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Organic Chemistry Basics

Subject: Chemistry
Topic: 9
Cambridge Code: 0620 / 0971 / 5070

Hydrocarbons​

Hydrocarbons - Compounds of carbon and hydrogen only

General Formulas​

Alkanes: CₙH₂ₙ₊₂

  • Single bonds
  • Saturated
  • C-C and C-H bonds

Cycloalkanes: CₙH₂ₙ

  • Ring structure
  • Single bonds
  • Saturated

Alkenes: CₙH₂ₙ

  • Double bond (C=C)
  • Unsaturated
  • C-C, C=C, C-H bonds

Alkanes​

Alkanes - Hydrocarbons with single C-C bonds

Structure and Naming​

First 10 alkanes:

  1. Methane: CH₄
  2. Ethane: C₂H₆
  3. Propane: C₃H₈
  4. Butane: C₄H₁₀
  5. Pentane: C₅H₁₂
  6. Hexane: C₆H₁₄
  7. Heptane: C₇H₁₆
  8. Octane: C₈H₁₈
  9. Nonane: C₉H₂₀
  10. Decane: C₁₀H₂₂

Properties​

  • Combustion: Complete combustion in oxygen CnH2n+2+3n+12O2→nCO2+(n+1)H2OC_nH_{2n+2} + \frac{3n+1}{2}O_2 → nCO_2 + (n+1)H_2O

  • Inert: Unreactive with acids, bases, oxidizing agents

  • Hydrophobic: Insoluble in water

  • Boiling point: Increases with chain length

Isomerism​

Structural isomers - Same formula, different structures

Example: Butane (C₄H₁₀)

  • n-butane: CH₃-CH₂-CH₂-CH₃ (straight chain)
  • Isobutane: CH₃-CH(CH₃)-CH₃ (branched)

Different properties despite same formula

Alkenes​

Alkenes - Hydrocarbons with C=C double bond

Structure​

CnH2nC_n H_{2n}

  • Ethene: CH₂=CH₂
  • Propene: CH₃-CH=CH₂
  • Butene: CH₃-CH₂-CH=CH₂

Properties​

Double bond characteristics:

  • Electron-rich (π bond)
  • Reactive to electrophiles
  • Planar around double bond

Reactions of Alkenes​

Addition Reactions - Electrons from C=C break, forming two new single bonds

Hydrogenation (Addition of H₂): Alkene+H2→Alkane\text{Alkene} + H_2 → \text{Alkane}

  • Catalyst: Ni, Pd, Pt
  • Saturates the double bond

Hydration (Addition of H₂O): Alkene+H2O→Alcohol\text{Alkene} + H_2O → \text{Alcohol}

  • Catalyst: H₂SO₄
  • Example: Ethene + H₂O → Ethanol

Halogenation (Addition of X₂): Alkene+Br2→Dibromoalkane\text{Alkene} + Br_2 → \text{Dibromoalkane}

  • No catalyst needed
  • Decolorizes brown Br₂ (test for C=C)

Polymerization: n×Monomer→Polymern \times \text{Monomer} → \text{Polymer}

  • Addition polymerization
  • Example: Ethene → Polyethylene

Functional Groups​

Functional group - Characteristic group of atoms in molecule

Alcohols (-OH)​

General structure: R-OH

Nomenclature: -ol ending

  • Methanol: CH₃OH
  • Ethanol: CH₃CH₂OH
  • Propanol: CH₃CH₂CH₂OH

Properties:

  • Hydrogen bonding (high bp, soluble)
  • Oxidation reactions
  • Aldehyde or ketone (depending on position)

Carboxylic Acids (-COOH)​

General structure: R-COOH

Nomenclature: -oic acid ending

  • Methanoic acid: HCOOH
  • Ethanoic acid: CH₃COOH
  • Propanoic acid: CH₃CH₂COOH

Properties:

  • Acidic (pH < 7)
  • Ionic bonding with bases (salts)
  • Oxidation of aldehydes

Aldehydes (-CHO)​

General structure: R-CHO

Nomenclature: -al ending

  • Methanal: HCHO
  • Ethanal: CH₃CHO
  • Propanal: CH₃CH₂CHO

Properties:

  • Oxidized to carboxylic acids
  • Reduced to alcohols
  • Nucleophilic addition

Ketones (C=O)​

General structure: R-CO-R'

Nomenclature: -one ending

  • Propanone: CH₃-CO-CH₃

Isomerism​

Structural Isomerism​

Chain isomerism: Different carbon chain arrangements

  • Examples: n-butane vs isobutane

Position isomerism: Double bond or group at different position

  • Examples: 1-butene vs 2-butene

Functional group isomerism: Different functional groups

  • Examples: Ethanol (alcohol) vs dimethyl ether (ether)

Stereoisomerism​

Geometric isomerism: cis-trans about C=C

  • Cis: Same side
  • Trans: Opposite sides

Combustion​

Complete combustion - Burns in oxygen, produces CO₂ and H₂O

CxHy+(x+y4)O2→xCO2+y2H2OC_xH_y + (x + \frac{y}{4})O_2 → xCO_2 + \frac{y}{2}H_2O

Example: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Energy: Highly exothermic (used as fuels)

Polymers​

Polymers - Long chains of repeated units

Addition Polymerization​

Monomers with C=C bonds join:

  • Ethene → Polyethylene
  • Propene → Polypropylene

Condensation Polymerization​

Monomers release small molecules (H₂O) while joining:

  • Diols + Diacids → Polyesters
  • Amino acids → Proteins

Key Points​

  1. Hydrocarbons: C and H only
  2. Alkanes: CₙH₂ₙ₊₂, single bonds
  3. Alkenes: CₙH₂ₙ, C=C bond (unsaturated)
  4. Functional groups determine properties
  5. Structural isomers: Same formula, different structure
  6. Addition reactions: C=C double bond breaks
  7. Combustion: All produce CO₂ + H₂O

Practice Questions​

  1. Name alkanes and alkenes
  2. Draw structural formulas
  3. Identify functional groups
  4. Predict reactions of alkenes
  5. Identify isomers
  6. Write combustion equations
  7. Explain polymer formation

Revision Tips​

  • Learn alkane naming (prefixes)
  • Know functional group names
  • Understand isomerism types
  • Practice addition reactions
  • Know combustion products
  • Draw structures clearly
  • Know polymer types