3.4 Alkenes

Overview of Alkenes

• Alkenes are unsaturated hydrocarbons with the general formula CnH2n.
• They contain a carbon-carbon double bond (C=C), which is planar and has a bond angle of 120°.
• Positional isomers can occur, requiring numbers in naming (e.g., But-1-ene, But-2-ene).

Bonds in Alkenes

• C=C double bond consists of one sigma (σ) bond and one pi (π) bond.
• Pi bonds have high electron density and are susceptible to electrophile attacks.

Stereoisomerism

• Alkenes can exhibit E-Z stereoisomerism due to restricted rotation around the C=C bond.
  • E-Z requirements: Restricted rotation and two different groups/atoms attached to each end of the double bond.
  • Naming: Priority groups determine E (entgegen) or Z (zusammen) isomers.

Electrophilic Addition Reactions

• Electrophiles: Electron pair acceptors that react with high electron density in C=C of alkenes.

Reaction with Bromine

• Change: Alkene → Dihalogenoalkane
• Reagent: Bromine (Br2)
• Mechanism: Electrophilic addition
• Product: 1,2-dibromoethane
• Process: Br2 approaches, forms dipole, creates carbocation intermediate.

Reaction with Hydrogen Bromide

• Change: Alkene → Halogenoalkane
• Reagent: Hydrogen bromide (HBr)
• Conditions: Room temperature
• Products: Major and minor products (Markovnikov's Rule)
• Stability: Major product via more stable carbocation.

Reaction with Sulfuric Acid

• Stage 1: Alkene → Alkyl hydrogensulfate
  • Reagent: Concentrated H2SO4
  • Mechanism: Electrophilic addition
• Stage 2: Alkyl hydrogensulfate → Alcohol
  • Reagent: Water
  • Reaction: Hydrolysis
  • Products: Major and minor (similar to HBr reaction)

Testing for Alkenes

• Bromine Water Test: Decolorization indicates presence of C=C bond.

Industrial Hydration of Alkenes

• Process: Alkenes converted to alcohols in one step.
• Conditions: High temperature (300-600°C), high pressure (70 atm), concentrated H3PO4 catalyst.

Addition Polymers

• Formed from alkenes through addition polymerization.
• Poly(alkenes): Unreactive due to strong C-C and C-H bonds.
• Examples:
  • Polyethene from ethene.
  • Polypropene from propene.
• Properties: Recycle-friendly; can be rigid or flexible depending on plasticizer use.

Special Cases

• Poly(chloroethene) (PVC): Rigid without plasticizers, flexible with plasticizers.
• Used in uPVC window framing and electrical insulation.

Notes adapted from N Goalby chemrevise.org guide on alkenes.