In-Depth Notes on Hydrocarbons, Alkenes, Alkynes, and Arenes

Hydrocarbons: Overview

• Hydrocarbons only contain carbon and hydrogen.
• Types of hydrocarbons include alkanes, alkenes, and alkynes.

Alkenes

• Alkenes are unsaturated hydrocarbons with at least one carbon-to-carbon double bond.
• Also referred to as olefins.
• More reactive than alkanes due to presence of the double bond.

Nomenclature

• Identify the longest carbon chain containing the double bond.

Preparation of Alkenes

1. From Alkynes:

   • Partial reduction of alkynes using Lindlar's Catalyst.
   • Lindlar's Catalyst: Partially deactivated palladised charcoal.
   • Cis alkenes formed via reduction of alkynes; Trans alkenes with sodium in liquid ammonia.

2. From Vicinal Dihalides:

   • Reaction with zinc to form alkenes; known as dehalogenation.

3. From Alkyl Halides:

   • Alkyl halides react with alcoholic potassium hydroxide to yield alkenes; this process is known as dehydrohalogenation.
   • Rate of reaction depends on the halogen atom and the alkyl group: Iodine > Bromine > Chlorine.

4. From Alcohols:

   • Alcohols heated with concentrated sulfuric acid lead to formation of alkenes through acidic dehydration, which is a β-elimination reaction.

Physical Properties of Alkenes

• Higher alkenes are colorless and odorless.
• Ethene: Colorless with a faint sweet smell.
• Alkenes are partially polar and insoluble in water.
• Soluable in nonpolar solvents like benzene, and petroleum ether.
• Boiling Point and Melting Point:
  • Changes with size; increased carbon size increases molecular size and hence intermolecular forces.
  • Gaseous at lower carbon numbers; liquids at middle; solids at higher numbers.
• Cis vs Trans isomers:
  • Cis isomers generally have higher boiling/melting points than Trans isomers.

Chemical Properties of Alkenes

• Addition of Dihydrogen:

  • Forms alkanes in the presence of catalysts (Ni, Pt, Pd).

• Addition of Halogens:

  • Halogens add to alkenes forming vicinal dihalides; the reaction with bromine solution can test for unsaturation.

• Addition of Hydrogen Halides:

  • Forms alkyl halides depending on Markovnikov's Rule, where the negative part of the addendum goes to the carbon with fewer hydrogen atoms.

• Anti Markovnikov Effect:

  • In the presence of peroxides, Br adds to the carbon with more H-atoms.

• Addition of Water:

  • Produces alcohols under acidic conditions; follows Markovnikov's Rule.

• Oxidation Reactions:

  • Reacts with Baeyer's reagent (KMnO₄) to produce vicinal glycols; decolorization indicates unsaturation.

• Polymerization:

  • Large numbers of ethene molecules react at high temperature and high pressure in the presence of a catalyst to form polymers (e.g., polyethylene).

Alkynes

• Alkynes are hydrocarbons with at least one triple bond.
• First stable member: Ethyne (Acetylene).

Preparation of Alkynes

1. From calcium carbide by treating it with water.
2. From vicinal dihalides through dehydrohalogenation.

Physical Properties of Alkynes

• Similar to alkenes but with different physical states.
• Insoluble in water; float on it. Fairly soluble in polar solvents.

Chemical Properties of Alkynes

• Weak acids; can react with strong bases.
• Addition of dihydrogen forms alkanes using the same catalysts as in alkenes.
• Addition of halogens leads to gem-dihalides formation.
• Addition of water in the presence of sulfuric acid forms carbonyl compounds at elevated temperatures.

Arenes (Aromatic Hydrocarbons)

• Characterized by alternating double and single bonds.
• Categories: Benzoids (benzene derivatives) and Non-Benzoids.
• Structure and Stability:
  • Resonance stabilization in benzene; delocalized pi electrons.
• Physical Properties:
  • Soluble in non-polar solvents; immiscible with water; burn with a sooty flame.

Chemical Properties of Arenes

1. Electrophilic Substitution Reactions:

   • Halogenation, Nitration, Sulphonation, among others.
   • Involves generation of electrophiles and formation of carbocation intermediates.

2. Meta vs Ortho-Para Directing Groups:

   • Meta directing (e.g., -NO₂) increase density at the meta position.
   • Ortho-para directing (e.g., -OH) guide electrophilic attacks.

3. Carcinogenicity & Toxicity:

   • Carcinogenic substances can cause cancer, while toxicity measures potentially harmful effects.