Chemistry topic 5

Alkenes

  • Unsaturated hydrocarbons with at least one carbon-carbon double bond.
  • General formula: C<em>nH</em>2nC<em>nH</em>{2n}
  • Cycloalkenes: Alkenes with carbon atoms in a closed ring.
  • High electron density makes them susceptible to electrophiles.
  • Composed of a σ bond and a π bond.

Test for Alkenes

  • Bromine water changes from orange-brown to colorless in the presence of alkenes.
  • The C=C bond accepts bromine atoms, becoming saturated.

Geometric Isomerism (Stereoisomers)

  • Different spatial arrangement due to limited rotation around a double carbon bond.
  • E-Z Isomerism: Functional groups are either 'together' or 'apart'.
    • E isomer (entgegen): Functional groups on opposite sides.
    • Z isomer (zusammen): Functional groups together on the same side.

Cahn-Ingold-Prelog (CIP) Priority Rules

  • Prioritize groups based on the atomic number (Ar) of the first directly bonded atom to the carbon with the double bond.
  • Higher Ar = higher priority.
  • Used to determine E or Z isomer.

Determining Complex E/Z Isomers

  • Step 1: Apply CIP priority rules to first attached atoms.
  • Step 2: If the first atoms are the same, look at the second atoms attached.
  • Step 3: Deduce E or Z based on the position of the highest priority groups.

Cis- and Trans- Isomers

  • Cis-: Groups are on the same side.
  • Trans-: Groups are on different sides.
  • Limited to cases with hydrogen atoms for comparison; E/Z isomerism is used otherwise.

Reactions of Alkenes

  • The carbon-carbon double bond opens up to form single bonds.

Alkanes

  • Alkenes undergo electrophilic addition with hydrogen to produce alkanes.
  • Requires a nickel catalyst.

Halogenoalkanes

  • Alkenes undergo addition reactions with halogens to form di-substituted halogenoalkanes, and with hydrogen halides to form mono-substituted halogenoalkanes.

Alcohols

  • Alkenes undergo addition reactions with steam to form alcohols.
  • Requires an acid catalyst (e.g., phosphoric acid).
  • Diols can be formed via oxidation with acidified potassium manganate(VII) (KMnO4).

Electrophilic Addition

  • Alkenes undergo electrophilic addition at the double bond.

Electrophiles

  • Electron acceptors attracted to areas of high electron density.
  • Examples: HBr, Br2, H2SO4

Mechanism

  • Double bond breaks, forming a carbocation.
  • Alkene + Halogen → Dihalogenoalkane
  • Alkene + Hydrogen Halide → Halogenoalkane

Electrophilic Addition Example: Propene + Hydrogen Bromide

  • HBrHBr is polar and the electron pair in the double bond attracts Hδ+H^{\delta+}
  • A covalent bond forms between carbon and hydrogen, creating a carbocation intermediate.
  • The hydrogen joins to the carbon atom bonded to the most hydrogen atoms.
  • The bromide ion bonds to the carbon atom joined to the most carbon atoms.

Inductive Effects of Alkyl Groups

  • Carbocations with more alkyl groups are more stable.
  • Alkyl groups have a positive inductive effect, feeding electrons towards the positive charge.
  • The more stable carbocation forms the major product.

Addition Polymers

  • Produced from alkenes where the double bond breaks to form a repeating unit.
  • Monomers join to form long chain polymers.
  • The repeating unit must be shown with extended bonds.

Uses of Polymers

  • Unreactive hydrocarbon chains with strong, non-polar covalent bonds.
  • Used for manufacturing many everyday plastic products.

Disposal of Polymers

  • Non-biodegradable polymers pose disposal challenges.
  • Waste polymers can be recycled, used as feedstock for cracking, or incinerated.
  • Incineration can release toxic gases.
  • Scientists are developing biodegradable polymers.