filter paper.

  (Lab Manual, page 112.)

• The chemistry. The Lab Manual’s scheme (page 111) splits diazoaminobenzene with HCl into benzenediazonium chloride and aniline; the diazonium ion then couples with the excess aniline at its para position. Balanced, the second step gives the HCl back, so a small amount of aniline hydrochloride is enough. Overall, diazoaminobenzene turns into its isomer p-aminoazobenzene (reasoned).

• Diazoaminobenzene, C₆H₅N=N–NHC₆H₅, is a pale yellow solid made from benzenediazonium chloride and aniline (Wikipedia, “1,3-Diphenyltriazene”, the article on diazoaminobenzene). In it the diazonium group has joined the nitrogen of aniline rather than its ring (reasoned from the structure).

• NCERT slip (textbook, page 276): NCERT’s scheme for coupling with aniline, with H⁺ over the arrow, shows Cl⁻ and H₂O as the other products. No oxygen enters this reaction, so no water can form: the products are aniline yellow and HCl, as in the second equation above. Water is right only in NCERT’s phenol scheme just above it, where OH⁻ is a reactant. In an exam, write HCl (reasoned).
• Route 2, direct coupling (alternative procedure). Make benzenediazonium chloride from 2 mL of aniline as in Experiment 10.4. Mix 4 mL of aniline with 4 mL of 1.0 M HCl, and add this cold mixture slowly to the cold diazonium solution. Filter the yellow compound and dry it between folds of filter paper (Lab Manual, page 113).
• Note on the amounts (Laboratory Manual, page 113): the Lab Manual calls the aniline in HCl a solution of aniline hydrochloride, but 4 mL of 1.0 M HCl holds only 4 × 1.0/1000 = 0.0040 mol of HCl. Beyond the NCERT text, aniline’s density is 1.02 g mL⁻¹ (PubChem, citing the US National Institute for Occupational Safety and Health), so 4 mL of aniline is about 4.08 g, or 4.08/93 = 0.0439 mol. The acid can turn only about one aniline molecule in eleven into aniline hydrochloride, and aniline itself is not soluble in water, as NCERT puts it (Exercise 9.3 (ii), page 278; beyond the NCERT text, only about 36 g dissolves per litre at 25 °C, HSDB via PubChem), so expect a cloudy mixture with an oily layer, not a clear solution (reasoned). Stir it well before adding it; in a practical exam, use the amounts you are given.
• Coupling with aniline or another aromatic amine is carried out in a weakly acidic medium, not in the alkaline solution used for β-naphthol (Lab Manual, page 112). The mixture must remain weakly acidic: in strongly acidic solution most of the aniline is protonated, and its positively charged nitrogen deactivates the ring (reasoned from NCERT page 273).
• NCERT slip (Laboratory Manual, page 111): the Lab Manual says the reaction is carried out “only in weekly acidic conditions”; it means weakly acidic, as page 112 says.
• Note on the temperature (Laboratory Manual, page 112): the procedure warms the mixture at about 40–45 °C, but the precautions on the same page say about 40–50 °C. The two ranges overlap, so this is a loose end rather than an error; keeping to 40–45 °C satisfies both (reasoned).

Aniline yellow (p-aminoazobenzene)

• Aniline yellow is C₆H₅–N=N–C₆H₄–NH₂, with –NH₂ para to the azo group, C₁₂H₁₁N₃: the same formula as diazoaminobenzene (reasoned).
• Like other azo compounds, it owes its colour to the extended conjugated system of two rings joined through –N=N– (NCERT page 276).
• Purifying it: the Lab Manual recrystallises a small portion of the crude product from carbon tetrachloride and takes the melting point of the perfectly dry sample (page 112). Carbon tetrachloride is hazardous (see Safety). Our safety advice: the instructor chooses an institution-approved purification method and solvent; do not substitute one without validation.
• Beyond the NCERT text (Wikipedia, “Aniline Yellow”):
  • a yellow azo dye that looks like an orange powder;
  • melting point 123–126 °C;
  • the first azo dye, first made in 1861.

Safety

• The Lab Manual marks aniline and carbon tetrachloride as toxic, and HCl as corrosive. Sodium nitrite is marked as an oxidiser and toxic, and acetic acid as flammable and corrosive (“causes severe burns”) (pages 110, 112 and 113).
• NCERT notes some evidence that exposure to carbon tetrachloride causes liver cancer in humans (Unit 6, page 187).
• Beyond the NCERT text: IARC’s published evaluation classifies p-aminoazobenzene as possibly carcinogenic to humans (Group 2B), based on animal evidence. Treat the product as hazardous and consult the actual supplier SDS.
• So handle the product and the solvent with gloves, in a fume cupboard, under a teacher’s supervision (reasoned).

Solved examples

Common mistakes to avoid

Quick revision summary

• Diazotisation: 2 mL aniline in 6.5 mL conc. HCl + 6.5 mL water, 0–5 °C, 1.6 g NaNO₂ in 8 mL water.
• Route 1: 3 g diazoaminobenzene + 7 mL aniline + 1.5 g aniline hydrochloride, 40–45 °C for one hour; 30 min; 9 mL glacial acetic acid + equal water; filter; the Lab Manual recrystallises from carbon tetrachloride (page 112), a toxic solvent, so use the institution-approved method.
• Route 2: diazonium solution + 4 mL aniline in 4 mL 1.0 M HCl, cold and slowly; filter the yellow dye.
• Aniline yellow, p-aminoazobenzene, C₆H₅N=NC₆H₄NH₂: coupling at the para position, an electrophilic substitution.

Questions students ask

What is the IUPAC name of aniline yellow?

The IUPAC name of aniline yellow is 4-(phenyldiazenyl)aniline, a point beyond the NCERT text: the –N=N– link is named as diazenyl, so the name reads as aniline carrying a phenyldiazenyl group, C₆H₅–N=N–, at C-4, para to its –NH₂ group (Wikipedia, “Aniline Yellow”; PubChem; reasoned). NCERT calls it p-aminoazobenzene (page 276), naming it instead from azobenzene, C₆H₅N=NC₆H₅, with an amino group at the 4-position, and it is also listed as 4-aminoazobenzene and C.I. Solvent Yellow 1 (PubChem).

Why is aniline yellow made from diazoaminobenzene rather than by direct coupling?

Aniline yellow is made from diazoaminobenzene because direct coupling of benzenediazonium chloride with aniline does not go cleanly to the dye, a point beyond the NCERT text: the diazonium ion can attack either a ring carbon or the nitrogen of aniline, and here the C-coupling that gives aniline yellow competes with the N-coupling that gives diazoaminobenzene (Wikipedia, “Azo coupling”). Warming diazoaminobenzene in aniline with a little aniline hydrochloride turns the N-coupled compound into the C-coupled one, in good yield (Lab Manual, page 111), and the same rearrangement is listed as a way of manufacturing the dye (PubChem, from the Hazardous Substances Data Bank).

What is aniline yellow used for?

It has been used as an oil-soluble dye and as an intermediate in dye chemistry. In this lesson its main role is to demonstrate electrophilic azo coupling and an extended conjugated system. An industrial colouring application does not imply suitability for food, cosmetics or unsupervised handling.

Is aniline yellow toxic or carcinogenic?

Aniline yellow (p-aminoazobenzene) is a hazardous laboratory dye. IARC’s 1987 evaluation classifies it as possibly carcinogenic to humans (Group 2B), with sufficient animal evidence and no human data in that evaluation. This classification is not a safe-exposure threshold. Avoid inhalation and skin contact and use the institution’s approved fume-hood and waste procedures.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Which of these, diazotised and then coupled with β-naphthol, gives an azo dye: p-toluidine, benzylamine, N-methylaniline? (Compare Lab Manual Unit 10, Discussion Question (vii), page 113.)

   Show answerHide answer

   Only p-toluidine, an aromatic primary amine. Benzylamine has its –NH₂ on a CH₂ group, so it behaves as an aliphatic primary amine and gives no stable diazonium salt. N-Methylaniline is a secondary amine (reasoned from NCERT page 274).

2. Q2. Why does the diazonium ion attack aniline at the para position?

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   –NH₂ is a powerful activating group that directs ortho and para (NCERT page 272; see directive influence), and the coupling is an electrophilic substitution (NCERT page 276). The para carbon is also the less crowded of the two (reasoned).