AKQA Alkenes Revision Notes

May 9, 2024

AKQA Alkenes Revision - AQA Specification

Introduction to Alkenes

  • Unsaturated hydrocarbons with double bonds.
  • General formula: CNH2N, applicable for molecules with one double bond.
  • Can undergo addition reactions.

Examples

  • Ethene (CH2=CH2): Unsaturated, contains a double covalent bond.
  • But-1,3-diene: Contains two double bonds.
  • Cycloalkenes: Alkenes in a cyclic structure, having two fewer hydrogens than their straight-chain counterparts.

Electrophilic Addition Reactions

  • Alkenes react with electrophiles due to their high electron density at the double bond.
  • Electrophiles: Electron pair acceptors, attracted to the electron-rich double bonds in alkenes.
  • Reactions often involve polar molecules or ions with a positive charge (e.g., H+, NO2+).

Key Mechanisms

  1. Bromine Water Test: Addition of bromine water turning from brown to colorless indicates the presence of an alkene.
  2. Hydrogen Halides Addition: Alkenes react with HBr, HCl, forming halogenoalkanes.
  3. Sulfuric Acid Addition: Forms alkyl hydrogen sulfates, leading to alcohol production through hydrolysis.

Markovnikov's Rule

  • Determines the major product in addition reactions with unsymmetrical alkenes, predicting that the electrophile adds to the less substituted carbon atom, leading to the most stable carbocation intermediate.

Polymerization

  • Alkenes can form polymers through addition polymerization processes.
  • Examples: Polyethylene, Polypropylene, and PVC.
  • Polymers can be further modified with plasticizers to change their physical properties.

Sustainability and Impact

  • Issues with polymer degradation and sustainability.
  • Importance of recycling and sustainable usage of polymers to mitigate environmental impact.

Key Takeaways

  • Alkenes feature prominently in electrophilic addition reactions, polymer chemistry, and have significant industrial applications.
  • Understanding the reaction mechanisms and properties of alkenes is crucial for designing synthetic routes and materials with desired properties.