CBSE Class 11 Chemistry Chapter 12 Notes: Organic Chemistry: Basic Principles

Chapter Introduction

Organic Chemistry — Some Basic Principles and Techniques is the gateway chapter to organic chemistry. It introduces the language, conventions, and conceptual framework you will use throughout Class 11 and 12. The chapter covers the tetravalency of carbon, hybridization, IUPAC nomenclature, isomerism, electronic effects (inductive, resonance, hyperconjugation), and the methods used to purify and analyse organic compounds. This is a high-weightage chapter — expect direct questions on naming, isomer identification, electronic effects, and Lassaigne's test in every CBSE exam.

Key Definitions

Tetravalency and Hybridization in Carbon

Carbon has the electronic configuration 1s² 2s² 2p². In the ground state, it has two unpaired p-electrons (can form only two bonds). In the excited state, one 2s electron is promoted to 2p, giving four unpaired electrons (2s¹ 2p³), enabling tetravalency.

Hybridization Types

Key points:

• In sp³ hybridization: 4 sigma bonds, no pi bonds
• In sp² hybridization: 3 sigma bonds (from hybrid orbitals) + 1 pi bond (from unhybridised p-orbital)
• In sp hybridization: 2 sigma bonds + 2 pi bonds (two unhybridised p-orbitals)
• Sigma (σ) bonds: formed by head-on overlap; stronger; free rotation possible
• Pi (π) bonds: formed by lateral overlap; weaker; restrict rotation

Hybridization in Common Molecules

Structural Representations

Organic molecules can be represented in three ways:

1. Complete structural formula: Shows all atoms and bonds (e.g., H—C—C—H for ethane) 2. Condensed formula: Groups are written together (e.g., CH₃—CH₃ or CH₃CH₃) 3. Bond-line formula (skeletal): Lines represent C—C bonds; endpoints and intersections are carbon atoms; H atoms on C are not shown (most useful for complex molecules)

Example: n-Butane (C₄H₁₀)

• Complete: H₃C—CH₂—CH₂—CH₃ (condensed)
• Bond-line: /\/\ (zigzag with 4 carbons)

Classification of Organic Compounds

Based on Carbon Skeleton

Based on Functional Groups

Priority order (decreasing): —COOH > —SO₃H > —COOR > —COX > —CONH₂ > —CHO > C=O > —OH > —NH₂ > C=C > C≡C > —X

Homologous Series

A homologous series is a group of organic compounds with the same functional group where each successive member differs by —CH₂—.

IUPAC Nomenclature — Step by Step

Rules for Naming

1. Identify the longest carbon chain containing the principal functional group (not necessarily the longest chain if the functional group is not on it) 2. Number the chain from the end that gives the lowest locant to the principal functional group 3. Identify substituents and assign locants 4. Use the lowest-sum rule: When equal locants are possible, assign the lowest set of locants (e.g., 2,3,5 not 3,4,6) 5. Arrange substituents alphabetically (ignoring di-, tri-, tetra- prefixes) 6. Use multiplicative prefixes for identical substituents: di- (2), tri- (3), tetra- (4) 7. Use appropriate punctuation: Commas separate numbers; hyphens separate numbers from words; no space between locant and word

Nomenclature Examples

Special Naming Conventions

• Alkenes/Alkynes: Both double and triple bonds present → use "en" before "yne" (e.g., but-1-en-3-yne)
• Branched alkanes: Choose the longest chain; if equal, prefer the chain with more substituents
• Cyclic compounds: Prefix "cyclo-" (e.g., cyclohexane, cyclopentanol)
• Aromatic compounds: Benzene ring as parent; substituents as prefixes (e.g., methylbenzene = toluene)
• Multiple identical groups: Use di-, tri-, tetra- before the suffix (e.g., ethane-1,2-diol)

Isomerism

Isomerism is the phenomenon where compounds have the same molecular formula but different arrangements of atoms.

Structural (Constitutional) Isomerism

Stereoisomerism

Conditions for geometrical isomerism:

• A double bond (C=C) or ring structure
• Two different groups on each carbon of the double bond
• cis: similar groups on the same side; trans: similar groups on opposite sides

Conditions for optical isomerism:

• Presence of a chiral centre (asymmetric carbon) — a carbon bonded to four different groups
• The molecule must be non-superimposable on its mirror image

Fundamental Concepts: Electronic Effects

Inductive Effect

The permanent polarisation of a sigma bond transmitted through the sigma bond chain.

Order of —I effect: —NO₂ > —CN > —COOH > —F > —Cl > —Br > —I > —OH > —OCH₃

Order of +I effect: —C(CH₃)₃ > —CH(CH₃)₂ > —C₂H₅ > —CH₃

Applications:

• Explains acidity: —I groups increase acidity (stabilise conjugate base); +I groups decrease acidity
• Explains reactivity of alkyl halides: +I effect of alkyl groups makes C more electron-rich

Resonance (Mesomeric Effect)

Delocalisation of π-electrons over conjugated systems, represented by multiple contributing structures (canonical forms).

Rules for resonance structures: 1. Nuclei must not change position (only electrons move) 2. Each structure must be a valid Lewis structure 3. Charge separation should be minimised 4. Structures with more covalent bonds are more stable 5. Negative charge on more electronegative atom = more stable

Applications:

• Stability of carbocations, carbanions, and free radicals
• Acid-base strength of phenols and anilines
• Directive influence of substituents in benzene

Hyperconjugation (No-Bond Resonance)

Delocalisation of σ-electrons of a C—H bond into an adjacent empty or partially filled p-orbital or π-orbital.

• Also called σ-π conjugation or no-bond resonance
• More hyperconjugative structures = more stable species
• Stability of carbocations: tertiary > secondary > primary > methyl (explained by hyperconjugation)
• Stability of alkenes: more α-hydrogens = more hypercongenation = more stable

Example: Propene (CH₃—CH=CH₂) has 3 hyperconjugative structures due to 3 α-hydrogens on the methyl group.

Electromeric Effect

Temporary polarisation of a π-bond in the presence of an attacking reagent.

• +E effect: Electron pair moves towards the attacking reagent (e.g., addition of H⁺ to C=C)
• —E effect: Electron pair moves away from the attacking reagent
• Operates only in the presence of an attacking reagent (unlike inductive effect, which is permanent)

Electrophiles and Nucleophiles

Key distinctions:

• Neutral molecules can be nucleophiles if they have lone pairs (e.g., H₂O, NH₃)
• Ambident nucleophiles: species that can attack through two different atoms (e.g., CN⁻ can attack through C or N; NO₂⁻ can attack through N or O)

Methods of Purification

Chromatography Types

Qualitative Analysis: Lassaigne's Test

Sodium Fusion Extract Preparation

A small piece of sodium is heated with the organic compound in a fusion tube. The fused mass is dissolved in water to give the sodium fusion extract (Lassaigne's extract).

Important notes:

• If N and S are both present, the fusion produces NaSCN (sodium thiocyanate) — not NaCN and Na₂S separately
• HNO₃ is added before AgNO₃ to decompose NaCN and Na₂S which would otherwise give false positives
• AgBr is pale yellow; AgI is yellow; AgCl is white — all insoluble in dilute HNO₃

Quantitative Analysis

Kjeldahl method steps: 1. Digestion: Organic compound + conc. H₂SO₄ + CuSO₄ (catalyst) → (NH₄)₂SO₄ 2. Distillation: (NH₄)₂SO₄ + excess NaOH → NH₃ (distilled into known volume of standard acid) 3. Titration: Residual acid titrated with standard NaOH

Limitation of Kjeldahl method: Does NOT work for compounds containing N in —NO₂, —N=N— (azo), or ring nitrogen (e.g., pyridine) because these N atoms are not converted to (NH₄)₂SO₄.

Common Mistakes to Avoid

1. Wrong hybridization assignment: Count the number of sigma bonds + lone pairs around carbon. 4 → sp³, 3 → sp², 2 → sp. Do not forget to count lone pairs. 2. Incorrect IUPAC naming: Always choose the chain that contains the principal functional group, not just the longest chain. Number from the end nearest to the functional group. 3. Confusing —I and +I effects: —I groups are electronegative and pull electrons (e.g., —NO₂, —CN). +I groups are alkyl groups that push electrons. Remember: —I increases acidity; +I decreases acidity. 4. Resonance vs hyperconjugation: Resonance involves π-electrons in conjugated systems; hyperconjugation involves σ-electrons of C—H bonds. They are different effects. 5. Lassaigne's test errors: If N and S are both present, sodium thiocyanate (NaSCN) forms, giving a blood red colour with Fe³⁺ — NOT Prussian blue. Always add HNO₃ before AgNO₃ to eliminate interference from CN⁻ and S²⁻. 6. Geometric isomerism conditions: Both substituents on each C of C=C must be different for geometric isomerism. CH₂=CHCl does NOT show geometric isomerism. 7. Functional group priority: When naming, —COOH always takes priority over —OH, —CHO, and C=O. The principal functional group determines the suffix. 8. Kjeldahl limitation: Remember that —NO₂, —N=N—, and ring nitrogen compounds cannot be analysed by the Kjeldahl method.

Exam Tips

• IUPAC naming (2–3 marks): Practise naming compounds with multiple functional groups. Always identify the principal functional group first.
• Isomerism identification (2–3 marks): Be ready to draw and identify chain, position, and functional isomers for a given molecular formula (e.g., C₄H₁₀O has 7 isomers).
• Hybridization (1–2 marks): Practise identifying hybridization of each carbon in molecules like CH₃—CH=CH—C≡CH (sp³, sp², sp², sp, sp)
• Electronic effects (2–3 marks): Know how —I, +I, —M, +M effects influence acidity/basicity and reactivity. Prepare comparison tables.
• Lassaigne's test (2–3 marks): Memorise the observations for N, S, and halogens separately, and for N+S together.
• Purification methods (1–2 marks): Know which method is suitable for which type of mixture and why.
• Quantitative analysis formulae (1–2 marks): Dumas and Kjeldahl formulae are frequently asked.

Quick Revision Notes

Hybridization Summary

• sp³: 4 regions → tetrahedral → 109.5° → alkanes (single bonds only)
• sp²: 3 regions → trigonal planar → 120° → alkenes, benzene (one π-bond)
• sp: 2 regions → linear → 180° → alkynes (two π-bonds)

IUPAC Priority Order: —COOH > —SO₃H > —COOR > —COX > —CONH₂ > —CHO > C=O > —OH > —NH₂ > C=C > C≡C > —X

Isomerism Quick Reference

• Same formula, different connectivity → Structural isomerism (chain/position/functional/metamerism)
• Same connectivity, different spatial arrangement → Stereoisomerism (geometric/optical)

Electronic Effects Quick Reference

• Inductive effect: through σ-bonds, permanent, decreases with distance
• Resonance: through π-bonds, permanent, does not decrease with distance
• Hyperconjugation: σ(C—H) → p/π, permanent, stabilises carbocations and alkenes
• Electromeric effect: through π-bonds, temporary, only during attack

Lassaigne's Test Summary

• N alone → Prussian blue (Fe₄[Fe(CN)₆]₃)
• S alone → Violet colour (sodium nitroprusside)
• N + S → Blood red (FeSCN)
• Halogens → AgX precipitate (after removing CN⁻ and S²⁻ with HNO₃)

Purification Methods Summary

• Sublimation → volatile solids (camphor)
• Crystallisation → solids from solutions
• Simple distillation → large bp difference (>30–40 K)
• Fractional distillation → small bp difference
• Steam distillation → steam-volatile, water-immiscible
• Chromatography → differential adsorption