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Two products X and Y are formed in the following reaction sequence. The suitable method for separation of products X and Y is :
C₆H₆ + CH₃Cl →(Anhydr. AlCl₃) W
W →(dil. HNO₃ + dil. H₂SO₄, warm) X + Y
Options
1
Continuous extraction
2
Differential extraction
3
Fractional distillation
4
Sublimation
Correct Answer
Option 3 : Fractional Distillation
Step-by-Step Solution
1

Step 1 — Friedel-Crafts Alkylation:

C₆H₆ + CH₃Cl →(Anhydrous AlCl₃) C₆H₅CH₃ (Toluene = W)

2

Step 2 — Nitration of Toluene (CH₃ is o/p director):

Toluene + dil. HNO₃/dil. H₂SO₄ (warm) → o-nitrotoluene (X) + p-nitrotoluene (Y)

CH₃ group activates the ring and directs incoming −NO₂ to ortho and para positions.

Major products: ~60% ortho + ~37% para (meta <5%)

3

Separation method:

o-Nitrotoluene: bp = 222°C (liquid at room temperature)

p-Nitrotoluene: mp = 52°C, bp = 238°C (solid at room temperature)

Both are liquids at high temperature. Difference in boiling points (16°C apart) → Fractional Distillation

Sublimation: only if one component is a solid that sublimes. Extraction: for separating organic/aqueous layers. Neither applies here.

W = Toluene (C₆H₅CH₃)
X = o-Nitrotoluene (bp 222°C)
Y = p-Nitrotoluene (bp 238°C)
Separation: Fractional Distillation (different boiling points)
Theory: Purification Methods in Organic Chemistry
1. Fractional Distillation

Used to separate two or more miscible liquids with different boiling points (difference <25°C needs a fractionating column). The mixture is heated — component with lower bp vaporises first, condenses in the fractionating column, while higher-boiling component stays. Multiple vaporisation-condensation cycles occur in the column, giving excellent separation. Used in: petroleum refining (separating petrol, kerosene, diesel, etc.), separation of air (liquid air → N₂ bp −196°C, O₂ bp −183°C, Ar bp −186°C), alcohol-water mixtures (ethanol bp 78°C, water bp 100°C). In this problem: o-nitrotoluene (bp 222°C) and p-nitrotoluene (bp 238°C) — 16°C difference → fractional distillation.

2. Simple Distillation

Used when the boiling point difference between two liquids is >25°C, or to separate a liquid from non-volatile dissolved solids. Example: separating water from salt solution, or chloroform (bp 61°C) from aniline (bp 184°C). The liquid with lower boiling point distils over first. Cannot be used for liquids with close boiling points — fractional distillation needed instead.

3. Sublimation

Used to purify solid substances that sublime (go directly from solid to vapour without melting) on heating. The pure sublimed solid collects on a cold surface above the mixture, leaving non-sublimable impurities behind. Examples of substances that sublime: iodine (I₂), naphthalene, anthracene, camphor, benzoic acid, ammonium chloride (NH₄Cl). This method is used when the desired compound sublimes but the impurity doesn't (or vice versa). NOT applicable to o-nitrotoluene and p-nitrotoluene (both are liquids/non-sublimable solids).

4. Solvent Extraction (Differential/Continuous)

📌 Differential extraction: Two immiscible liquids in a separating funnel. Organic compound partitions into organic layer (more soluble there). Layers separated by tap. Used when: compound is more soluble in organic solvent than in water.

📌 Continuous extraction: Used when the distribution coefficient is small — requires repeated extraction. Soxhlet extractor is used. Solvent continuously cycles through the mixture, extracting more compound each time.

📌 NOT applicable here: o and p-nitrotoluene are both soluble in organic solvents — can't separate them by extraction (same solubility properties).

5. Crystallisation and Recrystallisation

Used to purify solid organic compounds. The impure solid is dissolved in minimum hot solvent, filtered to remove insoluble impurities, then allowed to cool slowly. Pure compound crystallises out (less soluble at lower temperature) while soluble impurities remain in solution. The pure crystals are filtered, washed, and dried. Key: choose a solvent in which the compound has high solubility when hot but low solubility when cold. Common solvents: water, ethanol, acetone, chloroform. p-Nitrotoluene could theoretically be recrystallised, but this wouldn't help separate it from o-nitrotoluene (same crystal behaviour).

6. Chromatography — Modern Purification

Chromatography separates mixture components based on differential migration through a stationary phase (solid/liquid) while a mobile phase (liquid/gas) moves through it. Types: Column chromatography (large-scale separation), Thin Layer Chromatography (TLC — quick analysis), Paper chromatography (amino acids, plant pigments), Gas-liquid chromatography (GLC/GC — volatile organic compounds), High Performance Liquid Chromatography (HPLC — pharma). The Rf value (ratio of distance moved by component to distance moved by solvent) is characteristic of each compound in a given system.

7. Nitration of Aromatic Compounds

Nitration requires a nitrating mixture (conc. HNO₃ + conc. H₂SO₄). The electrophile generated is NO₂⁺ (nitronium ion): HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O. NO₂⁺ attacks the π electrons of benzene ring via EAS mechanism. For toluene: CH₃ is an activating o/p director → both ortho and para positions are attacked. Major products: o-nitrotoluene and p-nitrotoluene. Meta-nitrotoluene is a minor product (<5%). Higher temperature or more concentrated acids can give dinitration (2,4-dinitrotoluene, 2,6-dinitrotoluene, etc.).

8. Friedel-Crafts Alkylation — Key Points

C₆H₆ + RCl →(AlCl₃) C₆H₅R + HCl. AlCl₃ generates carbocation R⁺ which attacks benzene. Limitations: (1) Polyalkylation — alkylbenzene more reactive than benzene → di/trialkyl products form. (2) Carbocation rearrangement — R⁺ may rearrange to more stable form. (3) Doesn't work with deactivated rings (nitrobenzene). (4) Can't use with −NH₂ group (amine coordinates to Lewis acid). To avoid polyalkylation: use excess benzene or Friedel-Crafts acylation (no rearrangement + monosubstitution guaranteed since acyl group deactivates ring).

Frequently Asked Questions
1. Why is fractional distillation used and not simple distillation? ⌄
o-Nitrotoluene bp = 222°C, p-Nitrotoluene bp = 238°C — difference is only 16°C. Simple distillation cannot efficiently separate liquids with boiling point difference less than 25°C. Fractional distillation uses a fractionating column that provides multiple vaporisation-condensation cycles, effectively amplifying the small bp difference and giving clean separation. Simple distillation would give a mixture of both isomers in the distillate.
2. Why does nitration of toluene give mainly ortho and para products? ⌄
The methyl group (−CH₃) is an ortho/para director because: (1) It releases electrons to the ring through hyperconjugation and induction. (2) This electron density is highest at ortho and para positions (through resonance of the transition state). (3) The carbocation intermediate (Wheland intermediate) is most stable when NO₂⁺ attacks ortho or para positions — the electron-donating CH₃ stabilises these intermediates more. Meta position: CH₃ cannot stabilise the intermediate → much less product (<5%).
3. Why is para product often preferred over ortho despite both being o/p products? ⌄
Ortho has two positions (C2 and C6) while para has one position (C4). Statistically, ortho should give 2× more product. However: (1) Steric hindrance — ortho position is adjacent to bulky CH₃ group → electrophile prefers less hindered para. (2) Para product is more thermodynamically stable (less steric strain). So in practice: ortho ~58–60%, para ~37–40%, meta <5%. With bulky electrophiles or bulky substituents: para product dominates even more.
4. What is the Rf value in chromatography? ⌄
Rf = (distance moved by compound)/(distance moved by solvent front). Range: 0 to 1. Rf value is specific for a compound under given conditions (solvent, temperature, stationary phase). Compounds with Rf close to 1: highly soluble in mobile phase, less adsorbed on stationary phase → less polar compounds. Rf close to 0: strongly retained on stationary phase → more polar compounds. Same Rf for two compounds in different solvents suggests they might be the same compound. Rf cannot exceed 1.
5. What substances can be separated by sublimation? ⌄
Substances that sublime (go directly solid→vapour): Iodine (I₂), Camphor (C₁₀H₁₆O), Naphthalene (moth balls), Anthracene, Benzoic acid (partially), Ammonium chloride (NH₄Cl), Dry ice (CO₂), Aluminium chloride (AlCl₃). Application: purifying naphthalene from coal tar, purifying iodine, separating NH₄Cl from NaCl. NOT applicable to o and p-nitrotoluene — they are liquids (o-nitrotoluene) and a low-melting solid (p-nitrotoluene, mp 52°C) that don't sublime.
6. Why can't extraction separate o- and p-nitrotoluene? ⌄
Solvent extraction works by selectively dissolving one component in an immiscible solvent while the other component stays in the original solvent. o-Nitrotoluene and p-nitrotoluene have very similar chemical properties (both are mono-nitro derivatives of toluene, similar polarity, similar solubility in most solvents). No solvent would selectively dissolve only one isomer. Extraction separates chemically different compounds (e.g., organic compound from inorganic salts in water) — not structurally similar isomers.
7. What is steam distillation and when is it used? ⌄
Steam distillation is used to purify high-boiling, heat-sensitive organic compounds that are immiscible with water. Steam is passed through the mixture — the organic compound co-distils with water at a temperature below 100°C (below both boiling points). Example: aniline (bp 184°C) is steam distilled at ~98°C. Used when: compound would decompose at its normal boiling point. The distillate (organic + water) is separated in a separating funnel. Not applicable to nitrotoluene isomers — we need to separate two organic compounds from each other, not from water.
8. What is vacuum distillation (distillation under reduced pressure)? ⌄
Reducing pressure lowers boiling points (Clausius-Clapeyron equation: lower pressure → lower bp). Used for: very high boiling compounds (would decompose at their normal bp), heat-sensitive compounds, and viscous substances. Example: glycerol (bp 290°C) is distilled under reduced pressure to avoid decomposition. The boiling point at reduced pressure can be calculated: if bp at 1 atm = T₁, at pressure P₂ bp = T₂ (from vapour pressure tables). Used in pharmaceuticals and petroleum industry for high-boiling fractions.
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