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Organic preparations in the practical exam reward students who know why each step is there: why the ice bath, why pour into cold water, why the melting point. This post covers three preparations that appear in the syllabus, acetanilide, aniline yellow and iodoform, with the reactions, a worked yield calculation and the viva questions examiners ask.
Which preparations are in your syllabus?
The CBSE Chemistry curriculum for 2026-27 asks Class 12 students to prepare any one of acetanilide, di-benzalacetone, p-nitroacetanilide, and aniline yellow or 2-naphthol aniline dye. Iodoform is not on that CBSE list. The JEE (Main) 2026 syllabus and the NEET (UG)-2026 syllabus ask for "the chemistry involved in the preparation" of acetanilide, p-nitroacetanilide, aniline yellow and iodoform. So CBSE students need to do one of these, while entrance aspirants need to explain all of them. Check the documents for your own year.
How is acetanilide prepared?
Aniline is acetylated: its –NH2 group reacts with acetic anhydride to form an amide, acetanilide (C6H5NHCOCH3), and acetic acid.
C6H5NH2 + (CH3CO)2O → C6H5NHCOCH3 + CH3COOH
The NCERT Class 12 chapter Amines uses this same acetylation to protect the –NH2 group before substitution, because the lone pair on nitrogen in acetanilide is partly drawn into resonance with the carbonyl oxygen. In the lab, the reaction mixture is poured into ice-cold water so the product separates as a solid, which is filtered and recrystallised from hot water. PubChem's acetanilide entry gives its melting point as 114.3 °C. An impure sample melts lower and over a wider range, so the melting point is your purity check.
Worked example: percentage yield of acetanilide
Data from PubChem: aniline has a molar mass of 93.13 g/mol and a density of about 1.02 g/mL; acetic anhydride has 102.09 g/mol and about 1.08 g/mL; acetanilide has 135.16 g/mol.
You use 5.0 mL of aniline and 6.0 mL of acetic anhydride.
Aniline: 5.0 × 1.02 = 5.10 g; 5.10 / 93.13 = 0.0548 mol.
Acetic anhydride: 6.0 × 1.08 = 6.48 g; 6.48 / 102.09 = 0.0635 mol.
The ratio is 1 : 1, so aniline is the limiting reagent.
Theoretical yield: 0.0548 × 135.16 = 7.40 g.
If you obtain 5.6 g of dry product (a sample figure), yield = 5.6 / 7.40 × 100 = 75.7%.
How is aniline yellow made?
Aniline yellow is p-aminoazobenzene. It is made in two stages, both described in the NCERT Amines chapter.
Diazotisation. Aniline reacts with nitrous acid, made from sodium nitrite and hydrochloric acid, at 273–278 K to give benzene diazonium chloride. NCERT notes that if the temperature rises to 283 K, the salt is hydrolysed to phenol. That is why the beaker sits in an ice bath and the nitrite is added slowly.
Coupling. The diazonium salt reacts with aniline, and NCERT states that this gives p-aminoazobenzene. It is an electrophilic substitution at the para position.
NCERT explains the colour: azo compounds have an extended conjugated system, with both rings joined through –N=N–, so they are coloured and used as dyes. PubChem's entry for 4-aminoazobenzene gives its formula as C12H11N3 and its molar mass as 197.24 g/mol.
How is iodoform formed?
Iodoform, CHI3, comes from the haloform reaction. The NCERT Class 12 chapter Aldehydes, Ketones and Carboxylic Acids states that methyl ketones are oxidised by sodium hypohalite, and that the iodoform reaction with sodium hypoiodite detects a CH3CO group, or a CH3CH(OH) group that is oxidised to CH3CO. Ethanol and acetone both qualify. With acetone, iodine and sodium hydroxide:
CH3COCH3 + 3I2 + 4NaOH → CHI3 + CH3COONa + 3NaI + 3H2O
Check the balance: carbon 3 = 1 + 2, hydrogen 10 = 10, oxygen 5 = 5, sodium 4 = 4, iodine 6 = 6. Iodoform separates as a yellow solid with a distinctive smell. PubChem's iodoform entry lists its melting point as about 119 °C.
What will the viva ask?
Why is the acetanilide mixture poured into ice-cold water? (The product is poorly soluble in cold water and separates out.)
Why recrystallise? (To remove impurities; check with the melting point.)
Why keep diazotisation at 273–278 K? (Diazonium salts decompose to phenol at higher temperature.)
Why is aniline yellow coloured? (Extended conjugation through the –N=N– link.)
Which compounds give the iodoform test? (Those with CH3CO or CH3CH(OH) groups.)
Is methanol iodoform-positive? (No, it has no CH3CH(OH) group.)
Safety matters here. Aniline, acetic anhydride and sodium nitrite are hazardous. Wear gloves and goggles and follow your teacher's instructions on where to work.
The same functional groups appear in our guide to tests for organic functional groups, and the reactions behind aniline yellow are covered in amines and diazonium salts.
A practice routine before the practical
Write the equation for each preparation and name the reaction type.
Redo the yield calculation for 4.0 mL of aniline and check which reagent limits.
Write the reason for every temperature and every cold-water step.
Answer the viva questions aloud without notes.
When a reaction sequence feels like a list to memorise, slow down on the reasoning. The Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Before the lab, write the equation, the limiting reagent and the expected yield for your preparation on the first page of your record.
