CLASS 11 CHEMISTRY
UNIT 8 — ORGANIC CHEMISTRY
Some Basic Principles and Techniques
Conceptual • NCERT-Focused • NEET-Oriented
| THE CHAPTER IN ONE IDEA Organic chemistry becomes manageable when you connect structure → nomenclature → isomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements. → electron movement → reaction mechanism → purification → analysis. |
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STRUCTURE → carbon tetravalence + hybridisationhybridisationMixing of atomic orbitals to form equivalent hybrid orbitals with defined shapes. + σ/π bonding
REPRESENTATION → Lewis/dash + condensed + bond-line + 3-D wedge/dash
CLASSIFICATION → open chain / cyclic / aromatic + functional groups
NAMING → IUPAC parent + principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. + numbering + substituents
ISOMERISMisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements. → same molecular formula, different arrangement / spatial relationship
MECHANISM → bond fission + nucleophilenucleophileAn electron-rich species that donates an electron pair to form a bond./electrophileelectrophileAn electron-deficient species that accepts an electron pair. + electron displacement
PURIFICATION → sublimation / crystallisation / distillation / extraction / chromatographychromatographyA separation technique based on different affinities for stationary and mobile phases.
ANALYSIS → qualitative tests + quantitative estimation
8.1 General Introduction
Organic chemistry studies compounds of carbon. NCERT begins this unit by connecting organic chemistry to carbon's unique ability to form covalent bonds with itself (catenationcatenationThe ability of carbon atoms to bond with one another and form chains or rings.) and with elements such as H, O, N, S, P and halogens. Organic compounds occur in biological molecules and in materials such as fuels, polymers, dyes, clothing and medicines.
| HISTORICAL LOCK Wöhler's 1828 synthesis of urea from ammonium cyanate helped reject the 'vital force' idea. Kolbe's acetic-acid synthesis (1845) and Berthelot's methane synthesis (1856) reinforced laboratory synthesis from inorganic sources. |
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8.2 Tetravalence of Carbon: Shapes of Organic Compounds
8.2.1 Shapes of Carbon Compounds
Carbon tetravalence and molecular shapes are explained through its electronic configuration and hybridisationhybridisationMixing of atomic orbitals to form equivalent hybrid orbitals with defined shapes. of s and p orbitals. NCERT focuses on sp³, sp² and sp hybridisationhybridisationMixing of atomic orbitals to form equivalent hybrid orbitals with defined shapes. using methane, ethene and ethyne.
| HybridisationhybridisationMixing of atomic orbitals to form equivalent hybrid orbitals with defined shapes. | s-character | Typical example | Shape | Key bonding consequence |
|---|---|---|---|---|
| sp³ | 25% | CH₄ | Tetrahedral | σ bonds; longest/weakest among the three C hybrid orbitals |
| sp² | 33.3% | C₂H₄ | Trigonal planar | One unhybridised p orbital remains for π bonding |
| sp | 50% | C₂H₂ | Linear | Two unhybridised p orbitals remain for two π bonds |
| HYBRIDISATIONhybridisationMixing of atomic orbitals to form equivalent hybrid orbitals with defined shapes. TREND More s-character → orbital closer to nucleus → shorter and stronger bond. Carbon electronegativity also increases with increasing s-character: sp > sp² > sp³. |
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8.2.2 Characteristic Features of π Bonds
π bonding requires parallel orientation and sideways overlap of adjacent p orbitals.
In ethene, the p orbitals are perpendicular to the molecular plane and mutually parallel for overlap.
Rotation about a C=C bond is restricted because it disrupts maximum p–p overlap.
π-electron density lies above and below the plane of the bonding atoms.
π bonds are relatively accessible to attacking reagents and are important reactive centres in multiple-bond compounds.
| NEET LOCK Single bond = 1σ. Double bond = 1σ + 1π. Triple bond = 1σ + 2π. |
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8.3 Structural Representations of Organic Compounds
8.3.1 Complete, Condensed and Bond-Line Structural Formulae
| Representation | What is shown | NCERT-style use |
|---|---|---|
| Lewis / dot | Valence electrons and bonding pairs | Detailed electron bookkeeping |
| Dash structure | A dash represents a two-electron covalent bond | Single/double/triple bonds shown by one/two/three dashes |
| Complete structural formula | Atoms and bonds are explicitly displayed | Useful for seeing every H and bond |
| Condensed structural formula | Repeated groups are compressed | e.g. CH₃(CH₂)₆CH₃ |
| Bond-line formula | Carbon and attached H are omitted; carbon skeleton is drawn as lines | Vertices and line ends represent carbon atoms; heteroatoms are written |
| BOND-LINE RULE Every line junction and line end represents a carbon unless another atom is explicitly written. Hydrogens attached to carbon are omitted and inferred from carbon valency. |
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8.3.2 Three-Dimensional Representation
| Symbol | Meaning |
|---|---|
| Solid wedge | Bond projects out of the plane toward the observer |
| Dashed wedge | Bond projects behind the plane, away from the observer |
| Normal line | Bond lies in the plane of the paper |
Molecular Models
| Model | What it emphasises |
|---|---|
| Framework model | Only bonds connecting atoms; emphasises bonding pattern |
| Ball-and-stick / ball-and-spring | Atoms and bonds; useful for visualising connectivity and multiple bonds |
| Space-filling model | Relative atomic size based on van der Waals radii; shows occupied molecular volume |
8.4 Classification of Organic Compounds
| Main class | Sub-class | Examples / idea |
|---|---|---|
| Acyclic / open chain / aliphatic | Straight or branched chain | Ethane, isobutane, acetaldehyde, acetic acid |
| Cyclic / closed chain / ring | Alicyclic | Cyclopropane, cyclohexane, cyclohexene |
| Cyclic | Heterocyclic | Tetrahydrofuran; ring contains atoms other than carbon |
| Aromatic | Benzenoid | Benzene, aniline, naphthalene |
| Aromatic | Non-benzenoid | Tropone |
| Aromatic | Heterocyclic aromatic | Furan, thiophene, pyridine |
8.4.1 Functional Groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions.
A functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. is an atom or group of atoms attached to a carbon chain in a characteristic way and responsible for characteristic chemical properties. Compounds containing the same functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. generally undergo similar reactions.
| Functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. / class | Prefix when subordinate | Suffix when principal | Example |
|---|---|---|---|
| –COOH carboxylic acid | carboxy- | –oic acid | CH₃COOH |
| –SO₃H sulphonic acid | sulfo- | –sulphonic acid | RSO₃H |
| –COOR ester | alkoxycarbonyl- | –oate | CH₃COOCH₃ |
| –COCl acid chloride | chlorocarbonyl- | –oyl chloride | CH₃COCl |
| –CONH₂ amide | carbamoyl- | –amide | CH₃CONH₂ |
| –CN nitrile | cyano- | –nitrile | CH₃CN |
| –CHO aldehyde | formyl- | –al | CH₃CHO |
| >C=O ketone | oxo- | –one | CH₃COCH₃ |
| –OH alcohol | hydroxy- | –ol | CH₃CH₂OH |
| –NH₂ amine | amino- | –amine | CH₃NH₂ |
| C=C alkene | — | –ene | CH₂=CH₂ |
| C≡C alkyne | — | –yne | HC≡CH |
| –X halo | fluoro-/chloro-/bromo-/iodo- | — | CH₃Cl |
| –NO₂ nitro | nitro- | — | CH₃NO₂ |
| –OR ether | alkoxy- | — | CH₃OCH₃ |
8.4.2 Homologous Serieshomologous seriesA family of compounds with the same functional group and general formula; successive members differ by CH₂.
A homologous serieshomologous seriesA family of compounds with the same functional group and general formula; successive members differ by CH₂. is a family of compounds containing the same characteristic functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions..
Successive members differ in molecular formula by a –CH₂– unit.
Members can be represented by a general molecular formula.
Examples include alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids and amines.
A compound containing two or more identical or different functional groups is a polyfunctional compound.
8.5 Nomenclature of Organic Compounds
IUPAC nomenclature links a systematic name to structure so that the structure can be deduced from the name. Trivial/common names are still encountered and can be important, but systematic naming provides a general framework.
| CORE NAMING ALGORITHM Identify parent → identify principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. → choose suffix → number for lowest appropriate locants → name substituents/prefixes alphabetically → assemble the name. |
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8.5.1 IUPAC System — Parent, Prefix, Suffix
| Part | Meaning |
|---|---|
| Parent | Main carbon skeleton / parent hydrocarbon |
| Prefix | Substituents or subordinate functional groups |
| Word root | Number of carbon atoms in parent |
| Primary suffix | Saturation / multiple bond: –ane, –ene, –yne |
| Secondary suffix | Principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions.: –ol, –al, –one, –oic acid, etc. |
8.5.2 IUPAC Nomenclature of Alkanes
Straight-chain Alkanes
| C atoms | IUPAC root / name |
|---|---|
| 1 | methane |
| 2 | ethane |
| 3 | propane |
| 4 | butane |
| 5 | pentane |
| 6 | hexane |
| 7 | heptane |
| 8 | octane |
| 9 | nonane |
| 10 | decane |
| 11 | undecane |
| 12 | dodecane |
Alkanes are saturated hydrocarbons containing only C–C single bonds. The homologous members differ by CH₂.
Alkyl Groups
| Parent alkane | Alkyl group | Common name |
|---|---|---|
| methane | –CH₃ | methyl |
| ethane | –C₂H₅ | ethyl |
| propane | –C₃H₇ | propyl / isopropyl |
| butane | –C₄H₉ | n-butyl / sec-butyl / isobutyl / tert-butyl |
| neopentane | –CH₂C(CH₃)₃ | neopentyl |
| ALKYL RULE An alkyl group is obtained by removing H from an alkane. Replace –ane by –yl: methane → methyl, ethane → ethyl. |
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Branched-Chain Alkane Rules
Select the longest continuous carbon chain as the parent chain.
Number the parent chain from the end that gives substituents the lowest possible locants.
Name each alkyl substituent and write its position.
Different substituents are written in alphabetical order.
For identical substituents use di-, tri-, tetra-, penta-, etc.; separate locant numbers by commas.
When two chains of equal size compete, choose the chain with the greater number of side chains.
If two numbering directions are possible, use the lowest locant set; when equivalent, the substituent cited first alphabetically gets the lower number as shown by NCERT examples.
| PUNCTUATION LOCK Numbers are separated by commas; numbers and words by hyphens; there is no break between a substituent name and the parent name. |
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Cyclic Compounds
A saturated monocyclic compound is named by adding the prefix cyclo- to the corresponding alkane.
If side chains are present, the same lowest-locant logic is applied.
8.5.3 Compounds Containing Functional Groups
First identify the functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions.. It determines the appropriate suffix and the parent chain must contain it. Number the longest chain so the principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. receives the lowest possible number.
| Priority order (decreasing, selected NCERT groups) | Principal suffix / treatment |
|---|---|
| –COOH | –oic acid |
| –SO₃H | sulphonic acid |
| –COOR | –oate |
| –COCl | –oyl chloride |
| –CONH₂ | –amide |
| –CN | –nitrile |
| –CHO | –al |
| >C=O | –one |
| –OH | –ol |
| –NH₂ | –amine |
| C=C | –ene |
| C≡C | –yne |
| PRINCIPAL GROUP RULE In a polyfunctional compound, the highest-priority group becomes the principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. and receives the suffix. Other groups are named as prefixes (e.g. hydroxy-, oxo-, amino-, halo-, nitro-, alkoxy-). |
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NCERT examples:
HO–CH₂–CH₂–CH₂–CH₂–CH₂–CH(CH₃)–CH₃ → 6-methyloctan-3-ol
A chain with ketone groups at C-2 and C-4 → hexane-2,4-dione
A carboxylic acid with a ketone at C-5 → 5-oxohexanoic acid
A six-carbon chain with C=C at 1,3 and C≡C at 5 → hexa-1,3-dien-5-yne
| MULTIPLE SAME GROUPS Use di-, tri-, etc. before the class suffix: ethane-1,2-diol; hexane-2,4-dione. For multiple double/triple bonds, the final –e of the parent alkane is dropped before –diene/–diyne etc. |
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8.5.4 Nomenclature of Substituted Benzene Compounds
For a monosubstituted benzene, the substituent is written as a prefix to benzene: chlorobenzene, nitrobenzene, methylbenzene.
For disubstituted benzene, number the ring to give the lowest locants.
In traditional nomenclature, ortho (o-) = 1,2; meta (m-) = 1,3; para (p-) = 1,4.
For tri- or higher substituted benzene derivatives, use numerical locants rather than o-/m-/p-.
When benzene is attached to an alkane containing the principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions., benzene may be treated as a phenyl substituent, C₆H₅–.
| Common name | Systematic / accepted example |
|---|---|
| Toluene | methylbenzene |
| Anisole | methoxybenzene |
| Aniline | aminobenzene |
| Phenyl | C₆H₅– |
| o-dibromobenzene | 1,2-dibromobenzene |
| m-dibromobenzene | 1,3-dibromobenzene |
| p-dibromobenzene | 1,4-dibromobenzene |
8.6 IsomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements.
IsomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements. is the phenomenon in which two or more compounds have the same molecular formula but different properties. Such compounds are isomers.
| IsomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements. | Subtype | Defining difference |
|---|---|---|
| Structural isomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements. | Chain | Different carbon skeleton |
| Structural isomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements. | Position | Different position of substituent / functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. |
| Structural isomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements. | Functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. | Different functional groups |
| Structural isomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements. | Metamerism | Different alkyl groups on either side of the same polyvalent functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. |
| Stereoisomerism | Geometrical | Same connectivity; different spatial arrangement |
| Stereoisomerism | Optical | Same connectivity; non-superimposable spatial arrangements / mirror-image relationship |
8.6.1 Structural IsomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements.
Chain IsomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements.
Same molecular formula but different carbon skeletons. NCERT uses C₅H₁₂ as a classic example: pentane, isopentane (2-methylbutane) and neopentane (2,2-dimethylpropane).
Position IsomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements.
Same molecular formula and carbon skeleton, but the position of a substituent or functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. differs. Example: propan-1-ol and propan-2-ol.
Functional Groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. IsomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements.
Same molecular formula but different functional groups. Example: C₃H₆O can represent an aldehyde or ketone.
Metamerism
Different alkyl groups occur on either side of the same functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions.. NCERT example: C₄H₁₀O as methoxypropane and ethoxyethane.
8.6.2 Stereoisomerism
Stereoisomers have the same constitution and sequence of covalent bonds but differ in relative spatial positions of atoms/groups. NCERT divides stereoisomerism into geometrical and optical isomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements..
8.7 Fundamental Concepts in Organic Reaction Mechanism
An organic reaction can be viewed as a sequence in which a substrate reacts with an attacking reagent, passes through one or more intermediates, and gives product(s). A reaction mechanism describes the sequence of bond breaking/forming, electron movement, energetics and rates.
SUBSTRATE + ATTACKING REAGENT → [INTERMEDIATE(S)] → PRODUCT(S) + BYPRODUCT(S)
| SUBSTRATE vs REAGENT The molecule whose carbon participates in new bond formation is called the substrate; the other reactant is the reagent. If both reactants supply carbon to the new bond, the choice can be arbitrary and depends on the molecule being discussed. |
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8.7.1 Fission of a Covalent Bond
| Fission | Electron movement | Intermediate / species | Reaction type |
|---|---|---|---|
| Heterolytic cleavage | Shared electron pair goes to one fragment | CarbocationcarbocationAn organic ion in which a carbon atom carries a positive charge. + carbanioncarbanionAn organic ion in which a carbon atom carries a negative charge. / charged species | Ionic / heteropolar / polar |
| Homolytic cleavage | One electron goes to each atom | Free radicals | Free-radical / homopolar / nonpolar |
Carbocations
A carbocationcarbocationAn organic ion in which a carbon atom carries a positive charge. has a positively charged carbon with a sextet of electrons. It is classified as primary, secondary or tertiary according to the number of carbons directly attached to the positively charged carbon.
CarbocationcarbocationAn organic ion in which a carbon atom carries a positive charge. stability: methyl < primary < secondary < tertiary
Shape: positively charged carbon is generally sp² hybridised and trigonal planar.
Carbanions
A carbanioncarbanionAn organic ion in which a carbon atom carries a negative charge. is a carbon species carrying a negative charge. NCERT describes carbon in a carbanioncarbanionAn organic ion in which a carbon atom carries a negative charge. as generally sp³ hybridised with a distorted tetrahedral structure. Carbanions are reactive and unstable species.
Free Radicals
Homolytic cleavage produces neutral species containing an unpaired electron. Alkyl radicals are classified as primary, secondary and tertiary; NCERT gives the stability order:
methyl < primary < secondary < tertiary
| ARROW LOCK Curved full-headed arrow = movement of an electron pair. Fish-hook / half-headed curved arrow = movement of a single electron. |
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8.7.2 Substrate and Reagent; Nucleophiles and Electrophiles
| Species | Electron character | What it does | Examples from NCERT |
|---|---|---|---|
| NucleophilenucleophileAn electron-rich species that donates an electron pair to form a bond. | Electron-rich | Donates an electron pair to an electron-deficient centre | OH⁻, CN⁻, carbanions, neutral lone-pair donors |
| ElectrophileelectrophileAn electron-deficient species that accepts an electron pair. | Electron-deficient | Accepts an electron pair from an electron-rich centre | H⁺/carbocations, carbonyl carbon, alkyl-halide carbon |
| CENTRE RULE NucleophilenucleophileAn electron-rich species that donates an electron pair to form a bond. attacks an electrophilic centre (electron-deficient). ElectrophileelectrophileAn electron-deficient species that accepts an electron pair. attacks a nucleophilic centre (electron-rich). |
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In an alkyl halide, the C–X bond is polar and carbon carries partial positive character, making that carbon an electrophilic centre.
8.7.3 Electron Movement in Organic Reactions
Curved-arrow notation shows redistribution of electrons during a reaction.
For an electron pair, start the arrow at the electron source and point it to the destination.
For a single electron, use a fish-hook arrow.
Electron movement helps explain how bonds break, form and rearrange.
8.7.4 Electron Displacement Effects in Covalent Bonds
| Effect | Nature | When it operates |
|---|---|---|
| Inductive effect | Permanent polarisation transmitted through σ bonds | Ground-state electronic effect of electronegativity/substituents |
| ResonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. / mesomeric effect | Permanent delocalisation in a conjugated system | π bond ↔ π bond / lone pair interaction |
| Electromeric effect | Temporary complete shift of π electrons | Only in presence of an attacking reagent; disappears when reagent is removed |
| Hyperconjugation | Permanent delocalisation of σ(C–H) electrons into adjacent unsaturated/empty p orbital | Alkyl group directly attached to unsaturated system or carbocationcarbocationAn organic ion in which a carbon atom carries a positive charge. |
8.7.5 Inductive Effect
When atoms of different electronegativity form a covalent bond, electron density shifts toward the more electronegative atom. The resulting polarisation can be transmitted through adjacent σ bonds.
CH₃–CH₂–Cl → C–Cl polarisation → δ+ character on C-1
C-1 draws electron density from the adjacent C–C bond → smaller δ+ character at C-2
Effect continues through σ bonds but decreases rapidly and becomes very small after about three bonds.
| Group type | NCERT examples |
|---|---|
| Electron-withdrawing (−I) | Halogens, –NO₂, –CN, –COOH, –COOR, –OAr |
| Electron-donating (+I) | Alkyl groups such as –CH₃ and –CH₂CH₃ |
| INDUCTIVE ACIDITY LINK Electron-withdrawing groups stabilise negative charge on a conjugate base and can increase acidity; the effect weakens as the group is moved farther away. |
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8.7.6 ResonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. Structure
Some molecules cannot be adequately represented by one Lewis structure. Benzene is NCERT's key example: the actual C–C bond distances are equal and intermediate between a C–C single and C=C double bond.
| ResonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. principle | Rule |
|---|---|
| Canonical structures | Hypothetical contributing structures; they individually do not represent the real molecule |
| Actual structure | ResonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. hybrid |
| Nuclei | Remain in the same positions in all canonical forms |
| Unpaired electrons | Same number in each canonical form |
| Stability | More covalent bonds, complete octets, less charge separation and appropriate charge placement increase contributor stability |
| Energy | ResonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. hybrid is lower in energy than any individual canonical structure |
| RESONANCEresonanceRepresentation of electron delocalisation using two or more contributing structures. ENERGY The energy difference between the actual resonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. hybrid and the lowest-energy canonical structure is called resonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. stabilisation energy (resonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. energy). |
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8.7.7 ResonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. / Mesomeric Effect
NCERT defines resonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. effect as polarity produced by interaction of two π bonds or a π bond with a lone pair on an adjacent atom, transmitted through the conjugated system.
| Effect | Direction of electron displacement | NCERT examples |
|---|---|---|
| +R / +M | Electron density is transferred away from the substituent toward the conjugated system | –OH, –OR, –NH₂ and related lone-pair donating groups; halogens are listed as +R in NCERT |
| −R / −M | Electron density is transferred toward the substituent/group | –COOH, –CHO, >C=O, –CN, –NO₂ |
| CONJUGATED SYSTEM Alternating single and double bonds in an open-chain or cyclic system form a conjugated system. π electrons become delocalised and the system develops characteristic polarity. |
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8.7.8 Electromeric Effect (E effect)
It is temporary and occurs only in compounds containing a multiple bond when an attacking reagent approaches.
It is the complete transfer of a shared pair of π electrons to one atom of the multiple bond on the demand of the attacking reagent.
It is annulled when the attacking reagent is removed.
+E effect: π electrons shift toward the atom to which the attacking reagent becomes attached.
−E effect: π electrons shift toward the atom to which the attacking reagent does not become attached.
| EFFECT DIRECTION Inductive and electromeric effects can oppose each other; NCERT notes that when they operate in opposite directions, the electromeric effect predominates. |
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8.7.9 Hyperconjugation
Hyperconjugation is a stabilising interaction involving delocalisation of σ(C–H) electrons of an alkyl group directly attached to an unsaturated system or to an atom with an unshared/empty p orbital.
Adjacent σ(C–H) bond aligns with an empty p orbital or π system
↓
σ electrons become delocalised
↓
Positive charge/electron deficiency is dispersed
↓
Species becomes more stable
| CARBOCATIONcarbocationAn organic ion in which a carbon atom carries a positive charge. LOCK More alkyl groups attached to the positively charged carbon generally mean more hyperconjugative interaction and greater carbocationcarbocationAn organic ion in which a carbon atom carries a positive charge. stability. |
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NCERT's stability order remains:
CH₃⁺ < primary < secondary < tertiary
| WHY METHYL IS LEAST STABLE In methyl cation, the vacant p orbital is perpendicular to the plane containing the C–H σ bonds, so the required overlap for hyperconjugation is absent. |
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8.7.10 Types of Organic Reactions and Mechanisms
| Reaction type | General idea |
|---|---|
| Substitution | One atom/group is replaced by another |
| Addition | Atoms/groups add across a multiple bond |
| Elimination | Atoms/groups are removed to create a multiple bond |
| Rearrangement | Internal reorganisation of the carbon/electron framework |
8.8 Methods of Purification of Organic Compounds
Purification method depends on the nature of the compound and the impurities present. NCERT covers sublimation, crystallisation, distillation, differential extraction and chromatographychromatographyA separation technique based on different affinities for stationary and mobile phases.. Purity can also be checked through sharp melting or boiling points.
| Method | Principle / best use |
|---|---|
| Sublimation | Sublimable solid separates from non-sublimable impurities |
| Crystallisation | Difference in solubility at different temperatures |
| Distillation | Difference in volatility / boiling points |
| Differential extraction | Difference in solubility between immiscible solvents |
| ChromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. | Differential adsorption or partition between stationary and mobile phases |
8.8.1 Sublimation
On heating, some solids pass directly from solid to vapour without becoming liquid. Sublimation purifies sublimable compounds from non-sublimable impurities.
8.8.2 Crystallisation
Dissolve the impure solid in a solvent in which it is sparingly soluble at room temperature but appreciably soluble at higher temperature.
Concentrate the solution to obtain a nearly saturated solution.
Cool the solution so the pure compound crystallises out.
Remove crystals by filtration; the mother liquor retains impurities and some dissolved compound.
If needed, use a two-solvent system when the compound is highly soluble in one solvent and poorly soluble in another.
Activated charcoal can remove colour-imparting impurities.
| CRYSTALLISATION TRAP Repeated crystallisation may be necessary when impurities have solubilities comparable to that of the desired compound. |
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8.8.3 Distillation
| Type | When used | NCERT principle |
|---|---|---|
| Simple distillation | Volatile liquid from non-volatile impurities or liquids with sufficiently different boiling points | Lower-boiling component vaporises first, is condensed and collected |
| Fractional distillation | Liquids with close boiling points | Vapours pass through a fractionating column; repeated vaporisation/condensation enriches the more volatile component |
| Reduced-pressure distillation | Very high-boiling liquids or liquids that decompose at/below their normal boiling point | Lower external pressure lowers boiling temperature |
| Steam distillation | Steam-volatile, water-immiscible compounds | Total vapour pressure reaches atmospheric pressure at a temperature below the organic liquid's normal boiling point |
| FRACTIONAL DISTILLATION The fractionating column provides repeated heat exchange between rising vapour and descending condensate. Each successive vaporisation-condensation unit is called a theoretical plate. |
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| STEAM DISTILLATION A water-immiscible organic liquid can boil below its normal boiling point because the total vapour pressure is the sum of the water and organic vapour pressures. |
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8.8.4 Differential Extraction
If an organic compound is in an aqueous medium, shake it with an immiscible organic solvent in which the compound is more soluble.
Two layers form and are separated using a separating funnel.
The organic solvent is removed by distillation or evaporation to recover the compound.
If solubility in the organic solvent is low, continuous extraction can repeatedly use the same solvent.
8.8.5 ChromatographychromatographyA separation technique based on different affinities for stationary and mobile phases.
ChromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. separates mixture components because they distribute differently between a stationary phase and a mobile phase. NCERT divides the methods discussed here into adsorption and partition chromatographychromatographyA separation technique based on different affinities for stationary and mobile phases..
| ChromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. | Stationary phase / principle | Examples / key point |
|---|---|---|
| Adsorption | Different degrees of adsorption on an adsorbent | Silica gel and alumina; column chromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. and TLC |
| Partition | Continuous differential partition between stationary and mobile phases | Paper chromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. |
Column ChromatographychromatographyA separation technique based on different affinities for stationary and mobile phases.
A glass column is packed with an adsorbent stationary phase and fitted with a stopcock.
The mixture is placed on top; an appropriate liquid eluent flows slowly down.
Components travel at different rates according to their degree of adsorption.
More strongly adsorbed substances remain nearer the top; less strongly adsorbed components move farther down.
Thin Layer ChromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. (TLC)
A thin (~0.2 mm) layer of silica gel or alumina is coated on a glass plate.
The mixture is spotted about 2 cm above one end; the plate is developed in an eluent.
Components move different distances depending on adsorption.
Colourless compounds can be detected under UV light, in an iodine chamber, or with suitable spray reagents such as ninhydrin for amino acids.
Rf = distance moved by substance / distance moved by solvent front
Partition ChromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. / Paper ChromatographychromatographyA separation technique based on different affinities for stationary and mobile phases.
Paper chromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. uses special chromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. paper containing trapped water as stationary phase.
The solvent is the mobile phase and rises by capillary action.
Components partition differently between the two phases and appear at different heights.
The developed paper is the chromatogram.
| CHROMATOGRAPHYchromatographyA separation technique based on different affinities for stationary and mobile phases. USES Separation + identification + purification + purity testing. |
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8.9 Qualitative Analysis of Organic Compounds
Organic compounds contain carbon and hydrogen and may additionally contain O, N, S, halogens and P.
8.9.1 Detection of Carbon and Hydrogen
Heat the organic compound with copper(II) oxide. Carbon is oxidised to CO₂ and hydrogen to H₂O. CO₂ turns limewater milky; water turns anhydrous CuSO₄ blue.
C + 2CuO → CO₂ + 2Cu
H₂ + CuO → H₂O + Cu
CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O → limewater turns turbid
H₂O + anhydrous CuSO₄ → hydrated CuSO₄ → white → blue
8.9.2 Detection of Other Elements — Lassaigne's Test
Nitrogen, sulphur and halogens are converted from covalent form into ionic forms by fusing the organic compound with metallic sodium. The aqueous solution obtained is called sodium fusion extract.
| Element in organic compound | Ionic form in sodium fusion extract |
|---|---|
| Nitrogen | CN⁻ (sodium cyanide) |
| Sulphur | S²⁻ (sodium sulphide) |
| Halogen X | X⁻ (sodium halide) |
| N + S together | SCN⁻ may form sodium thiocyanate |
Nitrogen Test
Sodium fusion extract + FeSO₄ → ferrocyanide complex
Acidification / oxidationOxidationOxidation is the addition of oxygen or removal of hydrogen from a substance. Reduction is the removal of oxygen or addition of hydrogen to a substance. gives ferric ferrocyanide (Prussian blue)
Prussian blue confirms nitrogen
| PRUSSIAN BLUE LOCK The characteristic blue is associated with ferric ferrocyanide, Fe₄[Fe(CN)₆]₃. |
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Sulphur Test
| Test | Observation |
|---|---|
| Lead acetate after acidification with acetic acid | Black PbS precipitate → sulphur |
| Sodium nitroprusside | Violet colour → sulphur |
| N + S TOGETHER If both N and S are present, sodium thiocyanate can form and gives a blood-red colour with Fe³⁺. Excess sodium may decompose thiocyanate to cyanide and sulphide. |
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Halogen Test
Acidify sodium fusion extract with nitric acid, then add AgNO₃.
Cl⁻ → white AgCl, soluble in NH₄OH.
Br⁻ → yellowish AgBr, sparingly soluble in NH₄OH.
I⁻ → yellow AgI, insoluble in NH₄OH.
If N or S is present, nitric acid treatment is necessary to destroy CN⁻/S²⁻ that would otherwise interfere with the AgNO₃ test.
| WHY HNO₃ BEFORE AgNO₃? CN⁻ and S²⁻ from sodium fusion can react with Ag⁺ and interfere. Nitric acid destroys these interfering ions before the halide precipitation test. |
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Phosphorus Test
Oxidise phosphorus to phosphate using an oxidising agent such as sodium peroxide.
Acidify/boil with nitric acid and treat with ammonium molybdate.
Yellow colour/precipitate indicates phosphorus.
Alternatively, phosphoric acid can be precipitated with magnesia mixture and ignited to Mg₂P₂O₇.
8.10 Quantitative Analysis
Quantitative analysis determines the mass percentage of elements present in an organic compound. These percentages help determine empirical and molecular formulae.
8.10.1 Carbon and Hydrogen — Combustion Method
A known mass of compound is burnt in excess oxygen with CuO. Carbon becomes CO₂ and hydrogen becomes H₂O. Water is absorbed in weighed anhydrous CaCl₂ and CO₂ in weighed KOH solution.
| Quantity measured | Calculation |
|---|---|
| Mass of compound = m g; water = m₁ g | % H = (2 × 1 × m₁ × 100) / (18 × m) |
| Mass of compound = m g; CO₂ = m₂ g | % C = (12 × m₂ × 100) / (44 × m) |
| CARBON/HYDROGEN LOGIC The mass increase of the absorption tubes gives the mass of H₂O and CO₂ produced; stoichiometry converts those masses into H and C percentages. |
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8.10.2 Nitrogen
Dumas Method
Heat the nitrogen-containing compound with CuO in an atmosphere of CO₂.
Products include CO₂, H₂O and free N₂.
Nitrogen oxides, if formed, are reduced to N₂ by heated copper gauze.
CO₂ is absorbed by aqueous KOH; nitrogen volume is measured.
Correct collected N₂ volume to STP using the gas law.
At STP, 22,400 mL N₂ weighs 28 g.
% N = (28 × V_STP × 100) / (22400 × mass of compound in g)
| DUMAS PRESSURE CORRECTION For gas collected over water: dry-gas pressure = atmospheric pressure − aqueous vapour pressure. |
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Kjeldahl's Method
Heat the nitrogen-containing compound with concentrated H₂SO₄ so nitrogen is converted into ammonium sulphate.
Treat the resulting acid mixture with excess NaOH to liberate NH₃.
Absorb NH₃ in a known excess of standard H₂SO₄.
Determine the acid consumed by NH₃ from the difference between the initial and residual acid, using titration with standard alkali.
| KJELDAHL LIMITATION NCERT states that Kjeldahl's method is not applicable to compounds containing nitrogen in nitro or azo groups and to ring nitrogen such as pyridine, because that nitrogen does not convert to ammonium sulphate under the method's conditions. |
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8.10.3 Halogens — Carius Method
A known mass of organic compound is heated with fuming nitric acid in the presence of AgNO₃ in a hard glass Carius tube. The halogen forms AgX, which is filtered, washed, dried and weighed.
% halogen = [atomic mass of X × mass of AgX × 100] / [molar mass of AgX × mass of organic compound]
8.10.4 Sulphur
The organic compound is heated in a Carius tube with sodium peroxide or fuming nitric acid. Sulphur is oxidised to sulphuric acid and precipitated as BaSO₄ with excess BaCl₂. The precipitate is filtered, washed, dried and weighed.
% S = [32 × mass of BaSO₄ × 100] / [233 × mass of organic compound]
8.10.5 Phosphorus
Phosphorus is oxidised to phosphoric acid using fuming nitric acid. It is precipitated as ammonium phosphomolybdate, NH₄₃PO₄·12MoO₃, or alternatively as magnesium ammonium phosphate followed by ignition to Mg₂P₂O₇.
| Route | Useful relationship |
|---|---|
| Ammonium phosphomolybdate | % P = (31 × mass of precipitate × 100) / (1877 × mass of compound) |
| Mg₂P₂O₇ route | % P = (62 × mass of Mg₂P₂O₇ × 100) / (222 × mass of compound) |
8.10.6 Oxygen
Oxygen is usually obtained by difference: subtract the sum of the percentages of all other elements from 100.
% O = 100 − (%C + %H + %N + %S + %P + %halogens + other determined elements)
NCERT also describes a direct oxygen estimation based on converting the oxygen released from the compound to CO and then oxidising CO with iodine pentoxide, producing CO₂ and iodine.
NEET Problem-Solving Toolkit
1. HybridisationhybridisationMixing of atomic orbitals to form equivalent hybrid orbitals with defined shapes. from Bonding
4 electron domains / four σ-type directions around carbon → sp³
3 electron domains / trigonal planar carbon → sp²
2 electron domains / linear carbon → sp
More s-character → shorter/stronger bond and greater carbon electronegativity
2. Count σ and π Bonds
Every single bond → 1σ
Every double bond → 1σ + 1π
Every triple bond → 1σ + 2π
3. Name an Organic Compound
Principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. → parent chain containing it → numbering → multiple-bond locants → substituent locants → alphabetical order → final suffix
4. Identify IsomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements.
Same formula + different carbon skeleton → chain
Same skeleton + different position → position
Same formula + different functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. → functional-group isomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements.
Different alkyl groups on two sides of same polyvalent group → metamerism
Same connectivity + different spatial arrangement → stereoisomerism
5. Identify Mechanism Species
Electron pair donor → nucleophilenucleophileAn electron-rich species that donates an electron pair to form a bond.
Electron pair acceptor → electrophileelectrophileAn electron-deficient species that accepts an electron pair.
Heterolysis → ions / carbocationcarbocationAn organic ion in which a carbon atom carries a positive charge. / carbanioncarbanionAn organic ion in which a carbon atom carries a negative charge.
Homolysis → radicals
6. Identify the Electronic Effect
| Clue in question | Likely effect |
|---|---|
| Electronegativity transmitted through σ bonds | Inductive |
| π system / lone pair delocalisation | ResonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. |
| Multiple bond + attacking reagent + temporary shift | Electromeric |
| σ(C–H) delocalisation adjacent to π/empty p orbital | Hyperconjugation |
7. Choose Purification Method
| Situation | Method |
|---|---|
| Sublimable solid + non-sublimable impurity | Sublimation |
| Solid with temperature-dependent solubility difference | Crystallisation |
| Liquid + nonvolatile impurity / large b.p. difference | Simple distillation |
| Liquids with close b.p. | Fractional distillation |
| High b.p. / decomposes on heating | Reduced-pressure distillation |
| Steam-volatile + water-immiscible | Steam distillation |
| Aqueous compound + immiscible organic solvent | Differential extraction |
| Complex mixture / tiny quantities / identification | ChromatographychromatographyA separation technique based on different affinities for stationary and mobile phases. |
NEET TRAPS & NCERT LOCKS
Carbon's tetravalence is a structural concept; hybridisationhybridisationMixing of atomic orbitals to form equivalent hybrid orbitals with defined shapes. determines the geometry used in NCERT examples.
sp carbon has the greatest s-character among sp³, sp² and sp; it is therefore more electronegative and forms shorter, stronger bonds.
π bonds restrict rotation around C=C and are generally more exposed to attacking reagents.
In bond-line formulae, carbon and its attached hydrogens are omitted, but heteroatoms are written.
For IUPAC naming, the parent chain must contain the principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions..
The principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. gets the suffix; subordinate functional groups become prefixes.
o-/m-/p- correspond to 1,2 / 1,3 / 1,4 only in the traditional nomenclature of disubstituted benzene derivatives.
Isomers have the same molecular formula, not necessarily the same functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. or connectivity.
Heterolysis gives charged fragments; homolysis gives neutral radicals.
Full-headed curved arrow = electron pair; fish-hook = one electron.
NucleophilenucleophileAn electron-rich species that donates an electron pair to form a bond. donates an electron pair; electrophileelectrophileAn electron-deficient species that accepts an electron pair. accepts it.
Inductive effect travels through σ bonds and decreases rapidly with distance.
ResonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. contributors differ only in electron arrangement, not nuclear positions.
A resonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. hybrid is the real structure; individual canonical forms are not isolated molecules.
Electromeric effect is temporary and requires an attacking reagent.
Hyperconjugation is permanent and involves σ-electron delocalisation.
Simple distillation is not appropriate for liquids with very close boiling points; use fractional distillation.
Reduced-pressure distillation is useful for high-boiling or thermally decomposable liquids.
Steam distillation requires steam volatility and immiscibility with water.
In Lassaigne's test, sodium fusion converts covalent elements into ionic species.
Nitric acid treatment before AgNO₃ removes CN⁻/S²⁻ interference in the halogen test.
Kjeldahl method has the NCERT limitations for nitro, azo and ring nitrogen.
Oxygen is generally calculated by difference in the NCERT quantitative-analysis treatment.
ACTIVE RECALL
Why is carbon tetravalent and how does hybridisationhybridisationMixing of atomic orbitals to form equivalent hybrid orbitals with defined shapes. explain CH₄, C₂H₄ and C₂H₂?
How does s-character affect bond length, bond strength and carbon electronegativity?
Why is rotation around a C=C bond restricted?
What is the difference between complete, condensed and bond-line formulae?
What do solid wedge, dashed wedge and normal line mean?
Differentiate acyclic, alicyclic, aromatic and heterocyclic compounds.
What is a functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions. and what is a homologous serieshomologous seriesA family of compounds with the same functional group and general formula; successive members differ by CH₂.?
How do you select the parent chain in IUPAC nomenclature?
What is the rule for choosing the principal functional groupfunctional groupAn atom or group of atoms that gives an organic compound its characteristic reactions.?
Differentiate chain, position, functional-group and metameric isomerismisomerismThe existence of compounds with the same molecular formula but different structures or spatial arrangements..
What distinguishes stereoisomers from structural isomers?
What is heterolytic vs homolytic bond cleavage?
How are carbocations, carbanions and free radicals classified?
What is the difference between a nucleophilenucleophileAn electron-rich species that donates an electron pair to form a bond. and an electrophileelectrophileAn electron-deficient species that accepts an electron pair.?
How do full-headed and fish-hook curved arrows differ?
Compare inductive, resonanceresonanceRepresentation of electron delocalisation using two or more contributing structures., electromeric and hyperconjugation effects.
Why are resonanceresonanceRepresentation of electron delocalisation using two or more contributing structures. contributors not real molecules?
Why does inductive effect decrease rapidly with distance?
Why is tertiary carbocationcarbocationAn organic ion in which a carbon atom carries a positive charge. more stable than primary carbocationcarbocationAn organic ion in which a carbon atom carries a positive charge. in the NCERT framework?
When should you use crystallisation, fractional distillation, reduced-pressure distillation and steam distillation?
What is the difference between adsorption and partition chromatographychromatographyA separation technique based on different affinities for stationary and mobile phases.?
How is Rf defined in TLC/paper chromatographychromatographyA separation technique based on different affinities for stationary and mobile phases.?
What does Lassaigne's test accomplish chemically?
Why is HNO₃ used before AgNO₃ in the halogen test?
Differentiate Dumas and Kjeldahl methods for nitrogen estimation.
State the core calculation formulae for C, H, N, halogens, S and P estimation.