alkenes

• What are alkenes?

  unsaturated hydrocarbons that contain at least one C=C double bond
• What is the general formula of alkenes?

  CnH2n
• How is a  π bond formed?

  electrons in the adjacent p orbitals overlap above and below the carbon atoms

  they can only be made after a σ bond is formed
• What bond restricts the rotation of carbon atoms?


   π bond
• What is the angle and shape of a double bond?

  120 degrees

  trigonal planar
• Are alkenes more or less reactive than alkanes, why?

  alkenes are more reactive than alkanes due to a high electron density of double bonds and the fact the pi-bond is slightly easier to break
• What IMF do alkenes have?

  only London dispersion forces due to non-polar bonds
• Are alkenes soluble in water, why?

  No, alkenes are insoluble in water due to their non-polar bonds
• What are the types of isomers that can be formed using alkenes?

  E/Z isomers, due to the restricted rotation

  Cis-trans isomers, if two of the same substituents are attached to each carbon
• What is an electrophile?

  a species that are electron pair acceptors
• What is the most stable type of carbocation intermediate, why?

  Alkyl groups have a positive inductive effect, so the most stable carbocation is the one bonded to the most other carbon atoms e.g a tertiary carbocation
• What conditions are needed for the electrophilic addition of H2O to an alkene?

  Steam in the presence of an acid catalyst, usually phosphoric acid
• What is the reaction called between water and an alkene?


  hydration
• What are the product(s) of the electrophilic addition to H2O to an alkene (hydration reaction)?

  an alcohol
• What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene?

  hydrogen halide gases must be at room temperature
• What is the reaction called when a halogen is added to an alkene?

  Halogenation
• How does a molecule with a non-polar bond react as if it is an electrophile?

  C=C double bond with a high electron density induces a temporary dipole in the halogen molecule

  positively charged atom is attracted to the double bond
• How can an alkene be converted into an alkane, what is the reaction called, what are the required conditions?

  Alkene + hydrogen -> alkane

  • reaction is called hydrogenation
  • required conditions are 150 degrees celsius, nickel catalyst
• What is an addition polymer?


  many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule
• What are monomers, what form do they usually take?

  monomers are molecules which combine to form a polymer

  usually have a C=C bond which breaks to leave a repeating pattern
• What are the ways in which plastic can be disposed (6)?

  landfill

  • combustion
  • electricity generation
  • reuse
  • recycle
  • organic feedstock
• What are stereoisomers?

  molecules with the same structural formula, but the atoms are arranged differently in space
• What are the 2 main categories of stereoisomers?


  geometrical isomers and optical isomers
• single covalent bonds are fully rotational

  double covalent bonds have very restricted rotation, they can flex slightly but cannot rotate
• What are the 2 naming systems for geometrical isomers?

  cis/trans and E/Z
• What are the 2 conditions for an alkene to have cis/trans isomers?

  1)both carbon atoms on the double bond must be attached to 2 different groups

  2)at least one of those groups must be the same on both carbon atoms on the double bond
• If two of the same atoms/groups are on the same side of the molecule then it's the cis isomer

  If two of the same atoms/groups are on opposite sides of the molecule then it's the trans isomer
• What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene?

  hydrogen halide must be at room temperature
• What are stereoisomers?

  compounds with the same structural formula but with a different arrangement of atoms in space
• What is E-Z isomerism?

  A type of stereoisomerism which occurs due to the limited rotation around a double carbon bond.

  The limited rotation means that ‘high priority’ groups attached to the C=C can either be ‘together’ or ‘opposite’.
• Where are the high priority groups on the E isomer?

  on opposite sides of the double bond (one above, one below)
• Where are the high priority groups on Z isomers?

  together on the same side (both above or both below)

  on the zame zide!
• How do you know which groups are high priority in E/Z isomerism?


  The group on each side of the double bond with the higher atomic number /(Mr) have higher priority in E/Z isomerism.

  e.g if both the highest priority groups are on the same side e.g both on top, the molecule is the Z isomer
• What is Cis-Trans isomerism?

  2 of the substituent groups attached to each carbon atom on the C=C group are the same

  Cis- is used when the groups are on the same side

  Trans- is used when the groups are on different sides
• Why are alkenes reactive?

  As the carbon-carbon double bond in alkenes has a relatively low bond enthalpy.

  During their reactions, the double bond opens up to form single bonds to other atoms.

  These are called addition reactions.

  The pi bond is an area of high electron density, this attracts electrophiles.
• How can alkanes be produced from alkenes?

  Alkenes can undergo electrophilic addition with hydrogen to produce alkanes.

  The C=C bond opens up and forms single bonds to each of the hydrogen atoms.

  This reaction requires a nickel catalyst.
• What is an example of producing an alkane from an alkene?

  propene + hydrogen -> propane

  C2CHCH3 + H2 -> CH3CH2CH3

  aka a hydrogenation reaction
• What is catalytic hydrogenation used in?

  the manufacture of margarine from unsaturated vegetable oils
• What are haloalkanes?

  organic compound with single carbon bond and halogen functional groups
• How can you produce haloalkanes from alkenes?

  alkenes undergo addition reactions with halogens to form di-substituted haloalkanes and with hydrogen halides to form mono-substituted haloalkanes

  electrophilic addition mechanism