Organic Chemistry Some Basic Principles and Techniques Class 11 Notes | CBSE Chemistry Chapter 11

Table of Contents

• Key Concepts - Tetravalence, structures, classification, IUPAC names, isomerism, electron effects, mechanism, purification
• Weightage in Board & Entrance Exams
• Important Definitions
• Solved Examples
• Important Questions for Board Exams
• Quick Revision Points

Key Concepts

1. Tetravalence of Carbon and Hybridisation

Carbon has the electronic configuration 1s² 2s² 2p², giving it four valence electrons. To complete its octet it forms four covalent bonds - this property is called tetravalence, and it is why carbon builds endless chains, rings, and branches.

Carbon achieves tetravalence through hybridisation - mixing of atomic orbitals to form equivalent hybrid orbitals.

• sp³ hybridisation: 4 σ-bonds, tetrahedral, bond angle 109.5° (e.g. CH₄, all single bonds).
• sp² hybridisation: 3 σ-bonds + 1 π-bond, planar, bond angle 120° (e.g. C₂H₄, a C=C double bond).
• sp hybridisation: 2 σ-bonds + 2 π-bonds, linear, bond angle 180° (e.g. C₂H₂, a C≡C triple bond).

Key idea: A σ-bond forms by head-on overlap and allows rotation; a π-bond forms by sideways overlap and restricts rotation, giving rise to geometrical isomerism.

2. Structural Representation of Organic Compounds

The same molecule can be drawn in three standard ways. Knowing all three is a common 1-mark trap in exams.

• Complete (Lewis) structural formula: every bond shown - e.g. ethane drawn with all 7 bonds.
• Condensed formula: bonds omitted to save space - CH₃CH₃ for ethane, (CH₃)₂CHCH₃ for isobutane.
• Bond-line (zig-zag) formula: carbons at the ends and bends of lines, hydrogens on carbon not shown - used widely for rings and long chains.

[DIAGRAM: Butane shown three ways - full Lewis structure, condensed CH₃CH₂CH₂CH₃, and a zig-zag bond-line of three connected line segments.]

3. Classification of Organic Compounds

Organic compounds are first split into acyclic (open-chain) and cyclic (closed-chain) families.

• Acyclic / aliphatic: straight or branched chains (e.g. propane, isobutane).
• Alicyclic: carbon rings that are not aromatic (e.g. cyclohexane).
• Aromatic: contain benzene-like rings obeying Hückel’s rule of (4n+2) π-electrons (e.g. benzene, naphthalene).
• Heterocyclic: rings containing an atom other than carbon - N, O or S (e.g. pyridine, furan).

Functional Group and Homologous Series

A functional group is the atom or group of atoms that decides the chemical properties of a compound (e.g. –OH in alcohols, –COOH in acids). A homologous series is a family of compounds with the same functional group and general formula, where each member differs from the next by a –CH₂– unit and 14 u of mass (e.g. the alkanes CₙH₂ₙ₊₂).

4. IUPAC Nomenclature

The IUPAC name is built as Prefix + Word Root + Primary Suffix + Secondary Suffix. The word root tells the number of carbons in the longest chain (meth-, eth-, prop-, but-, pent-…); the primary suffix shows saturation (–ane, –ene, –yne); the secondary suffix names the functional group.

Rules for Naming

• Select the longest continuous carbon chain as the parent.
• Number the chain from the end that gives the lowest locants to the functional group, then to substituents (lowest-set rule).
• Name substituents alphabetically; use di, tri, tetra for repeats (these prefixes are ignored while alphabetising).
• The functional group gets the lowest possible number, taking priority over substituents.

Example: CH₃–CH(CH₃)–CH₂–OH is named 2-methylpropan-1-ol.

Priority Order of Functional Groups (highest first)

–COOH > –SO₃H > –COOR (ester) > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂.

5. Isomerism

Isomers are compounds with the same molecular formula but different structures or arrangements. Isomerism is split into two main branches.

Structural (Constitutional) Isomerism

• Chain isomerism: different carbon skeletons (n-butane vs isobutane).
• Position isomerism: functional group at different positions (propan-1-ol vs propan-2-ol).
• Functional isomerism: different functional groups (ethanol vs dimethyl ether, both C₂H₆O).
• Metamerism: different alkyl groups on either side of a functional group (diethyl ether vs methyl propyl ether).
• Tautomerism: dynamic interconversion of keto and enol forms.

Stereoisomerism

Same connectivity but different spatial arrangement - geometrical (cis–trans) isomerism due to restricted rotation about a C=C bond, and optical isomerism due to a chiral (asymmetric) carbon.

6. Fission of Covalent Bonds

A covalent bond can break in two ways, and which one occurs decides the whole mechanism.

• Homolytic fission: the bond breaks evenly, each atom keeping one electron, producing neutral free radicals (shown with single-barbed half-arrows). Favoured by heat or light in non-polar solvents.
• Heterolytic fission: the bond breaks unevenly, the more electronegative atom taking both electrons, producing a carbocation (C⁺) and an anion, or a carbanion (C⁻). Favoured in polar solvents.

Stability order: carbocations 3° > 2° > 1° > CH₃⁺; carbanions reverse this; free radicals 3° > 2° > 1°.

7. Nucleophiles and Electrophiles

Reagents attacking a substrate are of two kinds - and recognising them is a high-yield exam skill.

• Nucleophile (Nu⁻): “nucleus-loving”, electron-rich, donates an electron pair. Examples: OH⁻, CN⁻, NH₃, H₂O.
• Electrophile (E⁺): “electron-loving”, electron-deficient, accepts an electron pair. Examples: H⁺, NO₂⁺, carbocations, AlCl₃.

Electron movement is shown with curved arrows: a full curved arrow shows the shift of an electron pair, while a half-headed (fish-hook) arrow shows the shift of a single electron.

8. Electronic Displacement Effects

These four effects explain why molecules react where and how they do. They are the heart of every organic mechanism question.

(a) Inductive Effect (I)

A permanent shift of σ-bond electrons along a chain due to an electronegativity difference. It is transmitted through bonds and weakens after three carbons.

• –I groups (electron-withdrawing): –NO₂, –CN, –COOH, halogens.
• +I groups (electron-donating): alkyl groups (–CH₃, –C₂H₅).

(b) Resonance / Mesomeric Effect (M)

Delocalisation of π or lone-pair electrons across a conjugated system, shown by canonical (resonance) structures connected by a double-headed arrow ↔. The real molecule is a resonance hybrid, more stable than any single structure (lower energy = resonance energy).

• +M / +R groups push electron density into the system: –OH, –NH₂, –OR, halogens.
• –M / –R groups withdraw electron density: –NO₂, –CHO, –COOH, –CN.

(c) Electromeric Effect (E)

A temporary, complete transfer of a π-electron pair that occurs only in the presence of an attacking reagent and reverses once the reagent is removed.

(d) Hyperconjugation

Delocalisation of σ(C–H) electrons of an alkyl group into an adjacent empty p-orbital or π-bond - sometimes called “no-bond resonance”. More α-hydrogens means more hyperconjugation, which is why carbocation and alkene stability rise with more alkyl groups.

9. Types of Organic Reactions

Organic reactions fall into four broad classes - knowing one example of each is worth easy marks.

Reactions are further classed by the attacking reagent as nucleophilic, electrophilic, or free-radical.

10. Methods of Purification of Organic Compounds

An organic compound must be pure before its structure can be studied. The method chosen depends on the nature and impurities of the compound.

Chromatography has two main types: adsorption (column, TLC - based on differing adsorption on silica/alumina) and partition (paper - based on differing solubility between two phases). Purity is finally confirmed by a sharp, fixed melting or boiling point.

11. Qualitative Analysis

Qualitative analysis detects which elements are present besides C, H and O.

• Carbon and hydrogen: heating the compound with dry CuO turns C into CO₂ (turns lime water milky) and H into H₂O (turns anhydrous CuSO₄ blue).
• Nitrogen, sulphur, halogens: detected by Lassaigne’s test - the compound is fused with sodium to convert covalent N, S, X into ionic NaCN, Na₂S, NaX, which are then identified by colour tests (Prussian blue for N, black PbS for S, AgX precipitate for halogen).

12. Quantitative Analysis

Quantitative analysis finds how much of each element is present, giving the percentage composition and hence the empirical formula.

• Carbon and hydrogen (Liebig’s method): mass of CO₂ gives % C; mass of H₂O gives % H. %C = (12/44) × (mass CO₂/mass compound) × 100.
• Nitrogen: Dumas method (measures N₂ gas volume) or Kjeldahl’s method (converts N to (NH₄)₂SO₄, then NH₃ is titrated).
• Halogens (Carius method): compound is heated with fuming HNO₃ and AgNO₃; the AgX precipitate is weighed.
• Sulphur (Carius): oxidised to H₂SO₄ and precipitated as BaSO₄.

From the percentages we get the empirical formula (simplest whole-number ratio of atoms), and using the molar mass, the molecular formula (n × empirical formula).