2.5 - Hydrocarbons

• What is crude oil composed of?

  A mixture of different hydrocarbons
• How is crude oil separated into its different hydrocarbons?

  Fractional distillation
• How does fractional distillation work?

  Separates components of a mixture based on their boiling points.
• Where in the fractionation column do the short chain hydrocarbons with the lowest boiling points removed and why?

  Near the top as they condense in the low temperatures there
• What is cracking?

  Processing large less useful molecules to produce smaller and more useful ones
• What two products does cracking produce?

  Alkanes and alkenes
• What are the two main types of cracking?

  Catalytic and thermal
• How does thermal cracking work?

  High heat and pressure are used to break down the molecules. The reaction also involves free radicals.
• Give the reaction conditions for the chlorination of alkanes?

  UV light, presense of chlorine gas
• Why is UV light needed for the chlorination of alkanes?

  The UV light causes homolytic fission of the chlorine molecule to produce very reactive chlorine free radicals that will react with the hydrocarbon
• What are the three steps in the reaction mechanism for the chlorination of alkanes (free radical substitution)?

  1. Chain initiation
  2. Chain propagation
  3. Chain termination
• Describe the difference in reactivity between alkanes and alkenes.

  All of the covalent bonds in alkanes are sigma bonds. Alkenes contain a double carbon bond which is made up of a sigma bond and a pi bond. Sigma bonds are stronger than pi bonds and so alkanes are more stable/less reactive than alkenes.
• How is a sigma bond formed?

  A sigma bond is formed from the overlap of s-orbitals
• How is a pi bond formed?

  A pi bond is formed from the sideways overlap of p-orbitals
• What is the typical reaction for an alkene, due to the presense of the double carbon bond?

  Electrophilic addition
• What two examples of electrophilic addition do I need?

  Hydrogen bromide

  Bromine
• Electrophilic addition of hydrogen bromide results in 2 products, what are they?

  • 2-bromo(alkane)
  • 1-bromo(alkane)
• Why is the major product of electrophilic addition of HBr 2-bromo(alkane)?

  The secondary carbocation is the more stable arrangement
• How can the structure of a carbocation be determined?

  Decide based on the number of R-groups attached to the carbon that the electrophile has bonded to.
• Tests for carbon-carbon bond (alkene):

  • Bromine water (brown->colourless)
  • Potassium manganate - acidic cond. (purple-> colourless) OR alkaline cond. (purple-> dark green)
• What are the conditions for catalytic hydrogenation of an alkene?

  • Nickel catalyst
  • 300 degrees celsius
• What is the inducstrial use of alkene hydrogenation?

  Unsaturated oils and fats are hydrogenated in the process known as hardening - this produces products like margarine
• What is polymerisation?

  The combination of a very large number of molecules called monomers to form a larger molecule called a polymer
• Why can alkanes not undergo polymerisation?

  They are saturated/don't contain a double bond
• Reaction conditions for polymerisation of ethene:

  • 2000 atm pressure
  • 200 degrees celsius
  • presense of oxygen
• What is the general formula for alkanes?

  Cn H2n+2
• What is the general formula for alkenes?

  Cn H2n
• What is the functional group of the alkenes?

  Alkenes have a carbon-carbon bond as their functional group
• What is the functional group of the alkanes?

  Alkanes do not have a functional group.