Organic Chemistry - Some Basic Principles and Techniques — Chemistry Class 11 Notes (CBSE & HBSE)
Free NCERT Chemistry notes for Organic Chemistry - Some Basic Principles and Techniques (Class 11) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Organic Chemistry - Some Basic Principles and Techniques (CBSE & HBSE)
This foundational chapter introduces the language and tools of organic chemistry: the tetravalence of carbon and its hybrid orbitals, systematic IUPAC nomenclature, isomerism, the electronic effects (inductive, resonance, hyperconjugation, electromeric) that explain reactivity, the three reactive intermediates, the classification of organic reactions, and the experimental techniques of purification. Both CBSE and HBSE examine nomenclature, electronic effects (especially stability ordering of intermediates), and purification techniques heavily.
Tetravalence of Carbon, Hybridisation and IUPAC Nomenclature
Tetravalence and Hybridisation
Carbon has the configuration 1s\u00b2 2s\u00b2 2p\u00b2. By promoting one 2s electron to the empty 2p orbital, carbon achieves four unpaired electrons and forms four covalent bonds — its tetravalence.
| Hybridisation | s-character | Geometry | Bond angle | Example |
|---|---|---|---|---|
| sp\u00b3 | 25% | tetrahedral | 109.5\u00b0 | CH\u2084 (single bonds) |
| sp\u00b2 | 33% | trigonal planar | 120\u00b0 | C\u2082H\u2084 (one double bond) |
| sp | 50% | linear | 180\u00b0 | C\u2082H\u2082 (triple bond) |
A sigma (\u03c3) bond forms by head-on overlap; a pi (\u03c0) bond forms by sidewise overlap of unhybridised p orbitals. A double bond = 1\u03c3 + 1\u03c0; a triple bond = 1\u03c3 + 2\u03c0. Greater s-character means a shorter, stronger bond and higher electronegativity of that carbon.
IUPAC Nomenclature
The IUPAC name is built as: prefix(es) + word root + primary suffix + secondary suffix.
- Select the longest continuous chain containing the principal functional group (parent chain).
- Number from the end giving the lowest locant to the principal characteristic group.
- Cite substituents alphabetically as prefixes with locants.
| Group | Prefix | Suffix |
|---|---|---|
| \u2013COOH | carboxy | -oic acid |
| \u2013CHO | oxo / formyl | -al |
| >C=O | oxo | -one |
| \u2013OH | hydroxy | -ol |
| \u2013NH\u2082 | amino | -amine |
Tip: Seniority of functional groups for choosing the suffix: \u2013COOH > \u2013SO\u2083H > esters > acid halides > amides > nitriles > \u2013CHO > >C=O > \u2013OH > \u2013NH\u2082. Only the most senior group becomes the suffix; the rest are prefixes.
Isomerism and Electron Displacement Effects
Isomerism
Isomers have the same molecular formula but different structures or arrangements.
- Structural (constitutional) isomerism: chain, position, functional group, and metamerism.
- Stereoisomerism: same connectivity, different spatial arrangement — geometrical (cis-trans) and optical (enantiomers).
Electron Displacement Effects
1. Inductive effect (I): permanent polarisation of a \u03c3 bond due to electronegativity difference; transmitted through the chain and weakening with distance.
- \u2013I groups (electron-withdrawing): \u2013NO\u2082, \u2013CN, \u2013COOH, halogens.
- +I groups (electron-releasing): alkyl groups (\u2013CH\u2083, \u2013C\u2082H\u2085).
2. Resonance (mesomeric) effect (M/R): delocalisation of \u03c0 or lone-pair electrons through conjugation, shown by resonance structures. \u2013M groups (\u2013NO\u2082, \u2013C=O) withdraw; +M groups (\u2013OH, \u2013NH\u2082, \u2013halogen) donate.
Tip: Resonance structures are not real molecules in equilibrium — the true structure is a single resonance hybrid. Stating that the molecule 'oscillates' between forms is a common error penalised in CBSE.
3. Hyperconjugation: delocalisation of \u03c3(C\u2013H) electrons into an adjacent empty p orbital or \u03c0 system (the 'no-bond resonance'). More \u03b1-hydrogens \u2192 greater stabilisation; explains the stability order of carbocations and alkenes.
4. Electromeric effect (E): a temporary, complete shift of a \u03c0-electron pair to one atom in the presence of an attacking reagent; reversible and exists only while the reagent is near.
Reaction Intermediates, Reaction Types and Purification Techniques
Reactive Intermediates
| Intermediate | Carbon electrons | Charge | Geometry | Stability order |
|---|---|---|---|---|
| Carbocation | 6 (sextet) | + | sp\u00b2, planar | 3\u00b0 > 2\u00b0 > 1\u00b0 > CH\u2083\u207a |
| Carbanion | 8 (lone pair) | \u2212 | sp\u00b3, pyramidal | CH\u2083\u207b > 1\u00b0 > 2\u00b0 > 3\u00b0 |
| Free radical | 7 (odd e\u207b) | neutral | sp\u00b2/planar | 3\u00b0 > 2\u00b0 > 1\u00b0 |
Note that carbanion stability runs opposite to carbocations: electron-donating alkyls destabilise the already electron-rich carbanion.
Bond Fission and Reagents
- Homolytic fission gives free radicals (each atom keeps one electron); favoured by heat/UV in non-polar media.
- Heterolytic fission gives ions (one atom keeps both electrons); favoured in polar media.
- Electrophiles are electron-pair acceptors (E\u207a); nucleophiles are electron-pair donors (Nu:\u207b).
Types of Organic Reactions
Substitution, addition, elimination, and rearrangement.
Methods of Purification
| Technique | Principle | Used for |
|---|---|---|
| Crystallisation | difference in solubility with temperature | solids with soluble impurities |
| Simple distillation | difference in boiling points (large gap) | liquid + non-volatile impurity |
| Fractional distillation | small b.p. differences | close-boiling liquid mixtures |
| Steam distillation | steam-volatile, water-immiscible | e.g. aniline |
| Differential extraction | partition between immiscible solvents | organic compound from aqueous solution |
| Chromatography | differential adsorption/partition | separation & purification of mixtures |
Tip: In chromatography, adsorption chromatography (column, TLC) separates by differing adsorption on a stationary phase, while partition chromatography (paper) separates by differing partition between two liquid phases. R\u1da0 = distance moved by component / distance moved by solvent front.
Frequently asked questions
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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Organic Chemistry - Some Basic Principles and Techniques.