Alkenes

• General formula

  CnH2n
• functional group

  carbon-carbon double bond, an area of high electron density
• shape

  trigonal planar, 120 degrees
• the nature of the carbon-carbon double bond

  it is a pi-bond, formed when p-orbitals overlap

  the presence of the pi bond means there is restricted bond rotation about the planar c=c bond
• diene


  two c=c bonds
• triene

  three c=c bonds
• cycloalkenes

  have two less hydrogen atoms
• position isomer of pentene

  pent-2-ene
• chain isomer of pentene

  2-methylbut-ene
• functional group isomer of pentene

  chain alkenes and cyclic alkanes are fg isomers

  cyclopentane
• stereoisomers

  compounds with the same structural formula but a different arrangement of atoms in space
• what type of stereoisomerism do alkenes exhibit and why?

  E/Z

  E- entegen=opposite

  Z- zusarnmen= together

  due to the restricted rotation about the planar c=c bond

  exampl- e-but-2-ene and z-but-2-ene
• For an alkene to exihbit e/z isomerism both carbon atoms of the c=c must be bonded to different groups
• How can you determine whether an alkene is the e or z isomer?

  • identify the highest priority group on each C atom of the c=c
  • if both highest priority groups are on the same side of the c=c it is the z isomer
  • if both highest priority groups are on opposite sides of the c=c it is the e isomer
• Cahn-Ingold-Prelog Rules

  • the atom with the highest atomic number has the highest priority group
  • if two atoms have the same atomic number, the next atom along should be compared, the first point of difference will be used to assign priority
• explain how the CIP rules can be used to deduce the full IUPAC name of the alkene

  Consider each C of the c=c group, left hand C of c=c is bonded to CH3 and Br. Br has a higher atomic number than C so Br is priority group

  Right hand C of c=c is bonded to CH3 and CH2CH3. C has higher atomic number than H so CH2CH3 is priority group

  it is therefore the Z isomer as both priority groups are on same side

  Z- 2- bromo-3-methylpent-2-ene
• why are alkenes reactive?

  because of the area of high electron density
• alkenes undergo addition reactions in which a small molecule is added across the c=c bond, forming a saturated molecule

  c=c + AB -> C-C

  The c=c breaks

  each c from c=c bonds to a new atom group

  rest of molecule stays same
• electrophile


  electron pair acceptor
• electrophilic addition

  alkenes react with electrophiles because electrophiles are attracted to the high electron density in the c=c bond

  covalent bonds are broken and formed producing a positively charged intermediate called a carbocation

  a mechanism shows the breaking and forming of covalent bonds using curly arrows
• curly arrows

  show the movement of a pair of electrons

  they must start from a lone pair or a covalent bond
• ethene and bromine

  observation: orange bromine water is decolourised

  
• electrophilic addition of alkenes with bromine overview

  1. Br2 acts as an electrophile
  2. the high electron density in the c=c group induces a temporary dipole in the Br2 molecule
  3. a pair of e- is accepted by the Br delta+ atom, breaking the Br-Br by heterolytic fission (covalent bond breaks unequally)
  4. a lone pair of electrons on the Br- forms a new C-Br bond with the carbocation
• electrophilic addition of conc. H2SO4 and alkenes


  product= ethane hydrogen sulfate

  
• hydrolysis

  breaking of a bond using water
• organic hydrogen sulfates are readily hydrolysed with warm water to form an alcohol
• 2 steps alkenes take to form alcohols

  1. alkene + h2so4 -> organic hydrogen sulfate
  2. organic hydrogen sulfate + water -> alcohol + h2so4

  role of h2so4= catalyst
• hydration of alkenes

  reaction with steam

  with a phosphoric acid catalyst
• carbocations

  classified as either primary 1 degree, secondary, 2 or tertiary

  based on the number of carbon atoms directly attached to the carbon with the positive charge
• carbocations

  3>2>1 order of stability

  alkyl groups have a tendency to release electrons which help to stabilise the positive charge of carbocations, they have a positive inductive effect

  
• addition to an unsymmetrical alkene produces what?

  • major product produced in largest amount
  • minor product produced in smallest amount
  • more product is formed from the most stable carbocation
• hydrogenation of alkenes

  when a H is added across the c=c bond, an alkane is produced

  reagent and conditions: H2 and Ni catalyst

  margarine is produced by the hydrogenation of long chain fats or oils which contain c=c bonds
• addition polymers

  • polymers are formed when many small molecules called monomers join together to form a very large molecule
  • alkenes acts as monomers in addition polymerisation reactions- the c=c bond breaks, allowing monomers to add together
  • example: ethene forms poly(ethene)
• repeating unit


  a specific arrangement of atoms that occurs in the structure over and over again
• formation of PVC

  monomer repeating unit of polymer

  
• uses of some poly(alkenes)


  poly(propene)- food containers

  orlon- synthetic fibres used in blankets, rugs and clothing

  teflon- non stick coatings

  poly(styrene)- foam packaging and insulation

  poly( 1,1-dichloroethene) - shower curtains, plastic films
• properties of poly(alkenes)

  • simple poly(alkenes) are unreactive. they have a backbone of single c-c bonds so are saturated
  • properties depend on their structure and intermolecular forces
• properties of poly(ethene)

  has van der waals' between neighbouring chains, more branches decreases points of contact so the vdws' are weakened which increases flexibility and lowers boiling point

  low density poly(ethene): branched, flexible, carrier bags

  high density poly(ethene): unbranched, rigid, crates and pipes
• properties of PVC

  Rigid and brittle

  contains polar C-Cl bonds so has permanent dipole-dipole forces between polymer chains

  addition of small molecules, called plasticisers, make the polymer more flexible as they force neighbouring chains apart, allowing them to slide over each other

  plasticisers get in between neighbouring chains and reduce the intermolecular forces