Unit 4: Aliphatic and Aromatic Hydrocarbons
Course Code: CHM-DSM-252
Paper Name: Fundamentals of Chemistry - II
Table of Contents
1. Alkanes: Preparation and Reactions
Alkanes are saturated hydrocarbons containing only single C-C bonds.
Methods of Preparation
- Catalytic Hydrogenation: Addition of hydrogen to alkenes or alkynes in the presence of catalysts like Ni, Pt, or Pd.
- Wurtz Reaction: Reaction of alkyl halides with sodium metal in dry ether to form higher alkanes.
- Kolbe's Synthesis: Electrolysis of aqueous solutions of sodium or potassium salts of fatty acids.
- From Grignard Reagent: Treatment of RMgX with water or alcohols to yield alkanes.
Chemical Reactions
- Free Radical Substitution (Halogenation): Alkanes react with halogens (like Cl2) in the presence of UV light or heat.
2. Alkenes and Alkynes: Synthesis and Addition
These are unsaturated hydrocarbons containing double or triple bonds.
Preparation Methods
- Elimination Reactions: Dehydrogenation of alkanes or dehydrohalogenation of alkyl halides following Saytzeff's rule.
- Acetylene Production: Prepared industrially from Calcium Carbide (CaC2) reacting with water.
Key Reactions
- Hydrogenation and Halogenation: Addition of H2 or Br2 (trans-addition).
- Hydration: Addition of water to form alcohols or carbonyl compounds.
- Ozonolysis: Cleavage of double/triple bonds with ozone to identify the position of unsaturation.
- Formation of Metal Acetylides: Terminal alkynes react with ammoniacal AgNO3 or CuCl to form precipitates, highlighting their acidic nature.
3. Rules of Addition
When unsymmetrical reagents add to unsymmetrical alkenes, specific rules govern the orientation.
Markownikoff's Rule
In the addition of HX to an alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms, while the halogen attaches to the carbon with fewer hydrogens.
Anti-Markownikoff's (Peroxide) Effect
In the presence of organic peroxides, the addition of HBr (only) to alkenes occurs in the opposite direction via a free radical mechanism.
4. Aromatic Hydrocarbons: Benzene
Benzene (C6H6) is the simplest aromatic hydrocarbon, exhibiting unique stability.
Preparation of Benzene
- From Phenol: Reduction of phenol by heating with Zinc dust.
- Decarboxylation: Heating sodium benzoate with soda lime.
- Cyclic Polymerization: Passing acetylene through a red-hot iron tube.
- From Benzene Sulphonic Acid: Hydrolysis with superheated steam.
5. Electrophilic Aromatic Substitution (EAS)
Benzene undergoes substitution reactions where an electrophile replaces a hydrogen atom on the ring.
| Reaction | Reagents | Electrophile |
|---|---|---|
| Nitration | Conc. HNO3 + Conc. H2SO4 | NO2+ (Nitronium ion) |
| Halogenation | Cl2/Br2 + AlCl3/FeCl3 | Cl+ / Br+ (Halonium ion) |
| Sulphonation | Fuming H2SO4 (Oleum) | SO3 (Sulfur trioxide) |
| Friedel-Crafts Alkylation | Alkyl Halide (R-X) + Anhydrous AlCl3 | R+ (Carbocation) |
| Friedel-Crafts Acylation | Acid Chloride (R-COCl) + Anhydrous AlCl3 | R-CO+ (Acylium ion) |
6. Exam Focus: Tips and FAQs
- Saytzeff's Rule: In elimination reactions, the more substituted alkene (the one with more alkyl groups attached to the double bond) is the major product.
- Aromaticity: Remember Hückel's Rule (4n+2 pi electrons) to identify aromatic compounds.
- Benzene Stability: Explain benzene's stability using resonance energy and delocalized pi electrons.
Frequently Asked Questions
Q: Why do terminal alkynes show acidic character?
A: The hydrogen attached to the sp-hybridized carbon has high s-character, making the C-H bond polar and the hydrogen easily removable by strong bases.
Q: What is the purpose of Ozonolysis?
A: It is used to locate the position of a double or triple bond in a molecule by analyzing the resulting carbonyl fragments.
Q: Define the Wurtz Reaction.
A: It is a method to prepare higher symmetrical alkanes by reacting two moles of alkyl halides with sodium metal in dry ether.