Introduction to Organic Chemistry

🔹 Formulas, functional groups and the naming of organic compounds

1. Definition of Hydrocarbon

• A hydrocarbon is a compound made up of only carbon (C) and hydrogen (H) atoms.
• Two types:
  • Alkanes: saturated hydrocarbons (only single bonds)
  • Alkenes/Alkynes: unsaturated hydrocarbons (double/triple bonds)

2. Alkanes as Simple Hydrocarbons

• Alkanes are the simplest class of hydrocarbons.
• They have no functional group, only C–C and C–H single bonds.
• General formula: CₙH₂ₙ₊₂
• Example: Methane (CH₄), Ethane (C₂H₆)

3. Functional Groups and Their Role

• Functional groups are specific groups of atoms within molecules that determine their chemical and physical properties.
• Examples :
  • Alcohols (–OH)
  • Carboxylic acids (–COOH)
  • Aldehydes (–CHO)
  • Ketones (C=O)
  • Esters (–COO–)
  • Amines (–NH₂)
  • Halogenoalkanes (–Cl, –Br, –I)
  • Alkenes (C=C)
  • Nitriles (–CN)

4. Types of Formulae

• Empirical formula: Simplest whole-number ratio of atoms (e.g. CH₂O)
• Molecular formula: Actual number of atoms (e.g. C₂H₄O₂)
• Structural formula: Shows how atoms are arranged (e.g. CH₃CH₂OH)
• Displayed formula: Shows all atoms and bonds
• Skeletal formula: Shows carbon skeleton and functional groups; C and H on C not shown.

5. Nomenclature (IUPAC Rules)

• Prefix + Root + Suffix
• Rules include:
  • Longest carbon chain
  • Numbering for lowest position of functional groups
  • Branches and substituents named alphabetically
• Examples:
  • CH₃CH₂CH₂OH → Propan-1-ol
  • CH₃COOH → Ethanoic acid
  • CH₃CH₂COOCH₃ → Methyl propanoate
• Limitations:
  • Up to 6 carbon atoms (except esters: 6+6, straight chain only)

6. Deducing Empirical/Molecular Formulas

• Given structural, displayed, or skeletal formula, count atoms to deduce:
  • Empirical formula (e.g. CH₂)
  • Molecular formula (e.g. C₂H₄)
  • Techniques: atom counting, molecular mass analysis

🔹 Characteristic Organic Reactions

1. Terminology of Organic Chemistry

• Homologous series: Group with same functional group and general formula (e.g. alkanes, alcohols)
• Saturated: Only single bonds (e.g. alkanes)
• Unsaturated: Contains double/triple bonds (e.g. alkenes)
• Homolytic fission: Bond breaks evenly → radicals
  • A–B → A• + B•
• Heterolytic fission: Bond breaks unevenly → ions
  • A–B → A⁺ + B⁻
• Free radical: Atom/molecule with unpaired electron
• Initiation, Propagation, Termination: Steps in free-radical substitution (e.g. CH₄ + Cl₂)
• Nucleophile: Electron-rich species that donates a lone pair (e.g. OH⁻, NH₃)
• Electrophile: Electron-deficient species that accepts a pair (e.g. H⁺, Br⁺)
• Reaction types:
  • Addition: Atoms added across a double bond
  • Substitution: Atom/group replaced by another
  • Elimination: Removal of atom/group to form multiple bond
  • Hydrolysis: Splitting with water
  • Condensation: Two molecules join with loss of small molecule
  • Oxidation and Reduction:
    • Oxidation: [O] (gain of oxygen or loss of hydrogen)
    • Reduction: [H] (gain of hydrogen or loss of oxygen)

2. Organic Mechanisms

• Free-radical substitution (e.g. methane + chlorine under UV)
  • Initiation: Cl₂ → 2Cl•
  • Propagation: CH₄ + Cl• → CH₃• + HCl, CH₃• + Cl₂ → CH₃Cl + Cl•
  • Termination: Cl• + Cl• → Cl₂
• Electrophilic addition (e.g. ethene + HBr)
• Nucleophilic substitution (e.g. halogenoalkane + OH⁻ → alcohol)
• Nucleophilic addition (e.g. carbonyl compound + HCN)
• Curly arrows represent electron pair movement from lone pair or bond to new bonding site

🔹 Shapes of Organic Molecules; σ and π Bonds

1. Molecular Structure

• Molecules can be:
  • Straight-chained (e.g. propane)
  • Branched (e.g. 2-methylpropane)
  • Cyclic (e.g. cyclohexane)

2. Shape and Bond Angles Based on Hybridisation

• sp hybridisation: Linear, 180° (e.g. ethyne)
• sp² hybridisation: Trigonal planar, 120° (e.g. ethene)
• sp³ hybridisation: Tetrahedral, 109.5° (e.g. methane)

3. σ and π Bonds

• σ (sigma) bond: Single covalent bond, head-on overlap
• π (pi) bond: Sideways overlap, occurs in double/triple bonds
• Example:
  • Ethene: one σ and one π bond between C=C

4. Planarity in Organic Molecules

• Molecules like ethene (C₂H₄) are planar due to sp² hybridisation and restricted rotation of π bonds

🔹 Isomerism: Structural and Stereoisomerism

1. Structural Isomerism

Same molecular formula, different structure:

• Chain isomerism: Different carbon skeleton (e.g. butane vs. methylpropane)
• Positional isomerism: Same functional group, different position (e.g. butan-1-ol vs butan-2-ol)
• Functional group isomerism: Different functional groups (e.g. alcohol vs. ether)

2. Stereoisomerism

Same structural formula, different spatial arrangement:

• Geometrical (cis/trans): Due to restricted rotation around double bond
  • Cis: Same groups on same side
  • Trans: Opposite sides
• Optical isomerism: Due to chiral centre

3. Geometrical Isomerism in Alkenes

• Restricted rotation around C=C due to π bond
• Required:
  • Two different groups on each carbon of the double bond

4. Chiral Centre and Optical Isomers

• Chiral centre: Carbon atom with four different groups
• Results in enantiomers: non-superimposable mirror images

5. Identification of Isomerism

• From structural formula, identify:
  • Chiral centre (optical isomerism)
  • Geometrical isomerism (in alkenes and some rings)

6. Deducing Possible Isomers

• Use molecular formula to draw and count possible:
  • Structural isomers
  • Stereoisomers (cis/trans and optical)

✅ Key Tips for Exams

• Master formula types: skeletal vs displayed vs molecular
• Always count atoms when deducing formulas
• Practice IUPAC naming rules thoroughly
• Understand and draw reaction mechanisms with correct curly arrows
• Learn to identify functional groups quickly

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