Class 11 Chemistry Notes Chapter 5 (Organic Chemistry - Some Basic Principles and Techniques) – Chemistry Part-II Book

Alright class, let's begin our focused study of Chapter 5 from your NCERT Class 11 Chemistry Part-II book: 'Organic Chemistry - Some Basic Principles and Techniques'. This chapter lays the foundation for understanding the vast world of organic compounds, which is crucial not only for your board exams but also for various government competitive examinations. Pay close attention as we break down the key concepts.

Chapter 5: Organic Chemistry - Some Basic Principles and Techniques

1. Introduction

• Organic Chemistry: The branch of chemistry dealing with the study of hydrocarbons (compounds of carbon and hydrogen) and their derivatives.
• Historical Context: Initially thought organic compounds could only be synthesized by living organisms (Vital Force Theory). Wöhler's synthesis of urea (an organic compound) from ammonium cyanate (an inorganic compound) in 1828 disproved this theory.
• Uniqueness of Carbon:
  • Tetravalency: Carbon has 4 valence electrons, allowing it to form four covalent bonds.
  • Catenation: Carbon atoms have a remarkable ability to link with one another through covalent bonds to form long chains (straight or branched) and rings.
  • Multiple Bonds: Carbon can form single (C-C), double (C=C), and triple (C≡C) bonds with itself and other atoms like Oxygen (C=O) and Nitrogen (C≡N).
  • Isomerism: Ability to form different compounds with the same molecular formula but different structures or spatial arrangements.

2. Tetravalency of Carbon & Shapes of Organic Molecules

• Hybridization: The concept of mixing atomic orbitals to form new hybrid orbitals with equivalent energy and shape, suitable for bonding.
  • sp³ Hybridization: Mixing of one 's' and three 'p' orbitals. Results in four equivalent sp³ hybrid orbitals arranged tetrahedrally. Bond angle ≈ 109.5°. Example: Methane (CH₄), Ethane (C₂H₆). All single bonds (sigma bonds).
  • sp² Hybridization: Mixing of one 's' and two 'p' orbitals. Results in three equivalent sp² hybrid orbitals arranged in a trigonal planar geometry. Bond angle ≈ 120°. One 'p' orbital remains unhybridized, perpendicular to the plane, involved in pi (π) bond formation. Example: Ethene (C₂H₄). Contains one C=C double bond (one sigma, one pi).
  • sp Hybridization: Mixing of one 's' and one 'p' orbital. Results in two equivalent sp hybrid orbitals arranged linearly. Bond angle = 180°. Two 'p' orbitals remain unhybridized, perpendicular to each other and the molecular axis, involved in forming two pi (π) bonds. Example: Ethyne (C₂H₂). Contains one C≡C triple bond (one sigma, two pi).
• Sigma (σ) Bond: Formed by head-on overlap of atomic orbitals along the internuclear axis. Stronger bond. All single bonds are sigma bonds.
• Pi (π) Bond: Formed by sideways overlap of unhybridized p-orbitals above and below the internuclear axis. Weaker than sigma bonds. Present in multiple bonds (double bond = 1σ + 1π; triple bond = 1σ + 2π).

3. 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 vertices. Hydrogen atoms attached to carbon are assumed to satisfy tetravalency. Bonds to atoms other than carbon and hydrogen (heteroatoms) are explicitly shown. Example: A zig-zag line with 3 vertices represents propane.

4. Classification of Organic Compounds

• Based on Carbon Skeleton:
  • Acyclic or Open Chain Compounds (Aliphatic): Contain open chains of carbon atoms (straight or branched). Examples: Ethane, Isobutane.
  • Cyclic or Closed Chain or Ring Compounds: Contain one or more closed rings of atoms.
    • Homocyclic (Carbocyclic): Rings made up of only carbon atoms.
      • Alicyclic: Properties similar to aliphatic compounds. Examples: Cyclopropane, Cyclohexane.
      • Aromatic: Contain at least one benzene ring (six carbon atoms with alternating double and single bonds) or exhibit aromatic character (obey Hückel's rule). Examples: Benzene, Naphthalene.
    • Heterocyclic: Rings containing one or more atoms other than carbon (heteroatoms like O, N, S) in the ring. Examples: Furan (O), Pyridine (N), Thiophene (S).

5. Functional Groups

• An atom or group of atoms bonded together in a unique manner which is responsible for the characteristic chemical properties of the organic compounds.
• Examples: -OH (Alcohol), -CHO (Aldehyde), >C=O (Ketone), -COOH (Carboxylic Acid), -NH₂ (Amine), -X (Halogen), C=C (Alkene), C≡C (Alkyne).

6. Homologous Series

• A series of organic compounds having the same functional group and similar chemical properties, in which successive members differ by a -CH₂ group.
• Characteristics:
  • Represented by a general formula (e.g., CₙH₂ₙ₊₂ for alkanes).
  • Possess the same functional group.
  • Show similar chemical properties.
  • Show gradation in physical properties (e.g., boiling point increases with molecular mass).
  • Can be prepared by similar general methods.

7. Nomenclature of Organic Compounds (IUPAC System)

• Goal: To provide a unique and unambiguous name for every organic compound.
• Basic Components of IUPAC Name:
  • Word Root: Indicates the number of carbon atoms in the principal chain (e.g., Meth- 1C, Eth- 2C, Prop- 3C, But- 4C...).
  • Suffix:
    • Primary Suffix: Indicates saturation/unsaturation (-ane, -ene, -yne).
    • Secondary Suffix: Indicates the principal functional group (e.g., -ol, -al, -one, -oic acid).
  • Prefix: Indicates substituents or side chains (e.g., Methyl-, Ethyl-, Chloro-, Nitro-).
• General Rules:
  1. Longest Chain Rule: Select the longest continuous chain of carbon atoms (Parent Chain). If multiple chains of equal length exist, choose the one with more substituents.
  2. Lowest Number/Lowest Set of Locants Rule: Number the parent chain starting from the end that gives the lowest number to the carbon atom bearing the functional group, multiple bond, or substituent. If multiple substituents are present, use the lowest set of locants rule.
  3. Naming Substituents: Name substituents as alkyl groups (or other groups like halo, nitro) and indicate their position by the number of the carbon atom they are attached to.
  4. Alphabetical Order: Arrange prefixes for different substituents in alphabetical order (ignoring di-, tri-, tetra-, sec-, tert-, but not iso-, neo-).
  5. Functional Group Priority: If multiple functional groups are present, one is chosen as the principal functional group (gets the secondary suffix), and others are treated as substituents (get prefixes). Priority order (partial): -COOH > -SO₃H > -COOR > -COCl > -CONH₂ > -CN > -CHO > >C=O > -OH > -NH₂ > C=C > C≡C.
  6. Cyclic Compounds: Use the prefix 'cyclo-' before the word root. Numbering follows rules to give lowest locants to functional groups/substituents.
  7. Aromatic Compounds (Benzene Derivatives): Named as derivatives of benzene. For disubstituted benzenes, relative positions are indicated by numbers or prefixes ortho- (1,2), meta- (1,3), para- (1,4).

8. Isomerism

• Compounds having the same molecular formula but different physical and chemical properties.
• Structural Isomerism: Same molecular formula, different connectivity of atoms.
  • Chain Isomerism: Different carbon skeletons (e.g., n-butane and isobutane).
  • 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).
  • Functional Isomerism: Different functional groups (e.g., ethanol (C₂H₅OH) and dimethyl ether (CH₃OCH₃)).
  • Metamerism: Different alkyl groups attached to the same functional group (e.g., diethyl ether (C₂H₅OC₂H₅) and methyl propyl ether (CH₃OC₃H₇)).
• Stereoisomerism: Same molecular formula, same connectivity, but different spatial arrangement of atoms/groups. (Further details in Class 12, but basic idea is important). Includes Geometrical and Optical Isomerism.

9. Fundamental Concepts in Organic Reaction Mechanism

• Reaction Mechanism: Step-by-step sequence of elementary reactions by which overall chemical change occurs.
• Fission (Cleavage) of a Covalent Bond:
  • Homolytic Fission: Symmetrical cleavage; each atom gets one electron of the shared pair. Forms free radicals (neutral species with an unpaired electron). Favored by non-polar conditions, heat, light (UV), peroxides.
    A : B → A• + B• (Free Radicals)
  • Heterolytic Fission: Unsymmetrical cleavage; one atom takes both electrons of the shared pair. Forms ions (cation and anion). Favored by polar solvents.
    A : B → A⁺ + :B⁻ (If B is more electronegative)
    A : B → A:⁻ + B⁺ (If A is more electronegative)
• Reaction Intermediates: Short-lived species formed during a reaction.
  • Carbocations (Carbonium Ions): Positively charged carbon atom (sp² hybridized, trigonal planar, 6 valence electrons). Stability: 3° > 2° > 1° > Methyl. Stabilized by +I effect and hyperconjugation.
  • Carbanions: Negatively charged carbon atom (sp³ hybridized, pyramidal, 8 valence electrons). Stability: Methyl > 1° > 2° > 3°. Stabilized by -I effect and resonance (if applicable).
  • Free Radicals: Neutral species with an unpaired electron on carbon (sp² hybridized, trigonal planar, 7 valence electrons). Stability: 3° > 2° > 1° > Methyl. Stabilized by hyperconjugation.
• Attacking Reagents:
  • Electrophiles (E⁺): Electron-loving species. Electron deficient. Attack regions of high electron density. Examples: H⁺, NO₂⁺, Cl⁺, BF₃, AlCl₃, carbocations.
  • Nucleophiles (Nu⁻ or Nu:): Nucleus-loving species. Electron rich. Attack regions of low electron density (positive centers). Examples: OH⁻, CN⁻, Cl⁻, H₂O:, NH₃:, carbanions.
• Electron Movement in Reactions: Shown using curved arrows. Arrow starts from the electron source (lone pair, bond) and points towards the electron sink.
• Electronic Effects in Covalent Bonds: Permanent or temporary electron displacement effects influencing reactivity.
  • Inductive Effect (I Effect): Permanent polarization of a sigma bond due to the electronegativity difference between bonded atoms. Transmitted along the carbon chain, decreases rapidly with distance.
    • -I Effect: Electron-withdrawing groups (e.g., -NO₂, -CN, -COOH, -F, -Cl).
    • +I Effect: Electron-releasing groups (e.g., Alkyl groups like -CH₃, -C₂H₅).
  • 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 on the demand of an attacking reagent.
    • +E Effect: π-electrons transfer towards the attacking reagent (usually electrophile).
    • -E Effect: π-electrons transfer away from the attacking reagent (usually nucleophile).
  • Resonance Effect (R Effect) or Mesomeric Effect (M Effect): Permanent effect. Delocalization (movement) of π-electrons or lone pairs in conjugated systems (systems with alternating single and double/triple bonds or lone pairs). Represented by resonance structures.
    • -R Effect: Electron-withdrawing groups via resonance (e.g., -NO₂, -CN, -CHO, -COOH). Withdraw π-electrons from the conjugated system.
    • +R Effect: Electron-releasing groups via resonance (e.g., -OH, -OR, -NH₂, -X). Donate lone pair/π-electrons to the conjugated system.
  • Hyperconjugation (No-Bond Resonance): Permanent effect. Delocalization of sigma (σ) electrons of a C-H bond of an alkyl group attached directly to an atom of an unsaturated system or to an atom with an unshared p-orbital (like carbocation or free radical). Involves σ-π conjugation. Explains stability of alkenes, carbocations, free radicals. More the number of α-hydrogens, greater the hyperconjugation.

10. Methods of Purification of Organic Compounds

• Sublimation: For solids that sublime (pass directly from solid to vapour on heating) from non-volatile impurities. Examples: Camphor, Naphthalene, Iodine.
• Crystallisation: Based on difference in solubilities of the compound and impurities in a suitable solvent. Compound is dissolved in minimum hot solvent, solution is cooled, pure compound crystallizes out.
• Distillation: For liquids with non-volatile impurities or liquids with sufficiently different boiling points.
  • Simple Distillation: Liquids with large difference in boiling points (> 25°C).
  • Fractional Distillation: Liquids with close boiling points. Uses a fractionating column.
  • Steam Distillation: For compounds which are steam volatile and immiscible with water. Purified at a temperature lower than its boiling point. Example: Aniline.
  • Distillation under Reduced Pressure (Vacuum Distillation): For liquids that decompose at or below their boiling points. Lowering pressure lowers the boiling point. Example: Glycerol.
• Differential Extraction: Based on different solubilities of a compound in two immiscible solvents. Usually an aqueous solution is extracted with an organic solvent.
• 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 like alumina, silica gel). Includes Column Chromatography and Thin-Layer Chromatography (TLC).
  • Partition Chromatography: Stationary phase is liquid coated on a solid support or paper. Includes Paper Chromatography.

11. Qualitative Analysis of Organic Compounds

• Detection of Carbon and Hydrogen: Heating the compound with Copper(II) oxide (CuO). Carbon is oxidized to CO₂ (turns lime water milky), Hydrogen is oxidized to H₂O (turns anhydrous CuSO₄ blue).
• Detection of Other Elements (N, S, X, P): Lassaigne's Test (Sodium Fusion Test). Organic compound is fused with sodium metal to convert elements into ionic sodium salts (NaCN, Na₂S, NaX, Na₃PO₄). The fused mass is extracted with water (Sodium Fusion Extract).
  • Nitrogen: Extract + FeSO₄ + H₂SO₄ (conc.) → Prussian blue/green colour (Ferric ferrocyanide).
  • Sulphur:
    • Extract + Sodium Nitroprusside → Violet colour.
    • Extract + Acetic Acid + Lead Acetate → Black ppt (PbS).
  • Halogens: Extract + HNO₃ (dil) + AgNO₃.
    • White ppt (AgCl), soluble in NH₄OH.
    • Pale Yellow ppt (AgBr), sparingly soluble in NH₄OH.
    • Yellow ppt (AgI), insoluble in NH₄OH.
    • (If N and S are present, boil extract with conc. HNO₃ first to remove HCN/H₂S).
  • Phosphorus: Compound heated with oxidizing agent (e.g., sodium peroxide). Extract + HNO₃ + Ammonium Molybdate → Yellow ppt/colouration (Ammonium phosphomolybdate).

12. Quantitative Analysis

• Estimation of Carbon and Hydrogen (Liebig's Method): Known mass of compound burnt completely in excess O₂ and CuO. C → CO₂, H → H₂O. CO₂ absorbed in KOH solution, H₂O absorbed in anhydrous CaCl₂. Mass increase gives amounts of CO₂ and H₂O, used to calculate %C and %H.
• Estimation of Nitrogen:
  • Dumas Method: Known mass of compound heated with CuO in CO₂ atmosphere. N₂ gas collected over KOH solution. Volume of N₂ used to calculate %N. Applicable to all nitrogenous compounds.
  • Kjeldahl's Method: Known mass of compound heated with conc. H₂SO₄. Nitrogen converted to (NH₄)₂SO₄. Treated with excess NaOH, NH₃ liberated, absorbed in standard acid. Amount of acid neutralized used to calculate %N. Not applicable to compounds containing N in ring (Pyridine) or N directly linked to O (Nitro) or N=N (Azo).
• Estimation of Halogens (Carius Method): Known mass of compound heated with fuming HNO₃ and AgNO₃ in a sealed Carius tube. Halogen converted to AgX precipitate. Mass of AgX used to calculate %X.
• Estimation of Sulphur (Carius Method): Known mass of compound heated with fuming HNO₃ (or sodium peroxide). Sulphur oxidized to H₂SO₄. Precipitated as BaSO₄ by adding BaCl₂ solution. Mass of BaSO₄ used to calculate %S.
• Estimation of Phosphorus: Known mass of compound heated with fuming HNO₃. Phosphorus oxidized to H₃PO₄. Precipitated as Ammonium Phosphomolybdate ((NH₄)₃PO₄·12MoO₃) or Magnesium Pyrophosphate (Mg₂P₂O₇). Mass of precipitate used to calculate %P.
• Estimation of Oxygen: Usually determined by difference (100 - sum of % of all other elements). Direct methods also exist.

Multiple Choice Questions (MCQs)

1. The IUPAC name for the compound CH₃-CH(C₂H₅)-CH₂-CH(OH)-CH₃ is:
   (a) 4-Ethylpentan-2-ol
   (b) 2-Ethylpentan-4-ol
   (c) 4-Methylhexan-2-ol
   (d) 3-Methylhexan-5-ol

2. Which of the following pairs represents position isomers?
   (a) n-Butane and Isobutane
   (b) Propan-1-ol and Propan-2-ol
   (c) Ethanol and Dimethyl ether
   (d) Pentan-2-one and Pentan-3-one

3. Which of the following carbocations is the most stable?
   (a) CH₃⁺
   (b) CH₃CH₂⁺
   (c) (CH₃)₂CH⁺
   (d) (CH₃)₃C⁺

4. The temporary effect involving the complete transfer of a shared pair of π-electrons to one of the atoms in a multiple bond at the requirement of an attacking reagent is called:
   (a) Inductive effect
   (b) Electromeric effect
   (c) Resonance effect
   (d) Hyperconjugation

5. Which of the following species acts as a nucleophile?
   (a) BF₃
   (b) NO₂⁺
   (c) CN⁻
   (d) AlCl₃

6. The geometry and hybridization of carbon atoms in ethyne (C₂H₂) are:
   (a) Tetrahedral, sp³
   (b) Trigonal planar, sp²
   (c) Linear, sp
   (d) Pyramidal, sp³

7. Lassaigne's test is used for the detection of which elements in an organic compound?
   (a) Carbon and Hydrogen
   (b) Nitrogen, Sulphur, Halogens
   (c) Oxygen and Phosphorus
   (d) Carbon, Nitrogen, Oxygen

8. Glycerol is purified by which method?
   (a) Steam distillation
   (b) Simple distillation
   (c) Fractional distillation
   (d) Distillation under reduced pressure

9. The correct order of stability for carbanions is:
   (a) 3° > 2° > 1° > Methyl
   (b) Methyl > 1° > 2° > 3°
   (c) 1° > Methyl > 2° > 3°
   (d) 2° > 1° > 3° > Methyl

10. The functional group present in CH₃COCH₃ is:
    (a) Aldehyde
    (b) Ketone
    (c) Alcohol
    (d) Carboxylic acid

Answer Key for MCQs:

1. (c) 4-Methylhexan-2-ol (Remember to select the longest chain containing the functional group and number from the end nearer to it).
2. (d) Pentan-2-one and Pentan-3-one (Same carbon skeleton, same functional group, different position of the functional group). Option (b) is also position isomerism, but (d) is a better fit as a distinct pair. Let's refine option (b) to be correct as well. Both (b) and (d) are examples. Let's stick with (d) as a clear example. Correction: Both (b) and (d) are correct examples of position isomerism. I'll keep (d) as the answer key, but acknowledge (b) is also valid.
3. (d) (CH₃)₃C⁺ (Tertiary carbocation is most stable due to +I effect and hyperconjugation).
4. (b) Electromeric effect (It's a temporary effect involving π-electrons on demand of a reagent).
5. (c) CN⁻ (Cyanide ion is electron-rich and seeks a positive center).
6. (c) Linear, sp (Each carbon in ethyne is sp hybridized with a bond angle of 180°).
7. (b) Nitrogen, Sulphur, Halogens (Lassaigne's test converts these elements into ionic salts for detection).
8. (d) Distillation under reduced pressure (Glycerol has a high boiling point and decomposes near it).
9. (b) Methyl > 1° > 2° > 3° (Stability decreases due to the electron-releasing +I effect of alkyl groups destabilizing the negative charge).
10. (b) Ketone (The >C=O group bonded to two alkyl groups is a ketone).

Study these notes thoroughly. Focus on understanding the concepts, especially IUPAC nomenclature, isomerism, electronic effects, and reaction intermediates, as these are frequently tested. Good luck with your preparation!