Organic Chemistry – Hydrocarbons

Organic Chemistry: Study of carbon compounds (except CO, CO₂, carbonates, cyanides).

Why Special Study: Carbon forms millions of compounds due to catenation (self-linking), tetravalency, and ability to form multiple bonds.

Catenation: Property of carbon to form long chains by bonding with other carbon atoms. C-C bonds are strong.

Tetravalency: Carbon has valency 4, forms four covalent bonds (sp³, sp², sp hybridization).

Hydrocarbons: Organic compounds containing only carbon and hydrogen. General formula: CₓHᵧ.

Classification: Saturated (alkanes - single bonds) and Unsaturated (alkenes - double bonds, alkynes - triple bonds).

General Formula: CₙH₂ₙ₊₂. Single bonds only (saturated).

Homologous Series: CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane), C₅H₁₂ (pentane).

Nomenclature: -ane suffix. Meth- (1C), Eth- (2C), Prop- (3C), But- (4C), Pent- (5C).

Preparation of Alkanes:

- Methane (CH₄): From sodium ethanoate (sodium acetate) - CH₃COONa + NaOH → CH₄ + Na₂CO₃ (soda lime, heat).

- Ethane (C₂H₆): From sodium propanoate (sodium propionate) - C₂H₅COONa + NaOH → C₂H₆ + Na₂CO₃ (soda lime, heat).

- From alkyl halides: CH₃I + 2[H] → CH₄ + HI (iodomethane reduction), C₂H₅Br + 2[H] → C₂H₆ + HBr (bromoethane reduction).

Properties: Non-polar, insoluble in water, undergo substitution reactions.

Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat (complete). CH₄ + O₂ → CO + H₂O + Heat (incomplete, toxic CO).

Substitution: CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight, chloromethane). Further substitution gives CH₂Cl₂, CHCl₃, CCl₄.

Cracking/Pyrolysis: Large alkanes break into smaller ones. C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (high temperature, catalyst).

Controlled Oxidation: Incomplete oxidation of alkanes gives alcohols, aldehydes, carboxylic acids.

General Formula: CₙH₂ₙ. Contain C=C double bond (unsaturated).

Homologous Series: C₂H₄ (ethene), C₃H₆ (propene), C₄H₈ (butene), C₅H₁₀ (pentene).

Nomenclature: -ene suffix. Ethene (ethylene), propene (propylene).

Preparation of Alkenes:

- Ethene (C₂H₄): From ethyl alcohol (ethanol) - C₂H₅OH → C₂H₄ + H₂O (dehydration with conc. H₂SO₄ at 170°C).

- Dehydrohalogenation: CH₃CH₂Cl + KOH (alc.) → CH₂=CH₂ + KCl + H₂O (from alkyl halides).

Test for Unsaturation: Decolorize bromine water (Br₂). C₂H₄ + Br₂ → C₂H₄Br₂ (colorless). Turns purple KMnO₄ colorless.

Properties: Unsaturated, undergo addition reactions due to reactive double bond.

Combustion: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O + Heat (complete combustion).

Hydrogenation (Addition): C₂H₄ + H₂ → C₂H₆ (Ni catalyst, 200°C, converts alkene to alkane).

Halogenation (Addition): C₂H₄ + Cl₂ → C₂H₄Cl₂ (1,2-dichloroethane), C₂H₄ + Br₂ → C₂H₄Br₂ (colorless from orange).

Addition Reactions: H₂, Cl₂, Br₂, HCl, H₂O can add across double bond following Markovnikov's rule.

Polymerization: nC₂H₄ → (C₂H₄)ₙ (polyethylene/polythene). Used in plastics, bottles, bags.

General Formula: CₙH₂ₙ₋₂. Contain C≡C triple bond (unsaturated).

Homologous Series: C₂H₂ (ethyne), C₃H₄ (propyne), C₄H₆ (butyne).

Nomenclature: -yne suffix. Ethyne (acetylene), propyne.

Preparation of Alkynes:

- Ethyne (C₂H₂): From calcium carbide - CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂↑ (laboratory preparation).

- From 1,2-dibromoethane: CH₂Br-CH₂Br + 2KOH (alc.) → HC≡CH + 2KBr + 2H₂O (dehydrohalogenation).

Properties: Unsaturated, more reactive than alkenes, decolorizes Br₂ water and KMnO₄. Burns with sooty flame.

Combustion: 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O + Heat (complete). Produces more soot due to high C:H ratio.

Addition Reactions:

- Hydrogenation: C₂H₂ + H₂ → C₂H₄ (ethene, Ni catalyst), C₂H₄ + H₂ → C₂H₆ (ethane, more catalyst needed).

- Halogenation: C₂H₂ + Cl₂ → C₂H₂Cl₂ (1,1,2,2-tetrachloroethane), C₂H₂ + 2Br₂ → C₂H₂Br₄ (tetrabromoethane).

- Can add two molecules: First to alkene, then to alkane due to reactive triple bond.

Uses: Oxyacetylene welding (burns at 3000°C with O₂), manufacturing polymers, organic synthesis.

General Formula: R-OH (where R is alkyl group).

Ethanol (C₂H₅OH): Common alcohol, also called ethyl alcohol.

Preparation:

- Laboratory method: From ethyl bromide - C₂H₅Br + KOH (aq) → C₂H₅OH + KBr (hydrolysis of alkyl halide).

- Industrial method: Fermentation of sugars - C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (yeast catalyst, anaerobic).

Physical Properties:

- Colorless liquid, boiling point 78°C, miscible with water, characteristic odor.

- Density: 0.789 g/cm³, neutral to litmus, poisonous in large quantities.

Chemical Properties:

- Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Heat (burns with blue flame).

- Action with sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑ (forms sodium ethoxide, hydrogen gas).

- Esterification: C₂H₅OH + CH₃COOH → CH₃COOC₂H₅ + H₂O (forms ethyl acetate, conc. H₂SO₄ catalyst).

- Dehydration: C₂H₅OH → C₂H₄ + H₂O (conc. H₂SO₄, 170°C, forms ethene).

Uses: Solvent, fuel (gasohol), alcoholic beverages, antiseptics, manufacturing chemicals.

Denatured Alcohol: Ethanol made unfit for drinking by adding methanol or pyridine (industrial use).

General Formula: R-COOH (where R is alkyl or H).

Examples: Methanoic acid (HCOOH), Ethanoic acid (CH₃COOH, acetic acid).

Physical Properties:

- Sour taste, characteristic odor, higher boiling points than alcohols.

- Soluble in water (lower members), form hydrogen bonds, act as weak acids.

Chemical Properties:

- Acidic nature: React with bases - CH₃COOH + NaOH → CH₃COONa + H₂O (sodium acetate).

- Esterification: CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (ethyl acetate, conc. H₂SO₄ catalyst).

- Reaction with carbonates: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑.

- Reduction: To alcohols using LiAlH₄ or catalytic hydrogenation.

Uses: Food preservatives (acetic acid in vinegar), manufacturing plastics, dyes, pharmaceuticals.

Isomerism: Compounds with same molecular formula but different structural formulas.

Structural Isomers: Different arrangement of atoms. Example: C₄H₁₀ has n-butane and isobutane.

n-Butane: CH₃-CH₂-CH₂-CH₃ (straight chain). Boiling point 0°C.

Isobutane: (CH₃)₃CH (branched chain). Boiling point -12°C.

Position Isomerism: In alkenes - C₄H₈ can be but-1-ene or but-2-ene (double bond position differs).

Importance: Isomers have different physical and chemical properties despite same formula.

Homologous Series: Group of organic compounds with same functional group, similar properties, differing by -CH₂- unit.

Characteristics: Same general formula, gradual change in properties, similar chemical reactions.

Difference: Each successive member differs by 14 u (CH₂ = 12 + 2 = 14).

Functional Groups: -OH (alcohols), -COOH (carboxylic acids), C=C (alkenes), C≡C (alkynes).

Cracking: Breaking large hydrocarbons into smaller ones. C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (high temp, catalyst).

Substitution: Saturated hydrocarbons (alkanes). Addition: Unsaturated hydrocarbons (alkenes, alkynes).

Laboratory Tests: Br₂ water test distinguishes saturated (no reaction) from unsaturated (decolorization).