1. Introduction

Organic chemistry studies carbon-containing compounds, fundamental to biology, medicine, and materials science. Carbon’s tetravalency and catenation enable diverse organic structures.

2. Unique Properties of Carbon

• Catenation: Carbon forms strong covalent bonds with itself enabling long chains, branched, and cyclic structures.
• Tetravalency: Carbon forms four covalent bonds providing molecular diversity.
• Small atomic size: Enables strong sigma bonds with other atoms.
• Hybridization: Carbon exhibits sp³, sp², and sp hybridization, influencing molecule shape and bond angles.

3. Shapes and Hybridization

• Methane (CH₄): sp³ tetrahedral shape with 109.5° bond angle.
• Ethene (C₂H₄): sp² trigonal planar with 120° bond angle.
• Ethyne (C₂H₂): sp linear with 180° bond angle.

4. Structural Representations

Organic molecules can be drawn as:

• Lewis structures: Show all atoms, bonds, electron pairs.
• Condensed formulas: Show groups of atoms together, e.g. CH₃CH₂OH.
• Bond-line (skeletal) structures: Zig-zag lines representing carbon backbones simplifying drawing.

5. Classification of Organic Compounds

• Acyclic (Open-chain): Straight or branched chains.
• Cyclic: Carbon atoms arranged in rings.
• Aromatic: Cyclic compounds with conjugated π bonds, e.g. benzene.

6. Functional Groups & Homologous Series

Functional groups are reactive centers defining compound properties. Homologous series shares functional group differing by CH₂ units.

7. IUPAC Nomenclature Rules with Examples

1. Longest chain: Identify longest continuous carbon chain.
2. Number chain: Number starting nearest to a substituent.
3. Name substituents: Identify and name side groups.
4. Assign locants: Number substituents for lowest possible positions.
5. Multiple substituents: Use di-, tri-, tetra- prefixes.
6. Alphabetical order: Arrange substituents alphabetically.
7. Functional group priority: Give lowest number to the highest priority functional group.
8. Multiple bonds: Indicate position of double/triple bonds (–ene/–yne suffixes).

Examples:

• CH₃–CH₂–CH₂–CH₃: Butane
• CH₃–CH(CH₃)–CH₂–CH₃: 2-Methylbutane
• CH₂=CH–CH₂–CH₃: But-1-ene
• CH≡C–CH₂–CH₃: But-1-yne
• CH₃–CH₂–CH(CH₃)–Br: 3-Bromo-2-methylbutane

8. Isomerism

• Structural isomerism: Same molecular formula, different atom connectivity.
• Geometrical isomerism: Cis–trans isomers around double bonds.
• Optical isomerism: Non-superimposable mirror images due to chirality.

9. Electronic Effects (Detailed)

9.1 Inductive Effect (–I and +I)

Electron density shift through sigma bonds due to electronegativity differences.

• –I (electron withdrawing): Groups like –NO₂, –CN, –Cl pull electron density away, creating partial positive charge.
• +I (electron donating): Alkyl groups (–CH₃, –C₂H₅) push electron density, stabilizing positive centers.

Example: Halogen substituents in haloalkanes exert –I effect making carbons electrophilic.

9.2 Resonance Effect (–R and +R)

Electron delocalization through pi bonds or lone pairs across conjugated systems.

• –R (electron withdrawing): –NO₂, –CN groups pull electron density via resonance.
• +R (electron donating): –OH, –OR, –NH₂ donate electron density.

Example: Phenol’s –OH group increases electron density on aromatic ring via +R.

9.3 Electromeric Effect

Temporary polarization of pi electrons in presence of reagent: +E and –E effects.

Example: Electrophilic addition to alkene caused by polarization of double bond electrons.

9.4 Hyperconjugation

Delocalization of σ-electrons of C–H or C–C bonds into adjacent empty or π orbitals, stabilizing carbocations or radicals.

Example: Tertiary carbocations are more stable than primary due to more hyperconjugation.

10. Reaction Mechanisms in Organic Chemistry

Reaction mechanisms describe step-by-step how reactants convert into products by the breaking and forming of bonds, often via intermediates. Understanding mechanisms helps predict reaction behavior and outcomes.

Types of Primary Reaction Mechanisms

• Substitution Reactions: An atom or group in a molecule is replaced by another.
  Example: Halogenation of methane: CH₄ + Cl₂ → CH₃Cl + HCl.
• Addition Reactions: Atoms add across multiple bonds (double/triple bonds) converting unsaturated compounds to saturated or less unsaturated.
  Example: Hydrogenation of ethene: C₂H₄ + H₂ → C₂H₆.
• Elimination Reactions: Removal of atoms/groups from adjacent carbons resulting in formation of double/triple bonds.
  Example: Dehydrohalogenation: CH₃CH₂Br → CH₂=CH₂ + HBr.
• Rearrangement Reactions: Molecular reorganization leading to isomers by shifting atoms or bonds.
  Example: Pinacol rearrangement of 1,2-diols to ketones or aldehydes.

Mechanisms can be further classified into:

• Nucleophilic substitution (SN1 and SN2)
• Electrophilic substitution (characteristic of aromatic compounds)
• Radical reactions

11. Purification Techniques of Organic Compounds

Purification processes remove impurities and separate compounds based on differences in physical and chemical properties. Common methods include:

Crystallization

Used to purify solid compounds. The impure solid is dissolved in hot solvent; upon cooling, pure crystals form and can be collected. Insoluble impurities remain in solution.

Distillation

Separates liquids based on different boiling points.

• Simple Distillation: For liquids with significantly different boiling points.
• Fractional Distillation: For liquids with closer boiling points using a fractionating column.
• Steam Distillation: For temperature-sensitive liquids, distilled with steam.

Sublimation

Purifies solids that transition directly from solid to gas without melting.

Chromatography

Separates mixture components based on differential adsorption between mobile and stationary phases.

• Thin Layer Chromatography (TLC): Analytical and preparative tool.
• Column Chromatography: Preparative technique for pure compounds.
• Gas Chromatography: Separation of volatile compounds.
• Paper Chromatography: Analytical method for small amounts.

12. Qualitative and Quantitative Analysis of Organic Compounds

Qualitative Analysis

Determines the presence of elements:

• Test for Carbon and Hydrogen: Heating organic compounds with soda lime releases CO₂ and H₂O.
• Test for Nitrogen: Lassaigne’s test forms Prussian blue color indicating nitrogen presence.
• Test for Sulfur: Formation of black precipitate of PbS on treatment with lead acetate.
• Test for Halogens: Halide ions react with silver nitrate forming precipitates (AgCl, AgBr, AgI).

Quantitative Analysis

Determines the percentage composition of elements:

• Percentage of Carbon: From volume of CO₂ produced on combustion.
• Percentage of Hydrogen: From volume of H₂O produced.
• Percentage of Nitrogen: From gravimetric methods involving Kjeldahl process.
• Percentage of Sulfur and Halogens: Gravimetric/titration methods.

The data from elemental analysis helps to determine empirical and molecular formulas.

Important Questions & Answers