Chapter 8

chemistry • intermediate 12th

Chapter 8: Aliphatic Hydrocarbons

Complete notes for Chapter 8. Covers Hydrocarbons classification, Nomenclature, Alkanes, Alkenes, Alkynes properties, preparations, and reactions.

Introduction & Classification

Hydrocarbons: Compounds containing only Carbon and Hydrogen.

Saturated (Alkanes): Single bonds (sigma). General formula CnH2n+2. Also called Paraffins (little affinity).
Unsaturated: Double (Alkenes, CnH2n) or Triple (Alkynes, CnH2n-2) bonds.
Nomenclature: IUPAC rules for naming Alkanes, Alkenes, and Alkynes. Numbering chain to give substituents/multiple bonds lowest number.

Alkanes (Paraffins)

Preparation

Hydrogenation of Alkenes/Alkynes (Sabatier & Senderens). Catalyst Ni (200-300°C) or Pt/Pd (Room Temp).
From Alkyl Halides: Reduction with Zn/HCl or Hydrogenolysis.
Decarboxylation of Monocarboxylic Acids (Soda Lime).
Kolbe's Electrolytic Method (Symmetrical alkanes only).
From Carbonyls: Clemmensen (Zn-Hg/HCl) or Wolff-Kishner (N2H4/KOH).
Grignard Reagent: RMgX + H2O → RH + Mg(OH)X.

Reactions

Combustion: Highly exothermic. Used as fuel.
Halogenation: Free radical mechanism (Initiation, Propagation, Termination). Reactivity: F2 > Cl2 > Br2 > I2.
Nitration: Vapour phase nitration (450°C).

Alkenes (Olefins)

Preparation

Dehydrohalogenation of Alkyl Halides (Alcoholic KOH).
Dehydration of Alcohols (Alumina or H2SO4/H3PO4). Ease: Tert > Sec > Prim.
Dehalogenation of Vicinal Dihalides (Zn dust).
Kolbe's Electrolysis of Dicarboxylic Acid salts.
Partial Hydrogenation of Alkynes (Lindlar's Catalyst for Cis, Na/liq NH3 for Trans).

Reactions

Addition: H2 (Hydrogenation), HX (Markownikoff's Rule), H2SO4 (Formation of Alcohols), X2 (Via Bromonium ion, test for unsaturation).
Oxidation: KMnO4 (Bayer's Test - hydroxylation to Glycol), Ozonolysis (Locates double bond position), Epoxidation (Ag2O).
Polymerization: Ethene → Polyethene.

Alkynes (Acetylenes)

Preparation

Dehydrohalogenation of Vicinal Dihalides (KOH then NaNH2).
Dehalogenation of Tetrahalides (Zn dust).
Kolbe's Electrolysis of Unsaturated Dicarboxylic Acids.
Industrial: CaC2 + H2O → C2H2 + Ca(OH)2.

Reactions

Additions similar to Alkenes but consume 2 moles.
Acidic Nature: Terminal Alkynes have acidic H (sp hybrid C). React with Na, Ammonical AgNO3 (White ppt), Ammonical Cu2Cl2 (Red ppt). Used for identification and purification.

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