Lecture on Hydrocarbons

Introduction

• Lecturer: Surya
• Focus: Hydrocarbons chapter
• Previous topics covered: IUPAC, Isomerism, General Organic Chemistry (GOC)

What are Hydrocarbons?

• Organic compounds made of carbon and hydrogen.
• Derivatives are compounds made or derived from hydrocarbons.
• Example: Methane (CH4), the simplest hydrocarbon.

Types of Hydrocarbons

• Saturated Hydrocarbons: Single bonds (Alkanes)
• Unsaturated Hydrocarbons: Double or triple bonds
  • Alkenes: Double bonds
  • Alkynes: Triple bonds

Structure and Bonds

• Single bond (Alkanes): Sigma bond
• Double bond (Alkenes): One sigma bond, one pi bond
• Triple bond (Alkynes): One sigma bond, two pi bonds

Classification of Hydrocarbons

• Open Chain (Aliphatic): Straight or branched chains
• Cyclic (Aromatic): Closed ring structures

Alkanes

• Saturated hydrocarbons with single C-C bonds
• General formula: CnH2n+2
• Known as paraffins (low reactivity)
• Inert at room temperature
• Hybridization: sp3, Bond angle: 109.5°, Shape: Tetrahedral

Preparation of Alkanes

1. Hydrogenation of Unsaturated Hydrocarbons
   • Catalyst: Ni, Pd, Pt
   • Reduction of alkenes and alkynes to alkanes
2. From Alkyl Halides
   • Direct combination using Zn and H+
   • Wood's reaction: Haloalkanes with sodium in dry ether
3. Decarboxylation
   • Sodium salt of carboxylic acid + soda lime yields alkane + Na2CO3
4. Kolbe's Electrolysis
   • Electrolysis of sodium/potassium carboxylate to produce alkanes

Alkenes

• Unsaturated hydrocarbons with one or more C=C bonds
• Known as olefins
• General formula: CnH2n

Preparation of Alkenes

1. Dehydration of Alcohols
   • Using conc. H2SO4 or H3PO4 to form alkenes
2. Dehydrohalogenation of Alkyl Halides
   • Elimination reaction using alcoholic KOH
3. From Vicinal Dihalides
   • Elimination of halogens from adjacent carbon atoms

Alkynes

• Unsaturated hydrocarbons with C≡C bonds
• General formula: CnH2n-2

Preparation of Alkynes

1. From Calcium Carbide
   • Reaction with water yields acetylene
2. From Vicinal Dihalides
   • Use of alcoholic KOH and sodamide for elimination

Aromatic Hydrocarbons

• Also known as arenes
• Benzene-based structures

Preparation of Aromatic Compounds

1. From Alkynes
   • Cyclization to form benzene
2. From Phenol
   • Reduction using Zn
3. Decarboxylation
   • Sodium benzoate with soda lime

Chemical Properties

• Alkanes: Substitution reactions
• Alkenes/Alkynes: Addition reactions
• Aromatics: Electrophilic substitution reactions

Electrophilic Substitution

• Mechanism involves generation of electrophile, formation of carbocation, and removal of a proton.
• Examples: Chlorination, nitration, sulfonation, Friedel-Crafts alkylation/acylation

Conclusion

• Practice essential for mastering hydrocarbons.
• Understand mechanisms for better retention.
• Engage in solving problems to reinforce learning.

Note: For better understanding, it's advised to practice these reactions and mechanisms through exercises and problem-solving.