Chapter 12

• What are properties of alkenes?

  Very stable

  Main component in natural gases and crude oil

  Can be extracted using fractional distillation
• Effect of branching of alkenes boiling points?

  Increased branching means that there are fewer points of surface contact

  There are fewer London forces

  Molecules are therefore further apart

  Weaker London forces requires less energy is order to break

  Lower boiling points
• What is the bonding in alkanes?

  each carbon atom is joined to four other atoms by single covalent bonds - sigma bonds
• What is a sigma bond?

  Is the overlap of atomic orbitals directly between the bonding atoms

  representing a shared pair of electrons (a covalent bond)
• Define homolytic fission


  each of the bonded atoms takes one of the shared pair of electrons
• Define heterolytic fission


  one of the bonded atoms takes bother of the electrons from the bond
• What is a radical?

  a species with an unpaired electron
• What do you use to show movement of an electron pair when bonds are broken or made?

  Curly arrows
• What is an addition reaction?


  two reactant molecules join together to form one product (atom economy = 100%)
• What is a substitution reaction?


  an atom or group of atoms is replaced by a different atom or group of atoms
• What is an elimination reaction?

  a small molecule is removed from a large molecule
• What is the reactivity of alkanes?

  Sigma bonds are very strong

  Non-polar molecules - can't use heterolytic fission

  Electronegativity of carbon and hydrogen atoms are so similar that C-H bond is also considered non-polar

  Ea is very high
• Combustion of alkanes?

  Give out energy when reacted with oxygen.

  Show two different equations when showing incomplete combustion.

  (carbon dioxide OR carbon)

  Used as fuels because the energy is readily available - easy to transport
• What is needed for reaction of alkanes and halogens?

  UV radiation (aka - sunlight)
• What are observations of bromine reaction with C5H12

  Bromine is decolourised

  Moist litmus paper turns red (acidic)
• What is the reaction between halogens and alkanes?

  Substitution reaction - hydrogen replaced by bromine atom

  UV radiation is required for initial energy of reaction - no UV = bromine remains red-brown
• What are the stages of halogenation mechanism of alkanes?

  Initiation (formation of radicals via homolytic fission)

  Propogation (radicals on both sides of the equations - chain reaction)

  Termination (two radicals collide to form molecule with both electrons paired - stopping the chain reaction)
• What happens when further substitution occurs?

  Once haloaklane is formed in propogation, another bromine radical can attack it to form a dihaloaklane

  This can continue until all hydrogen atoms have been substituted with bromine

  We cannot control where on the chain the radical will attack!