Alkenes

General formula: C_nH_2n • Functional group: C=C double bond

Table of Contents

1. Introduction
2. Physical Properties
3. Chemical Properties (Key Reactions)
4. Methods of Preparation
5. Isomerism in Alkenes
6. Uses of Alkenes
7. Alkenes Quiz
8. References & Related Links

Introduction

Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). They follow the formula C_nH_2n, having two fewer hydrogens than the corresponding alkanes. The C=C bond imparts greater reactivity compared with alkanes.

Physical Properties

• Generally colorless gases or liquids at room temperature.
• Nonpolar; insoluble in water but soluble in organic solvents (e.g., benzene, ether).
• Boiling points increase with molecular mass due to stronger dispersion forces.

Chemical Properties (Key Reactions)

The C=C bond undergoes electrophilic addition and other transformations:

• Addition of Halogens: Br₂/Cl₂ → vicinal dihalides.
• Addition of HX: HCl/HBr/HI adds across C=C (Markovnikov’s rule).
• Hydrogenation: H₂ with Pt/Pd/Ni → corresponding alkanes.
• Hydration: Acid-catalyzed addition of water → alcohols.
• Polymerization: Chain growth to polymers (e.g., polyethylene).

Methods of Preparation

• Dehydration of Alcohols: Heat with conc. H₂SO₄ or alumina.
  Example: C₂H₅OH → C₂H₄ + H₂O
• Dehydrohalogenation of Alkyl Halides: Alcoholic KOH eliminates HX.
  Example: C₂H₅Br + KOH → C₂H₄ + KBr + H₂O
• Cracking of Petroleum Fractions: Thermal/catalytic cleavage of higher alkanes.
• Partial Dehydrogenation of Alkanes: Catalytic removal of H₂ at high temperature.
• From Alkynes: Lindlar-catalyzed partial hydrogenation → cis-alkenes.

Isomerism in Alkenes

• Cis: similar groups on the same side of the double bond.
• Trans: similar groups on opposite sides.

Restricted rotation around C=C leads to geometric (cis–trans or E/Z) isomerism when each double-bond carbon bears two different groups.

Uses of Alkenes

• Feedstocks for plastics, alcohols, epoxy compounds, and detergents.
• Ethene is used for artificial fruit ripening.
• Polymerization yields materials such as polyethylene and polypropylene for packaging and manufacturing.

Alkenes Quiz

1. What is the general formula of alkenes?
   Answer: C_nH_2n
   Because one double bond reduces the hydrogen count by two vs. alkanes.
2. Which reagent converts an alkyl halide to an alkene?
   Answer: Alcoholic KOH (dehydrohalogenation).
3. What type of isomerism is typical for alkenes?
   Answer: Geometric (cis–trans/E–Z).
4. Which catalysts hydrogenate alkenes?
   Answer: Pt, Pd or Ni.
5. Name the reaction where alcohols lose water to form alkenes.
   Answer: Dehydration of alcohols.
6. Product of ethene + Br₂ (in CCl₄)?
   Answer: 1,2-dibromoethane.
7. Polymer formed from ethene?
   Answer: Polyethylene.

Summary: Alkenes (C_nH_2n) are reactive unsaturated hydrocarbons. Their C=C bond drives hallmark additions (HX, X₂, H₂, H₂O), multiple preparation routes, geometric isomerism, and wide industrial use—especially as polymer feedstocks.

References & Related Links

Related topics: Methods of Preparation of Hydrocarbons • Alkanes • Aromatic Hydrocarbons

External reading: LibreTexts Organic Chemistry