Section-B: Organic Chemistry

Preparation of a Derivative

Objective: To identify an unknown organic compound.

Why prepare a derivative? Many organic compounds with the same functional group have similar properties. For example, many aldehydes are liquids with a wide range of boiling points. It's difficult to identify an unknown aldehyde just by its boiling point. By reacting the unknown compound with a specific reagent, we can convert it into a solid derivative.

This derivative has two key properties:

1. It is a solid: Solids are much easier to purify (by recrystallization) than liquids (which require distillation).
2. It has a sharp, characteristic melting point: We can compare the measured melting point of our purified derivative to a table of known melting points for different derivatives. A match confirms the identity of our original unknown compound.

The overall process in the exam is:

1. You are given an unknown organic compound.
2. You must first identify its functional group (e.g., -COOH, -OH, >C=O) using preliminary tests (not detailed in this syllabus, but implied).
3. Once the functional group is known, you select the correct reagent to prepare a suitable derivative (as listed in the syllabus).
4. You perform the reaction to synthesize the derivative.
5. You isolate the crude solid derivative by filtration.
6. You purify the derivative by recrystallization.
7. You dry the pure derivative and determine its melting point.
8. You report the melting point to the examiner.

Core Techniques: Recrystallization and Melting Point

Recrystallization (Purification)

Principle: This technique purifies a solid based on differences in solubility. The ideal solvent is one in which the solid derivative is highly soluble at high temperatures (boiling) and poorly soluble at low temperatures (ice-cold). The impurities are either insoluble at high temperatures (and can be filtered off hot) or very soluble at low temperatures (and stay in the solution).

Procedure

1. Choose a suitable solvent (e.g., ethanol, water, or a mix).
2. Place the crude solid in a flask and add a *minimum* amount of the solvent.
3. Heat the mixture gently (e.g., on a water bath) until the solid just dissolves, adding more solvent drop-by-drop if needed. You want a saturated solution at the boiling point.
4. If there are insoluble impurities, perform a "hot filtration" to remove them.
5. Allow the clear solution to cool slowly to room temperature, then in an ice bath.
6. Pure crystals of the derivative should form, leaving the soluble impurities in the "mother liquor" (the solution).
7. Filter the pure crystals using a Buchner funnel (vacuum filtration) and wash them with a small amount of ice-cold solvent.
8. Dry the crystals.

Melting Point Determination

Principle: A pure solid has a sharp, well-defined melting point (usually a narrow range of 0.5-1°C). Impurities broaden and depress (lower) the melting point.

Procedure

1. Ensure the crystal sample is perfectly dry and grind it into a fine powder.
2. Pack a small amount (2-3 mm high) into a capillary tube (sealed at one end).
3. Place the capillary tube in a melting point apparatus (e.g., Thiele tube or digital melting point device).
4. Heat the apparatus slowly, especially as you approach the expected melting point (about 1-2°C per minute).
5. Record the melting point range:
   • T1: The temperature when the first drop of liquid appears.
   • T2: The temperature when the last crystal turns into liquid.
6. Report the range, e.g., "120-121°C".

Derivative Preparations by Functional Group

a) -COOH (Carboxylic Acid)

Derivatives: Amide, Anhydride, or Ester.

1. Amide Preparation:
Principle: The carboxylic acid is first converted to a more reactive acid chloride, which then reacts with ammonia to form the amide.
Reaction 1: R-COOH + SOCl₂ (Thionyl chloride) → R-COCl (Acid chloride) + SO₂ + HCl
Reaction 2: R-COCl + 2 NH₃ → R-CONH₂ (Amide) + NH₄Cl

2. Ester (e.g., p-Nitrobenzyl ester):
Principle: Fischer esterification is too slow. A better method is reacting the sodium salt of the acid with an alkyl halide.
Reaction: R-COO⁻Na⁺ + (NO₂)-C₆H₄-CH₂Br → (NO₂)-C₆H₄-CH₂-OOCR + NaBr

b) >C=O (Aldehyde or Ketone)

Derivatives: 2,4-Dinitrophenylhydrazone (2,4-DNP) or Semicarbazone.

1. 2,4-DNP Derivative: (Most common)
Principle: The carbonyl group (C=O) undergoes a condensation reaction with 2,4-dinitrophenylhydrazine (Brady's reagent) to form a 2,4-dinitrophenylhydrazone, which is a highly colored (yellow, orange, or red) solid.
Reaction: R-CO-R' + H₂N-NH-C₆H₃(NO₂)₂ → R-C(R')=N-NH-C₆H₃(NO₂)₂ + H₂O
Procedure: Dissolve the aldehyde/ketone in a small amount of ethanol. Add Brady's reagent and shake. The solid derivative usually precipitates immediately. Filter, wash, and recrystallize from ethanol or ethyl acetate.

2. Semicarbazone Derivative:
Principle: Similar condensation reaction with semicarbazide hydrochloride.
Reaction: R-CO-R' + H₂N-NH-CO-NH₂ → R-C(R')=N-NH-CO-NH₂ + H₂O

c) -OH (Alcohol or Phenol)

Derivative: Benzoate.

Principle (Schotten-Baumann Reaction): The alcohol or phenol reacts with benzoyl chloride (C₆H₅COCl) in the presence of a base (e.g., NaOH solution) to form an ester (a benzoate). The base neutralizes the HCl produced.

Reaction (for alcohol): R-OH + C₆H₅COCl + NaOH → R-O-CO-C₆H₅ (Benzoate) + NaCl + H₂O

Procedure: The alcohol is mixed with 10% NaOH. Benzoyl chloride is added in small portions, and the mixture is shaken vigorously. The solid benzoate ester precipitates. It is filtered, washed with water (to remove base) and dilute HCl (to remove unreacted benzoyl chloride), and then recrystallized.

d) -NH₂ (Amine)

Derivative: Acetyl derivative (Acetylation).

Principle: The amine (primary or secondary) reacts with acetic anhydride (CH₃CO)₂O to form an N-substituted amide (an acetilide).
Reaction (for primary amine): R-NH₂ + (CH₃CO)₂O → R-NH-CO-CH₃ (e.g., Acetanilide) + CH₃COOH

Procedure: The amine is dissolved in dilute HCl, and then sodium acetate solution is added (to create a buffer). Acetic anhydride is added, and the mixture is shaken. The solid acetyl derivative precipitates. It is filtered, washed with water, and recrystallized from ethanol-water mixture.

e) -NO₂ (Nitro Compound)

Derivative: Reduction to an amine, followed by acetylation.

Principle: This is a two-step process. First, the nitro group (-NO₂) is reduced to an amine group (-NH₂) using a strong reducing agent (e.g., Sn/HCl). Second, the resulting amine is converted to its acetyl derivative as described in (d).

Reaction 1 (Reduction): R-NO₂ + 6 [H] --(Sn/HCl)--> R-NH₂ + 2 H₂O

Reaction 2 (Acetylation): R-NH₂ + (CH₃CO)₂O → R-NH-CO-CH₃ + CH₃COOH

Procedure: The nitro compound is refluxed with tin (Sn) and concentrated HCl. The reaction mixture is then made strongly basic with NaOH to liberate the free amine (R-NH₂). The amine is extracted (e.g., with ether) or steam distilled. Finally, the isolated amine is acetylated as described in the section above.