Class 11 Chemistry Notes Chapter 12 (Chapter 12) – Examplar Problems (English) Book

Alright class, let's get straight into Chapter 12: Organic Chemistry - Some Basic Principles and Techniques. This chapter is the bedrock for understanding organic chemistry, which is crucial for many government exams. Pay close attention to the fundamentals.

Chapter 12: Organic Chemistry - Some Basic Principles and Techniques: Detailed Notes

1. Introduction & General Concepts:

• Organic Chemistry: The study of hydrocarbons (compounds of carbon and hydrogen) and their derivatives.
• Tetravalency of Carbon: Carbon has 4 valence electrons, allowing it to form four covalent bonds.
• Catenation: The unique ability of carbon atoms to link with one another through covalent bonds to form long chains (straight or branched) and rings. This is due to the high C-C bond energy.
• Hybridization & Shapes:
  • sp³ Hybridization: Forms 4 single bonds (σ bonds). Tetrahedral geometry. Bond angle ≈ 109.5°. Example: Methane (CH₄), Ethane (C₂H₆).
  • sp² Hybridization: Forms 1 double bond (1 σ, 1 π) and 2 single bonds (σ bonds). Trigonal planar geometry. Bond angle ≈ 120°. Example: Ethene (C₂H₄).
  • sp Hybridization: Forms 1 triple bond (1 σ, 2 π) and 1 single bond (σ bond), OR 2 double bonds. Linear geometry. Bond angle = 180°. Example: Ethyne (C₂H₂).
• Sigma (σ) and Pi (π) Bonds:
  • σ Bond: Formed by head-on (axial) overlap of atomic orbitals. Stronger bond. Allows free rotation around the bond axis. All single bonds are σ bonds.
  • π Bond: Formed by sideways (lateral) overlap of unhybridized p-orbitals. Weaker bond. Restricts rotation around the bond axis. Present in multiple bonds (double bond = 1σ + 1π; triple bond = 1σ + 2π).

2. Structural Representations of Organic Compounds:

• Complete Structural Formula (Lewis Structure): Shows all atoms and all bonds connecting them.
• Condensed Structural Formula: Omits some or all covalent bonds and indicates identical groups attached to an atom by a subscript. Example: CH₃CH₂CH₃ for propane.
• Bond-Line Structural Formula (Zig-Zag Notation): Carbon atoms are represented by line ends and intersections. Hydrogen atoms attached to carbon are assumed to satisfy valency unless shown otherwise. Heteroatoms (O, N, S, Halogens) and hydrogens attached to them are shown. Example: A simple zig-zag line represents an alkane chain.
• 3-D Representation (Wedge-Dash):
  • Solid Wedge ( ): Bond projecting out of the plane towards the observer.
  • Dashed Wedge ( ): Bond projecting behind the plane away from the observer.
  • Normal Line (—): Bond lying in the plane of the paper.

3. Classification of Organic Compounds:

• Based on Structure:
  • Acyclic or Open Chain Compounds (Aliphatic): Carbon atoms form open chains (straight or branched). E.g., Butane, Isobutane.
  • Cyclic or Closed Chain or Ring Compounds: Carbon atoms form closed rings.
    • Alicyclic: Cyclic compounds resembling aliphatic compounds. E.g., Cyclohexane, Cyclohexene.
    • Aromatic: Contain at least one benzene ring (benzenoid) or exhibit aromatic character based on Hückel's rule (non-benzenoid). E.g., Benzene, Naphthalene, Aniline, Pyridine.
• Based on Functional Group:
  • Functional Group: An atom or group of atoms responsible for the characteristic chemical properties of an organic compound.
  • Homologous Series: A series of compounds with the same functional group and similar chemical properties, where successive members differ by a -CH₂ group. They have the same general formula.

4. Nomenclature of Organic Compounds (IUPAC System):

• Key Components: Word Root (indicates number of C atoms in parent chain) + Primary Suffix (indicates saturation/unsaturation) + Secondary Suffix (indicates principal functional group) + Prefix (indicates substituents or side chains).
• General Rules:
  1. Longest Chain Rule: Select the longest continuous carbon chain as the parent chain.
  2. Lowest Number/Locant Rule: Number the parent chain starting from the end that gives the lowest number to the substituent or functional group.
  3. Lowest Sum Rule: If multiple substituents are present, number from the end that gives the lowest sum of locants.
  4. Alphabetical Order: Name substituents alphabetically. Use di-, tri-, tetra- for identical substituents (these prefixes are not considered for alphabetization).
  5. Functional Group Priority: If multiple functional groups are present, one is chosen as the principal functional group (highest priority, determines secondary suffix), while others are treated as substituents (prefixes). Priority Order (simplified): -COOH > -SO₃H > -COOR > -COCl > -CONH₂ > -CN > -CHO > >C=O > -OH > -NH₂ > C=C > C≡C > -OR > -X (Halogen) > -NO₂ > -R (Alkyl).
  6. Cyclic Compounds: Use prefix 'cyclo-' before the word root. Numbering starts from the carbon bearing the principal functional group or substituent giving the lowest locant.

5. Isomerism:

• Compounds having the same molecular formula but different physical and/or chemical properties due to different arrangements of atoms.
• Structural Isomerism: Difference in the connectivity of atoms.
  • Chain Isomerism: Different carbon skeletons (parent chain length). E.g., n-butane and isobutane (C₄H₁₀).
  • Position Isomerism: Different positions of the same functional group or substituent on the same carbon skeleton. E.g., Propan-1-ol and Propan-2-ol (C₃H₈O).
  • Functional Isomerism: Different functional groups. E.g., Ethanol (alcohol) and Dimethyl ether (ether) (C₂H₆O).
  • Metamerism: Different alkyl groups attached to the same polyvalent functional group (like -O-, -S-, -NH-, -COO-). E.g., Diethyl ether (C₂H₅OC₂H₅) and Methyl propyl ether (CH₃OC₃H₇).
• Stereoisomerism: Same connectivity but different spatial arrangement of atoms/groups. (Detailed in Class 12, but basic concept needed).
  • Geometrical Isomerism (cis-trans): Arises due to restricted rotation around a C=C bond (or in cyclic compounds). Requires two different groups on each carbon of the double bond. cis: similar groups on the same side; trans: similar groups on opposite sides.

6. Fundamental Concepts in Organic Reaction Mechanism:

• Fission of a Covalent Bond:
  • Homolytic Cleavage (Homolysis): Symmetrical breaking; each atom gets one electron from the shared pair. Forms free radicals (neutral species with an unpaired electron). Favoured by non-polar conditions, heat, light (UV), peroxides.
  • Heterolytic Cleavage (Heterolysis): Unsymmetrical breaking; one atom takes both electrons. Forms ions.
    • Carbocation: Positively charged carbon (C⁺). Formed when the leaving group takes the electron pair.
    • Carbanion: Negatively charged carbon (C⁻). Formed when the carbon atom retains the electron pair.
    • Favoured by polar solvents.
• Stability of Intermediates:
  • Carbocations: 3° > 2° > 1° > Methyl (due to +I effect and hyperconjugation). Allylic and benzylic are highly stable due to resonance.
  • Carbanions: Methyl > 1° > 2° > 3° (due to +I effect destabilizing the negative charge). Allylic and benzylic are stabilized by resonance. Electron-withdrawing groups (-I, -R) stabilize carbanions.
  • Free Radicals: 3° > 2° > 1° > Methyl (similar reasoning to carbocations - hyperconjugation and +I effect). Allylic and benzylic are highly stable due to resonance.
• Attacking Reagents:
  • Electrophiles (E⁺): Electron-deficient species. Seek electron-rich centres. Can be positively charged (H⁺, NO₂⁺, CH₃⁺) or neutral with electron deficiency (BF₃, AlCl₃, SO₃). Lewis acids.
  • Nucleophiles (Nu⁻): Electron-rich species. Seek electron-deficient centres (positive charge or partial positive charge). Can be negatively charged (OH⁻, CN⁻, Cl⁻) or neutral with lone pairs (H₂O, NH₃, R-OH). Lewis bases.
• Electron Movement in Reactions: Shown using curved arrows. Arrow starts from the electron source (lone pair, bond) and points towards the electron sink (atom accepting electrons).
• Electronic Effects in Covalent Bonds: Permanent or temporary electron displacement effects influencing reactivity.
  • Inductive Effect (I Effect): Permanent polarization of a σ bond due to electronegativity difference. Transmitted along the chain but weakens rapidly with distance.
    • -I Effect: Electron-withdrawing groups (EWG): -NO₂, -CN, -COOH, -F, -Cl, -Br, -I, -OH, -OR etc.
    • +I Effect: Electron-donating groups (EDG): Alkyl groups (-CH₃, -C₂H₅ etc.).
  • Electromeric Effect (E Effect): Temporary effect. Complete transfer of a shared pair of π electrons to one of the atoms joined by a multiple bond in the presence of an attacking reagent.
    • +E Effect: π electrons transfer towards the attacking reagent (occurs when electrophile attacks).
    • -E Effect: π electrons transfer away from the attacking reagent (occurs when nucleophile attacks).
  • Resonance Effect (R Effect or Mesomeric Effect, M): Permanent effect. Delocalization (movement) of π electrons or lone pairs through a conjugated system (alternating single and multiple bonds/lone pairs). Represented by resonance structures (canonical forms). The actual structure is a resonance hybrid.
    • -R Effect (-M Effect): Group withdraws electrons from the conjugated system: -NO₂, -CN, -CHO, >C=O, -COOH, -SO₃H.
    • +R Effect (+M Effect): Group donates electrons to the conjugated system: -OH, -OR, -NH₂, -NR₂, -X (Halogens - note: halogens are -I > +R), -O⁻.
  • Hyperconjugation (No-Bond Resonance): Permanent effect. Delocalization of σ electrons of a C-H bond of an alkyl group directly attached to an atom of an unsaturated system (C=C, C≡C) or to an atom with an unshared p-orbital (carbocation, free radical). Involves overlap between σ(C-H) and adjacent π orbital or p-orbital. Stabilizes alkenes, carbocations, free radicals. Number of α-hydrogens determines the extent of hyperconjugation.

7. Methods of Purification of Organic Compounds:

• Sublimation: For solids that sublime (pass directly from solid to vapour on heating) from non-volatile impurities. E.g., Naphthalene, Camphor, Iodine, Anthracene.
• Crystallisation: Based on difference in solubilities of the compound and impurities in a suitable solvent. Compound is much more soluble at higher temperature than at lower temperature. Impurities are either insoluble or much more soluble.
• Distillation: For volatile liquids from non-volatile impurities OR liquids with sufficient difference in boiling points.
  • Simple Distillation: Difference in b.p. > 25-30 °C.
  • Fractional Distillation: Difference in b.p. is small. Uses a fractionating column (provides large surface area for repeated vaporization-condensation). E.g., Separating ethanol-water, components of crude oil.
  • Steam Distillation: For compounds that are steam volatile and immiscible with water. Purified at a temperature lower than its normal b.p. Useful for compounds that decompose near their b.p. E.g., Aniline, essential oils.
  • Distillation under Reduced Pressure (Vacuum Distillation): For liquids that decompose at or below their boiling points. Lowering the pressure lowers the boiling point. E.g., Glycerol.
• Differential Extraction: Based on different solubilities of an organic compound in two immiscible solvents. Usually aqueous and organic layers. Compound is extracted from one solvent layer to another in which it is more soluble.
• Chromatography: Based on differential adsorption or partition of components of a mixture between a stationary phase and a mobile phase.
  • Adsorption Chromatography: Stationary phase is solid adsorbent (silica gel, alumina). Mobile phase is liquid (solvent).
    • Column Chromatography: Stationary phase packed in a glass tube. Mixture applied at top, eluted with solvent. More strongly adsorbed components move slower.
    • Thin-Layer Chromatography (TLC): Stationary phase coated as a thin layer on glass plate. Used for checking purity, identifying compounds, monitoring reactions. R<0xE2><0x82><0x91> value (Retention factor) = (Distance travelled by substance) / (Distance travelled by solvent front).
  • Partition Chromatography: Based on continuous differential partitioning of components between stationary (liquid held on solid support) and mobile (liquid or gas) phases.
    • Paper Chromatography: Special paper (cellulose with adsorbed water) acts as stationary phase. Mobile phase is solvent. Similar principle to TLC.

8. Qualitative Analysis of Organic Compounds (Detection of Elements):

• Carbon & Hydrogen: Heat compound with dry Copper(II) oxide (CuO). Carbon → CO₂ (turns lime water milky), Hydrogen → H₂O (turns anhydrous CuSO₄ blue).
• Other Elements (N, S, X, P): Lassaigne's Test (Sodium Fusion Test): Fuse organic compound with metallic sodium to convert elements into ionic sodium salts (NaCN, Na₂S, NaX, Na₃PO₄). Extract the fused mass with distilled water to get Sodium Fusion Extract (SFE).
  • Nitrogen: SFE + FeSO₄ (freshly prepared) + conc. H₂SO₄ → Heat → Cool → Add few drops conc. H₂SO₄ along sides. Prussian blue/green colour/precipitate indicates N (formation of Ferric ferrocyanide, Fe₄[Fe(CN)₆]₃·xH₂O).
  • Sulphur:
    • SFE + Sodium Nitroprusside solution → Intense violet colour indicates S.
    • SFE + Acetic acid + Lead acetate solution → Black precipitate (PbS) indicates S.
  • Halogens (X): SFE + dil. HNO₃ (to decompose NaCN, Na₂S if present) → Boil → Cool → Add AgNO₃ solution.
    • White ppt (AgCl), soluble in NH₄OH → Cl present.
    • Pale yellow ppt (AgBr), sparingly soluble in NH₄OH → Br present.
    • Yellow ppt (AgI), insoluble in NH₄OH → I present.
  • Phosphorus: Compound + Oxidising agent (e.g., sodium peroxide) → Heat → Extract with water → Boil with conc. HNO₃ → Add Ammonium molybdate → Yellow colour/precipitate indicates P (formation of Ammonium phosphomolybdate).

9. Quantitative Analysis of Organic Compounds (Estimation of Elements):

• Carbon & Hydrogen (Liebig's Method): Known mass of compound burnt completely in excess O₂ and CuO. C → CO₂ (absorbed in KOH solution), H → H₂O (absorbed in anhydrous CaCl₂). Calculate % from mass increase of absorbents.
  • % C = (12/44) × (Mass of CO₂ formed / Mass of substance taken) × 100
  • % H = (2/18) × (Mass of H₂O formed / Mass of substance taken) × 100
• Nitrogen:
  • Dumas Method: Known mass of compound heated with CuO in CO₂ atmosphere. N₂ gas collected over KOH solution. Volume measured. Calculate % N. Applicable to all nitrogenous compounds.
    • % N = (28 / 22400) × (Volume of N₂ at STP / Mass of substance taken) × 100 (Volume in mL)
  • Kjeldahl's Method: Known mass of compound heated with conc. H₂SO₄ (+ K₂SO₄ + CuSO₄). N converted to (NH₄)₂SO₄. Treated with excess NaOH. NH₃ liberated, absorbed in standard acid. Unreacted acid titrated with standard base. Calculate % N.
    • Limitation: Not applicable to compounds containing N in nitro (-NO₂), azo (-N=N-) groups, or N present in a ring (e.g., Pyridine), as N in these is not completely converted to (NH₄)₂SO₄ under these conditions.
    • % N = 1.4 × Molarity of acid × Basicity of acid × (Vol. of acid taken - Vol. of base used for back titration) / Mass of substance taken (in g)
    • OR % N = 1.4 × Normality of acid × (Volume of acid consumed by NH₃) / Mass of substance taken (in g)
• Halogens (Carius Method): Known mass of compound heated with fuming HNO₃ and AgNO₃ in a sealed Carius tube. Halogen converted to AgX precipitate. Filter, wash, dry, weigh AgX. Calculate % X.
  • % Cl = (35.5 / 143.5) × (Mass of AgCl / Mass of substance) × 100
  • % Br = (80 / 188) × (Mass of AgBr / Mass of substance) × 100
  • % I = (127 / 235) × (Mass of AgI / Mass of substance) × 100
• Sulphur (Carius Method): Known mass of compound heated with fuming HNO₃ (or sodium peroxide). S oxidized to H₂SO₄. Precipitated as BaSO₄ by adding BaCl₂ solution. Filter, wash, dry, weigh BaSO₄. Calculate % S.
  • % S = (32 / 233) × (Mass of BaSO₄ / Mass of substance) × 100
• Phosphorus: Known mass of compound heated with fuming HNO₃. P oxidized to H₃PO₄. Precipitated as Ammonium phosphomolybdate [(NH₄)₃PO₄·12MoO₃] by adding ammonia and ammonium molybdate OR as MgNH₄PO₄ (which on ignition gives Mg₂P₂O₇). Weigh the precipitate. Calculate % P.
  • % P = (31 / 1877) × (Mass of (NH₄)₃PO₄·12MoO₃ / Mass of substance) × 100
  • % P = (62 / 222) × (Mass of Mg₂P₂O₇ / Mass of substance) × 100
• Oxygen: Usually determined by difference: % O = 100 - (Sum of % of all other elements). Can also be determined directly (Aluise's method): Compound heated in N₂ stream, O converts to CO, which is then analyzed.

Multiple Choice Questions (MCQs):

1. What is the hybridization of the carbon atoms numbered 1, 2, and 3 respectively in the compound: CH₂=C(CH₃)-CH=CH₂ ?
   (a) sp², sp, sp²
   (b) sp², sp², sp²
   (c) sp³, sp², sp²
   (d) sp², sp³, sp²

2. Which of the following pairs represents position isomers?
   (a) CH₃CH₂OH and CH₃OCH₃
   (b) CH₃CH₂CH₂Cl and CH₃CHClCH₃
   (c) CH₃CH₂CH₂CH₃ and (CH₃)₃CH
   (d) CH₃COCH₃ and CH₃CH₂CHO

3. Arrange the following carbocations in order of increasing stability:
   (I) (CH₃)₃C⁺
   (II) CH₃CH₂⁺
   (III) CH₂=CH-CH₂⁺
   (IV) C₆H₅CH₂⁺
   (a) II < I < III < IV
   (b) II < III < I < IV
   (c) II < I < IV < III
   (d) II < III < IV < I (Note: Benzylic and Allylic stability relative to 3° can vary, but generally Benzylic > Allylic ≈ 3°) Let's assume standard order Benzylic > Allylic > 3° > 2° > 1° for this question. A slightly better order might be Benzylic > Allylic ≈ 3°. Let's re-evaluate based on common textbook orders. Often Benzylic > Allylic > 3°. Let's stick to that. So, II < I < III < IV.

4. Which of the following species is an electrophile?
   (a) NH₃
   (b) H₂O
   (c) SO₃
   (d) CN⁻

5. The purification technique used for separating glycerol from spent-lye in soap industry is:
   (a) Steam distillation
   (b) Simple distillation
   (c) Fractional distillation
   (d) Distillation under reduced pressure

6. In the Lassaigne's test for nitrogen in an organic compound, the Prussian blue colour is due to the formation of:
   (a) Sodium cyanide
   (b) Ferrous sulphate
   (c) Ferric ferrocyanide
   (d) Sodium ferrocyanide

7. The inductive effect involves:
   (a) Delocalization of π electrons
   (b) Displacement of σ electrons
   (c) Delocalization of σ electrons
   (d) Displacement of π electrons

8. Hyperconjugation involves overlap between which orbitals?
   (a) σ - σ
   (b) π - π
   (c) σ - p (or π)
   (d) p - p

9. Which method cannot be used for the estimation of nitrogen in an organic compound containing a nitro group?
   (a) Dumas method
   (b) Carius method
   (c) Kjeldahl's method
   (d) Victor Meyer method

10. The IUPAC name for the compound CH₃-CH(OH)-CH₂-C(CH₃)₂-CHO is:
    (a) 5-Hydroxy-2,2-dimethylhexanal
    (b) 2-Hydroxy-5,5-dimethylhexanal
    (c) 2,2-Dimethyl-5-hydroxyhexanal
    (d) 5,5-Dimethyl-2-hydroxyhexanal

Answers to MCQs:

1. (b) sp², sp², sp² (C1 is sp², C2 is sp², C3 is sp²)
2. (b) CH₃CH₂CH₂Cl (1-chloropropane) and CH₃CHClCH₃ (2-chloropropane) differ only in the position of the Cl atom.
3. (a) II (1°) < I (3°) < III (Allylic) < IV (Benzylic). (Based on the common stability order: Benzylic > Allylic > 3° > 2° > 1° > Methyl)
4. (c) SO₃ is electron deficient at the sulfur atom and acts as an electrophile. NH₃ and H₂O are nucleophiles (lone pairs). CN⁻ is a nucleophile.
5. (d) Glycerol has a high boiling point and decomposes before reaching it, hence purified by distillation under reduced pressure.
6. (c) Ferric ferrocyanide, Fe₄[Fe(CN)₆]₃.
7. (b) Displacement of σ electrons along a carbon chain due to electronegativity differences.
8. (c) Overlap of σ electrons of C-H bond with adjacent empty or partially filled p-orbital or π-orbital.
9. (c) Kjeldahl's method fails for nitro compounds.
10. (a) Longest chain including CHO group is 6 carbons (hexanal). Numbering starts from CHO (C1). Substituents: -OH at C5 (Hydroxy), two -CH₃ at C2 (dimethyl). So, 5-Hydroxy-2,2-dimethylhexanal.

Remember to thoroughly revise these concepts and practice problems from the NCERT textbook and Exemplar book. Good luck with your preparation!