🔹 Isomerism (Class 11)

Definition:

Isomers are compounds having the same molecular formula but a different arrangement of atoms or different spatial arrangement, resulting in different properties.

1️⃣ Structural Isomerism

(Atoms have a different connection)

(A) Chain Isomerism

Definition:

Same molecular formula but a different carbon chain (straight or branched).

Example:

• C₄H₁₀
  • n-Butane → CH₃–CH₂–CH₂–CH₃
  • Isobutane → CH₃–CH(CH₃)–CH₃
• C₅H₁₂
  • Pentane
  • Isopentane
  • Neopentane

(B) Position Isomerism

Definition:

Same carbon chain and functional group, but the position of the functional group or multiple bond is different.

Example:

• C₃H₈O
  • 1-Propanol (–OH on C-1)
  • 2-Propanol (–OH on C-2)
• C₄H₈
  • 1-Butene
  • 2-Butene

(C) Functional Isomerism

Definition:

Same molecular formula but different functional groups.

Example:

• C₂H₆O
  • Ethanol (alcohol)
  • Dimethyl ether (ether)
• C₃H₆O
  • Propanal (aldehyde)
  • Acetone (ketone)

(D) Metamerism

Definition:

Same molecular formula and functional group but different alkyl groups on either side of a polyvalent atom (–O–, –NH–).

Example:

• C₄H₁₀O (ethers)
  • CH₃–O–C₃H₇
  • C₂H₅–O–C₂H₅
• C₅H₁₁N (amines)

(E) Ring–Chain Isomerism

Definition:

One compound is an open chain, while the other is a ring (cyclic).

Example:

• C₃H₆
  • Propene (open chain)
  • Cyclopropane (ring)
• C₄H₈
  • Butene
  • Cyclobutane

2️⃣ Stereoisomerism

(Atoms have the same connection, but 3D spatial arrangement is different)

(A) Geometrical Isomerism (cis–trans)

Definition:

Occurs due to restricted rotation around a C=C bond or ring.

Condition:
Each carbon of the double bond must have two different groups.

Example:

• 2-Butene
  • cis-2-butene
  • trans-2-butene
• Maleic acid (cis)
• Fumaric acid (trans)

(B) Optical Isomerism

Definition:

Compounds that rotate plane-polarized light due to a chiral carbon.

Chiral carbon:
Carbon attached to 4 different groups.

Types:

• Dextro (+) → right rotation
• Laevo (–) → left rotation

Example:

• Lactic acid
• Glyceraldehyde
• 2-Butanol

1️⃣ Crystallisation

📘 Definition (Exam Standard)

Crystallisation is the process of purification of an impure solid organic compound by dissolving it in a suitable hot solvent and then allowing it to cool slowly so that the pure compound separates out in the form of crystals, leaving impurities in the solution or removed by filtration.

📌 Principle

• Pure compound → crystallises
• Impurities → remain dissolved or removed as residue

🧪 Examples

1. Benzoic acid from water
2. Sugar from water
3. Copper sulphate from water
4. Acetanilide from hot water
5. Naphthalene from alcohol
6. Urea from water
7. Salicylic acid from water
8. Alum from water
9. Oxalic acid from water
10. Phthalic acid from alcohol

2️⃣ Sublimation

📘 Definition

Sublimation is a purification method used for those solid organic compounds which directly change from solid state to vapour state on heating and vapours on cooling give back the pure solid, leaving non-volatile impurities behind.

📌 Principle

• Solid → vapour → solid (no liquid phase)

🧪 Examples

1. Naphthalene
2. Camphor
3. Ammonium chloride
4. Iodine
5. Anthracene
6. Benzoic acid
7. Dry ice (CO₂)
8. Paradichlorobenzene
9. Chloral hydrate
10. Menthol

3️⃣ Simple Distillation

📘 Definition

Simple distillation is a method used to purify a liquid organic compound from non-volatile impurities or from another liquid having a large difference in boiling points, by heating the mixture and condensing the vapours of the liquid with the lower boiling point.

📌 Principle

• Difference in boiling points (≥ 25–30°C)

🧪 Examples

1. Acetone from impurities
2. Benzene from non-volatile solids
3. Toluene purification
4. Ether purification
5. Chloroform purification
6. Carbon tetrachloride
7. Methanol from salts
8. Ethanol from sugar solution
9. Hexane purification
10. Cyclohexane purification

4️⃣ Fractional Distillation ⭐⭐⭐

📘 Definition

Fractional distillation is a purification method used for separating two or more miscible liquids whose boiling points are close to each other by repeated vaporisation and condensation using a fractionating column.

📌 Principle

• Close boiling points (< 25°C difference)

🧪 Examples

1. Ethanol + water
2. Benzene + toluene
3. Acetone + methanol
4. Petroleum fractions
5. Crude oil refining
6. Hexane + heptane
7. Nitrobenzene + aniline
8. Liquid air separation
9. Xylene isomers
10. Chlorobenzene mixtures

5️⃣ Steam Distillation

📘 Definition

Steam distillation is a purification technique used for organic compounds which are immiscible with water, volatile in steam and have high boiling points, where the compound distils along with steam at a temperature lower than its normal boiling point.

📌 Principle

• Vapour pressure of mixture = atmospheric pressure

🧪 Examples

1. Aniline
2. Nitrobenzene
3. Turpentine oil
4. Clove oil
5. Eucalyptus oil
6. Essential oils
7. Bromobenzene
8. Chlorobenzene
9. Phenylamine
10. Oils from plants

6️⃣ Differential Extraction

📘 Definition

Differential extraction is a method of separation based on the difference in solubility of an organic compound in two immiscible solvents, where the compound is preferentially extracted into one solvent using a separating funnel.

📌 Principle

• Distribution coefficient

🧪 Examples

1. Iodine using CCl₄
2. Benzoic acid using NaHCO₃
3. Phenol using NaOH
4. Aniline using HCl
5. Organic dyes
6. Alkaloids extraction
7. Caffeine from tea
8. Oils from seeds
9. Acetic acid extraction
10. Drug extraction

7️⃣ Chromatography ⭐⭐⭐

📘 Definition

Chromatography is a purification technique based on the differential adsorption or distribution of components of a mixture between a stationary phase and a mobile phase, resulting in separation of components at different rates.

📌 Types

• Paper chromatography
• Column chromatography
• Thin layer chromatography (TLC)

🧪 Examples

1. Separation of ink dyes
2. Plant pigments
3. Amino acids
4. Sugars
5. Drugs
6. Vitamins
7. Antibiotics
8. Food colours
9. Alkaloids
10. Steroids

8️⃣ Fractional Crystallisation

📘 Definition

Fractional crystallisation is a method used to separate a mixture of two or more solids with different solubilities in a solvent by repeated crystallisation.

🧪 Examples

1. NaCl + KCl
2. Sugar + salt
3. Copper sulphate + ferrous sulphate
4. Alum mixtures
5. Benzoic acid mixtures
6. Organic acids mixture
7. Urea + sugar
8. Potash alum
9. Mixed salts