Alkenes

• Describe the structure of alkenes

  members of a homologous series

  hydrocarbons

  formula: CNH2N (non-cyclic alkenes)

  unsaturated

  contain a c-c double bond

  spatial arrangement around the double bond

  planar + bond angle 120 degrees
• Explain the hybridisation of orbitals

  3 orbitals (1s + 2 p's) hybridise to form three new orbitals

  All three orbitals are equivalent

  Remaining 2p is unchanged.
• What are the steps to the formation of alkenes?

  step 1: covalent bonds are formed by overlap of orbitals

  step 2: an sp2 orbital from each carbon overlaps to form a single c-c bond

  step 3: resulting in a sigma bond

  step 4: two 2p orbitals also overlap to form a second bond (pi bond), maximum overlap and the strongest bond
• How are cycloalkenes different ?

  2 less H atoms

  enables the formation of a ring

  formula: CnH2n-2
• How do alkenes react ?

  more reactive

  • due to double bond
  • area of dense electronegativity
  • susceptible to attack from electrophiles
• What is hydrogenation ?

  alkenes + hydrogen

  adding hydrogen across the double bond

  condition - Ni catalyst, heat
• What is hydration ?

  alkenes + steam

  Adding water (steam) across double bond forms alcohol

  conditions - H3PO4 catalyst, heat, high pressure
• What are hydrogen halides?

  alkenes + hydrogen halide

  adding hydrogen halide (Hx) across double bond forms Halogenoalkane

  conditions - RTP
• What is halogenation ?

  alkenes + halogens

  adding halogens (X2) across double bond forms dihalogenoalkane

  conditions - RTP
• What is oxidising ?

  alkenes + cold, dilute, acidified manganate (VII) ions

  adding manganate (VII) ions creates a diol

  conditions - reflux (heat)
• Mechanism of electrophilic addition

  first step

  A) bromine molecules collide with ethene

  B) double bond is electro dense, polarises bromine

  C) creates electrophile

  D) bond breaks between br atoms and new bond is formed

  
• mechanism of electrophilic addition

  second step

  A) S+

  B) S-

  C) +

  D) Br-

  E) 1,2-bromoethane

  
• mechanism of electrophilic addition in hydrogen halides

  1st step is the same

  Although, Hx is a permanent dipole = does not need to be polarised by the cloud of electrons

  A) not 50/50

  B) more major product

  C) inductive effect

  D) major product

  E) minor product

  
• What is the inductive effect ?

  tertiary, secondary, primary

  from most stable to least stable
• What is addition polymerisation ?

  creates polymers that are important to society

  the reaction has 100% atom economy
• What is polymerisation?

  using a catalyst to aid the reaction, thousands of monomers combine.

  double bond in alkenes molecules are broken to create a chain = polymer is formed

  e.g. polyethene
• polymerisation

  A) one repeating unit

  
• What are the issues with polymers ?

  they do not biodegrade (very slowly)

  branched polymers biodegrade even slower
• What are the bond angles within polymers?

  angles that exist alongs the chains

  e.g. polypropene = tetrahedral (109.5)