Class 11 Chemistry ⚗️

Chapter 14 — Basic Principles of Organic Chemistry

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⭐ Key Points 🔢 Formula / Quick Ref ✍️ 2 Mark Questions 📋 3 Mark Questions

Chapter 14 — Basic Principles of Organic Chemistry

Maharashtra HSC Board — Complete Notes

⭐
Chapter 14 — Quick Reference

All Key Points at a Glance

🧪 Introduction & Unique Nature of Carbon
  • Carbon forms an immense array of compounds — from methane (1C) to DNA (billions of C atoms)
  • Crude oil = complex mixture of hydrocarbons
  • Pharmaceutical industry produces medicines — mostly organic compounds
  • Tetravalency of carbon + ability to bond with itself = basis of organic chemistry
  • Carbon forms covalent bonds with H, O, N, S, halogens
  • Essential element in all organic compounds: Carbon (C)
✏️ Structural Representation of Organic Molecules
  • Structural formula: shows all atoms with symbols and all covalent bonds as dashes
  • Electron dot (Lewis) structure: all valence electrons shown as dots; two dots between atoms = one covalent bond
  • Condensed formula: bonds hidden, identical groups shown with subscript. Example: ethane = CH₃–CH₃ or CH₃CH₃
  • Bond line (zig-zag) formula: C and H symbols not written; C–C bonds shown as zig-zag lines; terminals = CH₃; intersection = C; heteroatoms written clearly
📐 3D Representation Methods
  • Wedge formula: solid wedge = bond coming towards reader; dashed wedge = bond going behind; normal line = bond in plane
  • Fischer projection (cross formula): main chain vertical; horizontal lines project towards reader; vertical lines go behind. Used in carbohydrate chemistry
  • Newman projection: view along C–C bond; front C = point; rear C = circle; 3 bonds radiate like spokes of wheel from each carbon
  • Sawhorse (andiron/perspective) formula: C–C bond as long slanting line; lower end = front C; upper end = rear C
🌿 Classification — Carbon Skeleton
  • Acyclic / Aliphatic (open chain): straight or branched carbon chains. Example: propane, isobutane
  • Cyclic (closed chain): ring structures
    → Homocyclic / Carbocyclic: ring of C atoms only
       → Alicyclic: properties similar to aliphatic (cyclobutane, cyclohexene)
       → Aromatic: special stability (benzenoid: benzene, naphthalene; non-benzenoid: tropone)
    → Heterocyclic: ring contains heteroatom (O, N, S). Examples: furan, pyridine, pyrrole
  • Heteroatoms: all atoms in organic molecules other than C and H
⚗️ Functional Groups & Homologous Series
  • Functional group: part of organic molecule which undergoes change during a reaction. Determines chemical properties of the compound
  • Key functional groups: –OH (alcohol), –COOH (carboxylic acid), –CHO (aldehyde), –CO– (ketone), –NH₂ (amine), –X (halide), –CN (nitrile), –NO₂ (nitro), C=C (alkene), –C≡C– (alkyne)
  • Homologous series: series of compounds of same family, same functional group, same carbon skeleton type, each member differing by –CH₂– unit
  • Homologue: individual member of a homologous series; all have same general formula
  • Members have similar chemical properties; physical properties (bp, mp, density) change gradually
🔤 IUPAC Nomenclature — Rules
  • IUPAC = International Union of Pure and Applied Chemistry (founded 1919)
  • Three parts of IUPAC name: parent hydrocarbon + branches + functional groups
  • Alkanes naming: based on number of carbon atoms (methane, ethane, propane…)
  • Alkyl groups: formed by removing one H from terminal C of alkane; 'ane' → 'yl' (methane → methyl, Me)
  • Rules: (1) Longest chain = parent; (2) Lowest locant number; (3) Alphabetical order of substituents; (4) di, tri, tetra prefix for identical substituents; (5) If equal length chains → choose chain with maximum substituents
  • Alkenes: 'ane' → 'ene'; Alkynes: 'ane' → 'yne'; multiple bond gets lowest locant
  • Cyclic: prefix 'cyclo' added (cyclopropane, cyclohexene)
  • Functional groups: as suffix (–ol, –al, –one, –oic acid, –amine) or prefix (nitro, halo, alkoxy)
  • Priority order: –COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > C=O > –OH > –NH₂ > C=C > –C≡C–
  • Benzene derivatives: mono (prefix + benzene), di (o–, m–, p– or 1,2 / 1,3 / 1,4), tri (numbers + alphabetical order)
🔄 Types of Isomerism
  • Isomerism: two or more compounds with same molecular formula; called isomers
  • Structural isomerism: same mol. formula, different structural formula
    → Chain isomerism: different carbon skeletons (butane vs 2-methylpropane)
    → Position isomerism: same functional group at different positions (but-1-ene vs but-2-ene)
    → Functional group isomerism: different functional groups (CH₃–O–CH₃ vs CH₃CH₂OH)
    → Metamerism: same functional group, unequal C distribution on either side (ethoxyethane vs methoxypropane)
    → Tautomerism: rapidly interconverting structural isomers; proton shifts (keto⇌enol)
  • Stereoisomerism: same structural formula, different spatial arrangement → geometrical + optical isomerism
⚡ Bond Cleavage & Reactive Intermediates
  • Substrate: reactant providing carbon to new bond
  • Reagent: brings about the change; Byproduct: unwanted product
  • Homolytic cleavage: one electron to each atom → two neutral free radicals (unpaired electron). Represented by fish-hook arrow (half headed). Favoured by UV light, peroxides, high temperature. Free radical = sp² hybridized, planar trigonal geometry
  • Heterolytic cleavage: both electrons to one atom → carbocation (+) + anion (–). Represented by full curved arrow. Favoured in polar solvents
  • Carbocation: C with sextet, positive charge, sp² hybridized, trigonal planar, vacant pz orbital. Stability: 3° > 2° > 1° > CH₃⁺
  • Carbanion: C with completed octet, negative charge. Formed when C bonds to electropositive atom
  • Free radical stability: 3° > 2° > 1° > CH₃•
🔌 Types of Reagents
  • Electrophile (E⁺): electron-seeking species; electron deficient; accepts electrons from substrate. Examples: Br⁺, CH₃⁺, AlCl₃ (vacant orbital)
  • Nucleophile (Nu:): nucleus-seeking species; electron-rich; donates electrons to substrate. Examples: OH⁻, H₂O (lone pair), NH₃
  • Electrophilic centre: electron-deficient atom in electrophile (e.g., Al in AlCl₃)
  • Nucleophilic centre: electron-rich atom in nucleophile (e.g., O in H₂O, N in NH₃)
🌀 Electronic Effects
  • Inductive effect: polarity induced in adjacent C–C single bonds due to polar covalent bond in molecule; permanent effect; operates through σ bond; decreases with distance (negligible beyond 3 bonds)
    → –I effect (electron withdrawing): –Cl, –Br, –I, –NO₂, –CN, –COOH; order: Cl > Br > I
    → +I effect (electron donating): alkyl groups (–CH₃, –C₂H₅)
  • Resonance / Mesomeric effect: polarity produced by interaction of conjugated π bonds; permanent; operates through π bond
    → +R effect: lone pair donated to π system (–OH, –OR, –NH₂, –halogens) → increases electron density at ortho, para positions
    → –R effect: electrons withdrawn from π system (–COOH, –CHO, –CN, –NO₂) → increases positive polarity at ortho, para positions
  • Electromeric effect: temporary effect; complete shift of π electrons on approach of reagent; disappears when reagent removed
  • Hyperconjugation: permanent effect; delocalization of σ electrons of C–H bond of α-alkyl group into adjacent empty p-orbital or π bond; also called 'no bond resonance' (σ–π conjugation); more α-H atoms → more stability. Effect of hyperconjugation > inductive effect
📐 Resonance Structures — Rules
  • Conjugated system: two or more multiple bonds alternating with single bonds
  • Resonance structures (canonical/contributing structures) are hypothetical; linked by double-headed arrow ↔
  • Resonance hybrid: actual molecule, more stable than any resonance structure; stability difference = resonance energy
  • Rules: (i) All atoms in same plane; (ii) same number of unpaired electrons; (iii) more stable (low energy) structure contributes more
  • More stable resonance structure: more covalent bonds; complete octets; less charge separation; –ve charge on electronegative atom; +ve on electropositive atom; more dispersal of charge
  • Benzene: resonance hybrid of two structures; all 6 C–C bonds equal at 138 pm (intermediate between C–C 154 pm and C=C 133 pm)
🔑 Primary, Secondary, Tertiary, Quaternary Carbon
  • Primary (1°): bonded to only 1 other carbon. Example: CH₃ in ethane
  • Secondary (2°): bonded to 2 other carbons. Example: middle C in propane
  • Tertiary (3°): bonded to 3 other carbons. Example: middle C in isobutane CH₃CH(CH₃)₂
  • Quaternary (4°): bonded to 4 other carbons. Example: middle C in neopentane C(CH₃)₄
  • Stability of carbocations and free radicals: 3° > 2° > 1° > methyl
🌿
Section 14.1–14.2

Introduction & Structural Representation

📖 Explanation

Of all elements, only carbon forms an immense array of compounds — from methane (1 carbon) to DNA (billions of carbons). Crude oil is a complex mixture of hydrocarbons. Medicines from the pharmaceutical industry are organic compounds. The tetravalency of carbon and its unique ability to bond with itself (catenation) makes organic chemistry a separate branch.

Ways to represent organic molecules:
• Structural formula: all atoms and bonds shown
• Electron dot (Lewis) structure: valence electrons shown as dots
• Condensed formula: bonds hidden, identical groups shown with subscript (CH₃CH₃)
• Bond line / zig-zag formula: C and H not written; C–C bonds as zig-zag; terminals = CH₃; intersections = C; heteroatoms written clearly

3D methods: Wedge formula, Fischer projection (carbohydrate chemistry), Newman projection (view through C–C bond), Sawhorse formula (slanting line for C–C bond).

✅ Short Answers
Q. What is condensed formula? Give example. (2 marks)
Condensed formula is a simplified structural formula where some or all covalent bonds are hidden, and the number of identical groups attached to an atom is indicated by a subscript.
Example: Condensed formula of ethane = CH₃–CH₃ or CH₃CH₃. Condensed formula of butane = CH₃(CH₂)₂CH₃.
Q. What is bond line or zig-zag formula? (2 marks)
In bond line (zig-zag) formula, symbols of carbon and hydrogen atoms are NOT written. C–C bonds are drawn as zig-zag lines. Terminals of zig-zag line represent methyl (–CH₃) groups. Each intersection represents a carbon atom bonded to appropriate number of H atoms. Heteroatoms and H bonded to heteroatoms are written clearly. Example: propane is shown as a simple zig-zag (∧).
Q. Explain Newman projection formula. (3 marks)
Newman projection formula is a method to represent a 3D molecule by looking through a C–C single bond.
Convention: Front carbon is represented by a point (dot); the rear carbon is represented by a circle around it. The remaining three bonds at each carbon are drawn like spokes of a wheel.
• Three bonds at front carbon radiate from the centre of the circle.
• Three bonds at rear carbon radiate from the circumference of the circle.
It shows the spatial arrangement of groups and is used to study conformations of molecules.
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Section 14.3

Classification of Organic Compounds

📖 Explanation

Organic compounds are classified in two ways:

(i) Based on carbon skeleton:
• Acyclic/Aliphatic: open chain, straight or branched (propane, isobutane)
• Cyclic: ring structures
  → Homocyclic (carbocyclic): ring of C only
    → Alicyclic: properties like aliphatic (cyclobutane, cyclohexene)
    → Aromatic: special stability (benzene = benzenoid; tropone = non-benzenoid)
  → Heterocyclic: ring has heteroatom (O, N, S) — furan, pyridine, pyrrole

(ii) Based on functional group:
A functional group is the part of an organic molecule which undergoes change during a reaction. Compounds with the same functional group form a family (alcohols, aldehydes, etc.).

Homologous series: Series of compounds of the same family, same functional group, same carbon skeleton, each member differing from the next by –CH₂– (methylene group). Individual members are called homologues. All have the same general formula. Physical properties change gradually; chemical properties are similar.

✅ Short Answers
Q. What are alicyclic compounds? Give examples. (2 marks)
Alicyclic compounds are cyclic compounds (closed chain) that exhibit properties similar to those of aliphatic compounds. Their ring is made entirely of carbon atoms (homocyclic) but their chemical behaviour resembles aliphatic compounds.
Examples: Cyclobutane, cyclohexene.
Q. What is a functional group? Give any four examples. (2 marks)
A functional group is a part of an organic molecule which undergoes change as a result of a reaction. It determines the chemical properties of the compound.
Examples:
1. –OH (alcohol/phenol)
2. –COOH (carboxylic acid)
3. –CHO (aldehyde)
4. –NH₂ (primary amine)
Q. Define homologous series. Write its characteristics. (3 marks)
Homologous series: A series of compounds of the same family in which each member has the same type of carbon skeleton and functional group, and differs from the next member by a constant difference of one methylene group (–CH₂–) in its molecular and structural formula.

Characteristics:
1. All members have the same general molecular formula.
2. Members have similar chemical properties.
3. Physical properties (bp, mp, density, solubility) vary gradually from one member to the next.
4. Each member differs from the adjacent member by –CH₂– unit (14 mass units).
Example: Aldehydes — HCHO, CH₃CHO, C₂H₅CHO, C₃H₇CHO; General formula: CₙH₂ₙO
Q. Distinguish between homocyclic and heterocyclic compounds. (2 marks)
Homocyclic (carbocyclic) compounds: Cyclic compounds in which the ring is made entirely of carbon atoms. Examples: benzene, cyclohexane, naphthalene.

Heterocyclic compounds: Cyclic compounds in which the ring contains one or more heteroatoms (O, N, S, etc.) in addition to carbon. Examples: furan (O), pyridine (N), thiophene (S), pyrrole (N).
🔤
Section 14.4

IUPAC Nomenclature

📖 Explanation

IUPAC (International Union of Pure and Applied Chemistry, founded 1919) developed a systematic, universally accepted method of naming organic compounds — giving each compound a unique name.

IUPAC name = Parent hydrocarbon + branches (prefix) + functional groups (prefix/suffix)

Rules for branched saturated hydrocarbons:
1. Select longest continuous chain = parent chain; other C atoms = side chains/branches
2. Number parent chain to give lowest locant numbers to substituents
3. Names of alkyl substituents added as prefix in alphabetical order
4. Substituent name separated from locant by hyphen
5. Identical substituents: di (2), tri (3), tetra (4) — prefixes di, tri, tetra ignored in alphabetizing
6. If equal length chains: choose chain with maximum substituents

Unsaturated hydrocarbons: 'ane' → 'ene' (alkene), 'yne' (alkyne). Multiple bond must be in parent chain and get lowest locant.

Monocyclic: prefix 'cyclo' (cyclopropane, cyclohexene)

Benzene derivatives:
• Mono: substituent as prefix + benzene (chlorobenzene)
• Di: o– (1,2), m– (1,3), p– (1,4) or numbers
• Special names: toluene (methylbenzene), aniline (aminobenzene), phenol (hydroxybenzene), anisole (methoxybenzene)
• If substituent >7 C: named as phenyl-substituted alkane

Priority order (principal functional group):
–COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > C=O > –OH > –NH₂ > C=C > –C≡C–

✅ Short Answers
Q. State rules for IUPAC nomenclature of branched saturated hydrocarbons. (3 marks)
1. Select the longest continuous chain as parent chain. Other C atoms form side chains/branches.
2. Number the parent chain from one end to give lowest possible locant numbers to substituents.
3. Names of alkyl substituents added as prefix in alphabetical order; each preceded by its locant number separated by hyphen.
4. If two or more identical substituents: use di, tri, tetra prefixes (ignored in alphabetizing).
5. If two chains of equal length: choose the one with maximum number of substituents.
6. Branched alkyl group without accepted trivial name: longest chain from point of attachment is base; C attached to parent chain is numbered 1; name enclosed in brackets.
Q. Give IUPAC names of disubstituted benzene isomers. (2 marks)
Three isomers of dichlorobenzene:
1. 1,2-dichlorobenzene (common name: o-dichlorobenzene)
2. 1,3-dichlorobenzene (common name: m-dichlorobenzene)
3. 1,4-dichlorobenzene (common name: p-dichlorobenzene)
IUPAC system uses numbers (1,2 / 1,3 / 1,4) instead of prefixes ortho/meta/para.
Q. Write IUPAC names of: (i) CH₃–CH(OH)–CO–CH₂CH₃ and (ii) CH₃–CH(NH₂)–CH₂–COOH (2 marks)
(i) Principal functional group = ketone (–CO–). Five carbon chain. –OH at C-2. IUPAC name: 2-hydroxypentan-3-one.
(ii) Principal functional group = carboxylic acid (–COOH). Four carbon chain. –NH₂ at C-3. IUPAC name: 3-aminobutanoic acid.
Q. What is meant by trivial names? Give examples. (2 marks)
Trivial names (common names) are old names of organic compounds developed as organic chemistry grew. They usually have some historical origin and are accepted due to long usage. They are still used for commonly used commercial compounds.
Examples: CH₃COOH = Acetic acid | CHCl₃ = Chloroform | CH≡CH = Acetylene | C₂H₅OC₂H₅ = Diethyl ether | HCHO = Formaldehyde | Methylbenzene = Toluene | Aminobenzene = Aniline
🔄
Section 14.5

Isomerism

📖 Explanation

Isomerism: The phenomenon of existence of two or more compounds possessing the same molecular formula. Such compounds are called isomers.

Two broad types:
1. Structural isomerism: same molecular formula, different structural formula.
  a. Chain isomerism: different carbon skeletons. Example: butane (CH₃CH₂CH₂CH₃) and 2-methylpropane [CH₃CH(CH₃)₂] — both C₄H₁₀.
  b. Position isomerism: same functional group at different positions. Example: but-1-ene and but-2-ene — both C₄H₈.
  c. Functional group isomerism: different functional groups with same mol. formula. Example: CH₃OCH₃ (dimethyl ether) and CH₃CH₂OH (ethanol) — both C₂H₆O.
  d. Metamerism: same functional group, unequal distribution of C on either side. Example: ethoxyethane and methoxypropane — both C₄H₁₀O.
  e. Tautomerism: same compound exists as two or more rapidly interconverting structural isomers. A proton shifts between atoms. Most common = keto-enol tautomerism.

2. Stereoisomerism: same structural formula, different spatial arrangement of groups → geometrical and optical isomerism.

✅ Short Answers
Q. What is isomerism? Name and explain its types. (3 marks)
Isomerism: Phenomenon of two or more compounds having the same molecular formula but different structural or spatial arrangements. Such compounds are called isomers.

Types:
1. Structural isomerism: same mol. formula, different structural formula.
  → Chain (different skeletons), Position (same group, different positions), Functional group (different groups), Metamerism, Tautomerism
2. Stereoisomerism: same structural formula, different spatial arrangement.
  → Geometrical isomerism, Optical isomerism
Q. What is tautomerism? Explain keto-enol tautomerism. (2 marks)
Tautomerism: The phenomenon where the same compound exists as a mixture of two or more structurally distinct molecules which are in rapid equilibrium with each other. Such interconverting isomers are called tautomers. In nearly all cases, a proton shifts from one atom to another in the molecule.

Keto-enol tautomerism: The α-hydrogen shifts from the keto form (C=O) to the enol form (C–OH).
–C(H)–C=O ⇌ –C=C–OH (keto form ⇌ enol form)
Q. Distinguish between chain isomerism and position isomerism with examples. (3 marks)
Chain isomerism: When compounds have the same molecular formula but different carbon skeletons (different parent chains).
Example: Butane (CH₃–CH₂–CH₂–CH₃) and 2-methylpropane [CH₃–CH(CH₃)–CH₃] — both have molecular formula C₄H₁₀ but different carbon skeletons.

Position isomerism: When compounds have the same molecular formula and same carbon skeleton, but the functional group (or double bond) is at a different position.
Example: But-1-ene (CH₂=CH–CH₂–CH₃) and But-2-ene (CH₃–CH=CH–CH₃) — both C₄H₈, same skeleton, but double bond at different position.
⚡
Section 14.6

Theoretical Basis of Organic Reactions — Cleavage, Reagents & Electronic Effects

📖 Explanation

During an organic reaction, a covalent bond at a carbon atom in the reactant is broken and a new covalent bond is formed. The overall organic reaction is a multi-step process involving intermediates. A sequential account of electron movements, bond cleavage, bond formation, energy changes, and rate constitute the reaction mechanism.

Types of bond cleavage:
• Homolysis: one electron to each atom → free radicals (neutral, unpaired electron). Fish-hook arrow (½ arrow). Favoured by UV, peroxides, high T. Reactions = free radical (nonpolar) reactions. Free radical = sp², trigonal planar.
• Heterolysis: both electrons to more electronegative atom → carbocation + anion. Full curved arrow. Favoured in polar solvents. Reactions = polar/ionic reactions.

Carbocation: sextet (6e⁻), +ve charge, sp², trigonal planar, vacant pz orbital. Stability: 3° > 2° > 1° > CH₃⁺.
Carbanion: octet, –ve charge, formed when C bonds to electropositive atom.

Electronic effects:
1. Inductive effect: permanent; sigma bond; polarity transmitted along C–C chain; decreases with distance (negligible after 3 bonds).
2. Resonance/Mesomeric effect: permanent; pi bond system; +R and –R effects.
3. Electromeric effect: temporary; multiple bond; disappears when reagent removed.
4. Hyperconjugation: permanent; σ–π conjugation; σ electrons of C–H of α-alkyl group delocalized; 'no bond resonance'; more α-H → more stable. Effect > inductive effect.

✅ Short Answers
Q. Distinguish between homolysis and heterolysis of covalent bond. (3 marks)
Homolysis (Homolytic cleavage):
• One electron goes to each bonded atom.
• Produces two neutral free radicals (species with unpaired electron).
• Represented by half-headed arrow (fish hook).
• Favoured by UV radiation, peroxides, high temperature.
• Reactions are called free radical / non-polar reactions.
• Free radical is sp² hybridized, trigonal planar.

Heterolysis (Heterolytic cleavage):
• Both electrons go to the more electronegative atom.
• Produces two charged species — a carbocation (+) and an anion (–).
• Represented by full curved arrow.
• Favoured in polar solvents.
• Reactions are called polar / ionic reactions.
Q. What are electrophiles and nucleophiles? Give examples. (2 marks)
Electrophiles (E⁺): Electron-loving (electron-seeking) species. They are electron-deficient and accept electrons from the substrate. Examples: positively charged ions (Br⁺, CH₃⁺) or neutral species with vacant orbitals (AlCl₃, BF₃).

Nucleophiles (Nu:): Nucleus-seeking species. They are electron-rich and donate electrons to the substrate. Examples: negatively charged species (OH⁻, CN⁻) or neutral species with lone pairs (H₂O, NH₃).
Q. What is inductive effect? Distinguish between +I and –I effect. (3 marks)
Inductive effect: When an organic molecule has a polar covalent bond in its structure, polarity is induced in adjacent carbon–carbon single bonds too. This permanent electron displacement along the sigma bond in the ground state is called the inductive effect. It decreases rapidly with distance and becomes negligible beyond 3 bonds.

–I effect (electron withdrawing inductive effect): The group withdraws electron density from the carbon chain. Example: –Cl, –Br, –I, –NO₂, –CN, –COOH. Order of –I effect: Cl > Br > I.

+I effect (electron releasing inductive effect): The group donates electron density to the carbon chain. Example: alkyl groups like –CH₃, –C₂H₅.
Q. What is resonance effect? Explain +R and –R effect with examples. (3 marks)
Resonance effect (Mesomeric effect): The polarity produced in a molecule by the interaction between conjugated π bonds (or between π bond and p orbital on attached atom) is called resonance effect. It is a permanent effect transmitted through conjugated π bonds.

+R effect: The substituent has a lone pair to donate into the π system → increases electron density at certain positions. Groups: –OH, –OR, –NH₂, –halogens. Example: In aniline, –NH₂ shows +R effect, increasing electron density at ortho and para positions.

–R effect: The substituent withdraws electrons from the π system → develops positive polarity at certain positions. Groups: –COOH, –CHO, –CN, –NO₂, –CO–. Example: In nitrobenzene, –NO₂ shows –R effect, developing positive polarity at ortho and para positions.
Q. What is hyperconjugation? Why is it called 'no bond resonance'? (3 marks)
Hyperconjugation is a permanent electronic effect that explains the stability of carbocations, free radicals, and alkenes. It is the delocalization of σ electrons of a C–H bond of an alkyl group directly attached to a carbon atom which is part of an unsaturated system or has an empty p-orbital.

It is called 'no bond resonance' because in the contributing structures (no-bond resonance structures), there is no covalent bond between carbon and one of the α-hydrogens (the H is shown as H⁺).

More α-hydrogens → more no-bond resonance structures → more stability. This is why carbocation stability is 3° > 2° > 1° > CH₃⁺. Hyperconjugation effect is usually stronger than inductive effect.
Q. What is resonance? State the rules for writing resonance structures. (3 marks)
Resonance: When the Lewis structure of a compound has a conjugated system of π bonds, two or more Lewis structures (resonance structures) can be written by delocalizing π electrons using curved arrows. The actual molecule is the resonance hybrid of all resonance structures, more stable than any one resonance structure. The extra stability is called resonance energy.

Rules for writing resonance structures:
(i) All atoms involved in the π conjugated system must lie in the same plane.
(ii) All resonance structures must have the same number of unpaired electrons.
(iii) More stable (low energy) resonance structures contribute more to the resonance hybrid.
(iv) A resonance structure is more stable if it has: more covalent bonds, complete octets, less charge separation, –ve charge on electronegative atom, +ve charge on electropositive atom, more dispersal of charge.

🔢 Quick Reference Table

🔤 IUPAC Name Endings Alkane → -ane (methane, ethane...)
Alkene → -ene (but-1-ene)
Alkyne → -yne (but-1-yne)
Alcohol → -ol (propan-1-ol)
Aldehyde → -al (ethanal)
Ketone → -one (propan-2-one)
Carboxylic acid → -oic acid
Amine → -amine
⚗️ First 10 Alkanes 1C: Methane (CH₄)
2C: Ethane (C₂H₆)
3C: Propane (C₃H₈)
4C: Butane (C₄H₁₀)
5C: Pentane (C₅H₁₂)
6C: Hexane (C₆H₁₄)
7C: Heptane (C₇H₁₆)
8C: Octane (C₈H₁₈)
9C: Nonane (C₉H₂₀)
10C: Decane (C₁₀H₂₂)
🔑 General Formulae Alkanes: CₙH₂ₙ₊₂
Alkenes: CₙH₂ₙ
Alkynes: CₙH₂ₙ₋₂
Aldehydes: CₙH₂ₙO
Alcohols: CₙH₂ₙ₊₂O
Cycloalkanes: CₙH₂ₙ
Benzene: C₆H₆
⚡ Bond Cleavage Summary Homolysis → Free radical (neutral)
Arrow: half-headed (fish hook)
Condition: UV, peroxides, high T
Reaction: Free radical / nonpolar
─────────────────
Heterolysis → Carbocation + Anion
Arrow: full curved arrow
Condition: Polar solvents
Reaction: Polar / ionic reactions
🏆 Stability Orders Carbocations: 3° > 2° > 1° > CH₃⁺
Free radicals: 3° > 2° > 1° > CH₃•
Carbanions: CH₃⁻ > 1° > 2° > 3°
─────────────────
–I effect of halogens: Cl > Br > I
Hyperconjugation > Inductive effect
🌀 Electronic Effects Inductive: σ bond, permanent, ↓ with distance
Resonance: π bond, permanent
Electromeric: π bond, temporary
Hyperconjugation: σ–π, permanent
─────────────────
+I: alkyl groups (donate e⁻)
–I: –Cl, –NO₂, –CN, –COOH (withdraw)
+R: –OH, –NH₂, –OR (donate lone pair)
–R: –COOH, –CHO, –CN, –NO₂ (withdraw)
📐 Important Alkyl Groups –CH₃ = methyl (Me)
–C₂H₅ = ethyl (Et)
–C₃H₇ = propyl (Pr)
–C₄H₉ = butyl (Bu)
–CH(CH₃)₂ = isopropyl
–C(CH₃)₃ = tert-butyl
C₆H₅– = phenyl (Ph)
🏷️ Common/Trivial Names CH₄ = Methane
CH₂=CH₂ = Ethylene
CH≡CH = Acetylene
CHCl₃ = Chloroform
CH₃COOH = Acetic acid
HCHO = Formaldehyde
CH₃CHO = Acetaldehyde
CH₃COCH₃ = Acetone
C₆H₅NH₂ = Aniline
C₆H₅OH = Phenol
🔄 Types of Isomerism Structural isomerism:
→ Chain (different skeleton)
→ Position (same group, diff. position)
→ Functional group (diff. groups)
→ Metamerism (unequal C distribution)
→ Tautomerism (proton shift, keto⇌enol)
Stereoisomerism:
→ Geometrical + Optical

✍️ 2 Mark Questions

✍️
Short Answer

2 Mark Questions — Chapter 14

Q1. Why is carbon unique in forming such a large number of compounds? (2 marks)
Carbon is unique because of two reasons:
1. Tetravalency: Carbon has valency of 4, allowing it to form 4 covalent bonds with other atoms (H, O, N, S, halogens).
2. Catenation: Carbon can bond with itself to form long chains, branched chains and rings — from small molecules like methane (1C) to DNA (billions of C atoms). No other element shows this to such an extent.
Q2. What is structural formula? What does it show? (2 marks)
Structural formula of a molecule shows all the constituent atoms denoted with their symbols and all covalent bonds therein represented by a dash joining mutually bonded atoms. It shows the complete bonding arrangement of all atoms in the molecule. Example: structural formula of CH₄ shows carbon bonded to 4 hydrogen atoms with 4 dashes.
Q3. What are heteroatoms? Give examples. (2 marks)
Carbon is an essential atom in any organic molecule. One or more hydrogen atoms are present in most organic molecules. All other atoms that are found in organic molecules are called heteroatoms. Examples: oxygen (O), nitrogen (N), sulphur (S), halogens (F, Cl, Br, I), phosphorus (P).
Q4. Distinguish between structural isomerism and stereoisomerism. (2 marks)
Structural isomerism: Two or more compounds have the same molecular formula but different structural formulae. Types: chain, position, functional group, metamerism, tautomerism.

Stereoisomerism: Two or more compounds have the same molecular formula and the same structural formula but different spatial (3D) arrangement of groups or atoms. Types: geometrical and optical isomerism.
Q5. What is metamerism? Give an example. (2 marks)
Metamerism: A type of structural isomerism where compounds have the same molecular formula and same functional group, but have unequal distribution of carbon atoms on either side of the functional group.
Example: Ethoxyethane (CH₃–CH₂–O–CH₂–CH₃) and methoxypropane (CH₃–O–CH₂–CH₂–CH₃) — both have molecular formula C₄H₁₀O and same ether (–O–) functional group.
Q6. What is a free radical? Give its geometry. (2 marks)
A free radical (or radical) is a neutral species with a single unpaired electron. It is formed by homolytic cleavage of a covalent bond. Free radicals are unstable and highly reactive — they seek an electron for pairing. A carbon free radical is sp² hybridized and has planar trigonal geometry. The unpaired electron is in the pz orbital.
Q7. What is a carbocation? State its stability order. (2 marks)
A carbocation (earlier called carbonium ion) is a carbon atom with a sextet of electrons and a positive charge. It is highly unstable and reactive, formed as intermediate in many organic reactions. It is sp² hybridized, trigonal planar, with a vacant pz orbital.
Stability order: (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺
(3° > 2° > 1° > methyl carbocation)
Q8. What is resonance energy? (2 marks)
The resonance hybrid (actual molecule) is more stable than any of the individual resonance structures. The difference between the actual energy of the real molecule and the lowest calculated energy of any single resonance structure is called resonance stabilization energy or resonance energy. Greater the resonance energy, more stable is the molecule. Resonance leads to stabilization of the actual molecule.
Q9. What is electromeric effect? (2 marks)
Electromeric effect is a temporary electronic effect exhibited by multiple-bonded groups in the excited state in the presence of a reagent. When a reagent approaches a multiple bond, the π electron pair gets completely shifted to one of the multiply bonded atoms, giving a charge-separated structure. This effect disappears when the reagent is removed from the reacting system. It is also called polarisability effect.
Q10. Give four differences between organic and inorganic compounds. (2 marks)
1. Organic compounds contain carbon as essential element; inorganic may or may not contain carbon.
2. Organic compounds have covalent bonds; inorganic often have ionic bonds.
3. Organic compounds generally have low melting/boiling points; inorganic compounds usually have high mp/bp.
4. Organic compounds are generally insoluble in water but soluble in organic solvents; inorganic compounds are mostly soluble in water.

📋 3 Mark Questions

📋
Long Short Answer

3 Mark Questions — Chapter 14

Q1. Explain classification of organic compounds based on carbon skeleton with diagram. (3 marks)
Organic compounds are classified based on carbon skeleton as:

1. Acyclic / Aliphatic (open chain): Molecules have open chains of carbon atoms — straight (propane) or branched (isobutane).

2. Cyclic (closed chain): Molecules formed by joining atoms in ring-like structures.
  a. Homocyclic (carbocyclic): ring of only C atoms.
    i. Alicyclic: properties like aliphatic (cyclobutane, cyclohexene).
    ii. Aromatic: special stability (benzenoid: benzene, naphthalene; non-benzenoid: tropone).
  b. Heterocyclic: ring contains heteroatoms (O, N, S).
    i. Aromatic heterocyclic: furan, thiophene, pyridine, pyrrole.
    ii. Non-aromatic heterocyclic: tetrahydrofuran, piperidine.
Q2. What is reaction mechanism? Explain the terms substrate, reagent, product and byproduct. (3 marks)
Reaction mechanism: A sequential account of (i) electron movements in each step, (ii) bond cleavage and/or formation, (iii) changes in energy and shapes of species, and (iv) rate of the overall reaction. This knowledge helps understand reactivity and plan synthetic strategies.

Substrate: The reactant that provides carbon to the new bond formed. It undergoes structural change.
Reagent: The other reactant which brings about the change in the substrate.
Product: The desired compound formed in the reaction.
Byproduct: Unwanted products formed along with the main product.
Example: CH₄ + Cl₂ →(hv) CH₃Cl + HCl
Substrate = CH₄, Reagent = Cl₂, Product = CH₃Cl, Byproduct = HCl
Q3. Explain stability of carbocations with reference to hyperconjugation. (3 marks)
Carbocation stability order: 3° > 2° > 1° > CH₃⁺

This is explained by hyperconjugation: Delocalization of σ electrons of C–H bond of an α-alkyl group (alpha = adjacent to the carbocation carbon) into the vacant p-orbital of the carbocation stabilizes it.

In no-bond resonance structures, there is no covalent bond between C and one of the α-H atoms. More α-hydrogens → more no-bond resonance structures → more stabilization.

t-Butyl cation has 9 α-H atoms (3 methyl groups) → most stable.
Ethyl cation has 3 α-H atoms → less stable.
Methyl cation has no α-C → no hyperconjugation → least stable.
Hence: (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺
Q4. Write resonance structures of benzene. Explain why all C–C bond lengths in benzene are equal. (3 marks)
Benzene has two resonance structures (Kekulé structures). Both show alternating single and double bonds but the positions of π electrons are different.

Structure I: C₁–C₂ single bond; Structure II: C₁=C₂ double bond.
These are linked by double-headed arrow (↔) and are called resonance structures.

The actual benzene molecule is the resonance hybrid of these two structures. Each C–C bond has both single and double bond character.

Expected bond lengths: C–C single = 154 pm; C=C double = 133 pm. But experimentally, all 6 C–C bonds in benzene are equal at 138 pm, which is intermediate between single and double bond length. This confirms that benzene is a resonance hybrid — the π electrons are delocalized over all 6 carbon atoms, represented as a dotted circle inside a regular hexagon.
Q5. Describe wedge formula and Fischer projection formula with diagrams. (3 marks)
Wedge formula: Used to represent 3D structure of organic molecules on plane paper using three types of lines:
• Solid wedge (▶): bond projecting above the paper towards the reader.
• Dashed wedge (----): bond going behind/below the paper away from the reader.
• Normal line (–): bond in the plane of the paper.

Fischer projection (Cross formula): A three-dimensional molecule is projected on the plane of paper by visualizing the molecule with its main carbon chain vertical. Each carbon on the vertical chain is represented by a cross (×).
• Horizontal lines = bonds projecting above (towards reader).
• Vertical lines = bonds going below/behind (away from reader).
Fischer projection formula is more commonly used in carbohydrate chemistry.
Q6. Explain all types of structural isomerism with one example each. (3 marks)
Structural isomerism: Same molecular formula, different structural formula. Types:

1. Chain isomerism: Different carbon skeletons.
Example: Butane and 2-methylpropane — both C₄H₁₀.

2. Position isomerism: Same functional group at different positions.
Example: But-1-ene and but-2-ene — both C₄H₈.

3. Functional group isomerism: Different functional groups.
Example: CH₃–O–CH₃ (dimethyl ether) and CH₃CH₂OH (ethanol) — both C₂H₆O.

4. Metamerism: Same functional group, unequal C distribution on either side.
Example: Ethoxyethane and methoxypropane — both C₄H₁₀O.

5. Tautomerism: Rapidly interconverting structural isomers; proton shifts.
Example: Keto form ⇌ Enol form in carbonyl compounds.
Q7. Write IUPAC rules for naming unsaturated hydrocarbons (alkenes and alkynes). (3 marks)
Additional rules for IUPAC names of alkenes and alkynes:

1. The longest continuous chain must include the carbon–carbon multiple bond.
2. Numbering must be done so that the multiple bond has the lowest possible locant number.
3. 'ane' of alkane is replaced by 'ene' for alkene and 'yne' for alkyne.
4. Position of multiple bond indicated by the smaller locant of the two multiply bonded carbons, placed before 'ene'/'yne'.
5. If multiple bond is equidistant from both ends, number from the end nearer to first branching.
6. Two double bonds: named as diene; two triple bonds: diyne (retain 'a' of 'ane').
7. If both double and triple bond present: number from end where multiple bond is nearer; 'en' ending comes before 'yne'; if tie, double bond gets lower number.
Q8. Explain positive (+R) and negative (–R) resonance effects with examples. (3 marks)
+R effect (Positive resonance / electron donating effect):
If a substituent group has a lone pair to donate to the attached π bond or conjugated π system, it shows +R effect. This increases electron density at certain positions.
Groups: –OH, –OR, –O⁻, –NHR, –halogens.
Example: In aniline (C₆H₅NH₂), –NH₂ donates lone pair into the benzene ring, increasing electron density at ortho and para positions. Hence aniline undergoes electrophilic substitution easily at ortho/para positions.

–R effect (Negative resonance / electron withdrawing effect):
If a substituent withdraws electrons from the attached π bond or conjugated π system, it shows –R effect. This develops positive polarity at certain positions.
Groups: –COOH, –CHO, –CO–, –CN, –NO₂, –COOR.
Example: In nitrobenzene, –NO₂ withdraws π electrons, developing positive polarity at ortho and para positions. Hence nitrobenzene undergoes electrophilic substitution at meta position.

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