IMChO 2008, Експеримент — Задача 1. Identification of organic compounds

Reagents, labware and equipment

p-DMABA

Part 1. Identification of organic compounds

Each participant is supplied with containers labeled A – G, each containing an individual compound from 7 listed below: p-nitrophenol, benzoic acid, salicylic acid, acetylsalicylic acid, paracetamol, streptocide, and analgin:

Identify the compounds using the reagents provided (decipher the compound in each vial). Identification is based on the ability of compounds to give more or less intensely colored products as a result of interaction with the corresponding reagents.

Tips:

1. Perform qualitative reactions in the 96-well plate (fig. 1/2008).
2. The unknown compounds can be subjected to analysis in the solid form. Transfer small amounts of compounds into the wells on spatula. Carefully wipe the spatula with paper towels.
3. Do not remove the vial stoppers; only pull out the pipettes from the stoppers. Do not pour high amount of the solutions into the wells (1-2 drops of a reagent are sufficient).
4. Follow both intensity of a product color as well as the time of its appearance.
5. Oxidation of the compounds by permanganate should be tested in neutral and alkaline media.

Fig. 1/2008. 96-well plate for qualitative reactions.

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6. One of the unknown compounds (that with aromatic ring most enriched with electrons) promptly reacts with nitrous acid at room temperature yielding a yellow nitro-derivative.
7. p-DMABA reacts with amines under acidic conditions affording Schiff bases. Most intensely colored products are formed with the compounds containing a primary amino group:

8. Many phenols interact with iron(III) ions, the coloration being most intense in the case of chelate formation.
9. The sample of acetylsalicylic acid may contain salicylic acid as an impurity.

Procedure

a) Identification of the unknown compounds

Place a small amount of the substance from vial A into each of the six wells of the 96-well plate. Add the alkali solution to the first well, neutral $\mathrm{KMnO_4}$ solution to the second well, $\mathrm{KMnO_4}$ and alkali solutions to the third well, $\mathrm{FeCl_3}$ solution to the forth well, $\mathrm{NaNO_2}$ and hydrochloric acid solutions to the fifth well, and p-DMABA solution to the sixth well. Repeat the same for the substances from vials B – G. Write down the observations in Table 1/2008.

Table 1/2008. A sample protocol for recording the observations.

Based on the study, identify the compounds in vials A – G (fill in the Table):

b) Confirmation of the identification results

Aromatic amines react with sodium nitrite in acidic medium affording diazonium salts (diazotization reaction), which, in turn, interact with electron enriched aromatic compounds with formation of intensely colored products (azo coupling reaction). The reaction takes place in weakly acidic or weakly alkaline medium; the deprotonated forms of the products usually possess the strongest coloration.

Questions and assignments

1. Write down the equation of a diazotization reaction, denoting the aromatic moiety as Ar.

2. Write down the equation of an azo coupling reaction, denoting the aromatic moiety as Ar'.

To confirm the identification you have accomplished, study the possibility of diazotization of one of the compounds A – G followed by azo coupling with another compound from the same list (A – G). Remember that an intensely colored product is formed. Complete the Table (only one colored product is scored).