Chapter 8: Organic Chemistry – Some Basic Principles and Techniques

8.1 General Introduction

Organic chemistry is the study of carbon compounds (excluding simple substances like CO, CO₂, carbonates, and cyanides).

Vital Force Theory (Berzelius): Organic compounds could only be made by living organisms.

Disproved by Wöhler (1828): Synthesised urea from ammonium cyanate — an inorganic compound.

\[ \text{NH}_4\text{CNO} \xrightarrow{\text{heat}} \text{NH}_2\text{CONH}_2 \]

Unique Properties of Carbon

• Tetravalency: Carbon forms 4 bonds
• Catenation: Carbon atoms bond with each other to form long chains, branches, and rings
• Multiple bonding (single, double, triple bonds)
• Small size → strong bonds

8.2 Methods of Purification

Chromatography

Principle: Components of a mixture are distributed between a stationary phase and a mobile phase based on their differential adsorption.

Types:

• Column chromatography: Stationary phase = column packed with alumina/silica
• Thin Layer Chromatography (TLC): Stationary phase = thin layer of silica on glass plate
• Paper chromatography: Stationary phase = water trapped in paper

\[ R_f = \frac{\text{Distance moved by component}}{\text{Distance moved by solvent front}} \]

8.3 Qualitative Analysis of Organic Compounds

Detection of Elements

Quantitative Analysis

Carbon and Hydrogen (Liebig’s method):

\[ \text{% C} = \frac{12 \times \text{Mass of CO}_2}{44 \times \text{Mass of compound}} \times 100 \]

\[ \text{% H} = \frac{2 \times \text{Mass of H}_2\text{O}}{18 \times \text{Mass of compound}} \times 100 \]

Nitrogen:

• Dumas’ method: N₂ gas collected over KOH
• Kjeldahl’s method: \(\text{% N} = \frac{1.4 \times M \times V}{\text{Mass of compound}}\)

where \(M\) = molarity of acid, \(V\) = volume of acid used.

8.4 Classification and IUPAC Nomenclature

Classification of Organic Compounds

IUPAC Nomenclature

General format: Prefix + Root word + Suffix

Functional Groups

8.5 Isomerism

8.6 Electronic Displacement Effects in Covalent Bonds

1. Inductive Effect (permanent)

The shifting of σ-electrons along a chain of atoms due to difference in electronegativity.

• −I effect (electron-withdrawing): −NO₂, −CN, −COOH, −F, −Cl, −Br, −I, −OH, −NH₂
• +I effect (electron-donating): alkyl groups (−CH₃, −C₂H₅, etc.)

Order of +I effect: \(-\text{C(CH}_3)_3 > -\text{CH(CH}_3)_2 > -\text{C}_2\text{H}_5 > -\text{CH}_3 > -\text{H}\)

2. Resonance (Mesomeric) Effect

Delocalisation of π-electrons or lone pairs in conjugated systems.

• +M effect (electron-donating to the ring): −OH, −NH₂, −OCH₃, −OR
• −M effect (electron-withdrawing from the ring): −NO₂, −CN, −CHO, −COOH

3. Electromeric Effect (temporary)

Temporary transfer of π-electrons to one atom on demand of an attacking reagent.

4. Hyperconjugation (σ–π conjugation)

The delocalisation of σ-electrons of C–H bonds adjacent to a multiple bond or positive charge.

• Stability of carbocations: \(\text{3°} > \text{2°} > \text{1°} > \text{CH}_3^+\) (more hyperconjugation → more stable)

8.7 Homolytic and Heterolytic Fission

Homolytic Fission (free radicals)

\[ \text{A:B} \xrightarrow{\text{UV/heat}} \text{A}· + \text{B}· \]

Produces free radicals (species with unpaired electrons). Common in non-polar solvents and gas phase.

Heterolytic Fission (ions)

\[ \text{A:B} \rightarrow \text{A}^+ + \text{B}^- \quad \text{or} \quad \text{A}^- + \text{B}^+ \]

Produces carbocations (\(\text{C}^+\)) and carbanions (\(\text{C}^-\)).

Stability of carbocations: \(\text{3°} > \text{2°} > \text{1°} > \text{CH}_3^+\) (due to hyperconjugation and +I effect)

Stability of carbanions: \(\text{CH}_3^- > \text{1°} > \text{2°} > \text{3°}\) (opposite order)

8.8 Types of Organic Reactions

Electrophiles: Electron-loving species (Lewis acids): H⁺, NO₂⁺, Cl⁺, BF₃, AlCl₃

Nucleophiles: Nucleus-loving species (Lewis bases): OH⁻, CN⁻, NH₃, H₂O, R−O⁻

Practice Questions

Multiple Choice Questions (MCQs)

1. The IUPAC name of (CH₃)₃C–CH₂–CH(CH₃)₂ is:

 (a) 2,2,4-Trimethylpentane

 (b) 2,4,4-Trimethylpentane

 (c) 2,2,4-Trimethylhexane

 (d) 2-Tert-butyl-3-methylbutane

2. Which of the following shows +I effect?

 (a) −NO₂

 (b) −Cl

 (c) −CH₃

 (d) −COOH

3. Which is the most stable carbocation?

 (a) \(\text{CH}_3^+\)

 (b) \(\text{(CH}_3)_2\text{CH}^+\)

 (c) \(\text{(CH}_3)_3\text{C}^+\)

 (d) \(\text{C}_2\text{H}_5^+\)

4. Lassaigne’s test is used to detect:

 (a) C and H only

 (b) N, S, and halogens

 (c) Molecular formula

 (d) Functional groups

5. The number of structural isomers of C₄H₁₀ is:

 (a) 1

 (b) 2

 (c) 3

 (d) 4

Short Answer Questions (2–3 Marks)

6. What is the inductive effect? Arrange the following in order of increasing +I effect: −H, −CH₃, −C₂H₅, −(CH₃)₃C.

7. Draw all the structural isomers of C₅H₁₂ and give their IUPAC names.

8. Distinguish between electrophiles and nucleophiles with examples.

9. Explain hyperconjugation with a suitable example.

10. Write the IUPAC names of: (i) CH₃CH(OH)CH₂CHO (ii) CH₂=CHCH₂Br

Long Answer Questions (5 Marks)

11. (a) Explain resonance with the example of benzene. What are the conditions for resonance?

 (b) Compare the stability of the following carbocations and explain:

  \(\text{CH}_3^+,;\text{C}_2\text{H}_5^+,;\text{(CH}_3)_2\text{CH}^+,;\text{(CH}_3)_3\text{C}^+\)

12. (a) What are the different types of isomerism in organic chemistry? Give one example of each.

 (b) Write the IUPAC name of:

  \(\text{CH}_3\text{CH}_2\text{C(CH}_3)_2\text{CH}_2\text{CH(CH}_3)\text{CH}_2\text{OH}\)

13. (a) Describe the following purification methods with diagrams: (i) Distillation (ii) Crystallisation.

 (b) How is nitrogen estimated by Kjeldahl’s method?

Assertion-Reason Questions

14. Assertion (A): Tertiary carbocations are more stable than primary carbocations.

Reason (R): Hyperconjugation and +I effect of alkyl groups stabilise carbocations.

15. Assertion (A): −NO₂ group shows −I and −M effects.

Reason (R): −NO₂ is an electron-withdrawing group.

Answer Key