10.2.3 The free radical thermal cracking of alkanes

What is the free radical mechanism for the cracking of alkanes?

what free radicals are formed in cracking reactions of alkanes?

what is the reaction mechanism for converting alkanes to alkenes?

What is the importance of cracking to the petrochemical industry?

How is cracking used in the oil industry?

The free radical mechanism for the cracking of alkanes to give lower alkanes, alkenes and hydrogen is given in a multi-step sequence and explained in detail.

• What is cracking? Why is it done?

  • Cracking is a type of chemical reaction in which hydrocarbon molecules, usually saturated hydrocarbons called alkanes, are broken down into smaller molecules by the application of heat, pressure and catalysts.
  • Cracking is done to provide a greater variety of hydrocarbon molecules from oil fractions, that perhaps otherwise would be of little value. Apart from the fact that oil does not contain valuable chemical stocks like alkenes, the distribution of alkanes in oil doesn't fit the commercial profile of hydrocarbon needs e.g. not enough lower alkane petrol or diesel fuel molecules.
  • Typical example of a cracking reaction ...
  • octane ==> hexane + ethene
  • C₈H₁₈ C₆H₁₄ + C₂H₄
  • 
  • Cracking reactions involve complex steps mainly involving the formation and reaction of free radicals - h9ighly reactive species with an unpaired electron.

• What is the reaction mechanism of hydrocarbon alkanes being cracked to form alkenes, smaller alkanes and hydrogen?

• e.g. CH₃CH₂CH₃ ==> CH₄, CH₂=CH₂, CH₃CH₃, CH₂=CHCH₃, H₂  [mechanism 31]

  • The equation is not meant to be balanced, but just to show the variety of possible products.

  • Balanced equations e.g. two possible cracking reaction outcomes of propane, and all will happen, but adjustment of temperature, pressure or catalyst can bias the product composition towards a desired outcome.

    • propane ===> methane + ethene

      • CH₃CH₂CH₃ ===> CH₄ + CH₂=CH₂

    • propane ==> propene + hydrogen

      • CH₃CH₂CH₃ ===> CH₂=CHCH₃ + H₂

    • See mechanism 31 below to illustrate the idea.

• When alkane hydrocarbons are heated to a high temperature (450-900^oC, with/without superheated steam) they are thermally decomposed or 'cracked' to form mainly alkanes of lower C number, alkenes of equal or smaller C number and hydrogen.

mechanism 31 - free radical thermal cracking of alkanes (red dot = unpaired electron)

• Cracking occurs via free radical reaction mechanisms and the diagram above illustrates the free radical reactions that can result from heating even a simple molecule like propane to a high temperature.

  • The free radical mechanistic steps show how alkenes, lower alkanes and hydrogen can all be formed.

• When the temperature is high enough, the kinetic energy of the particles is sufficient to cause bond fission on collision, and this initiates a free radical chain reaction.

• Step (1) is the initiation step when a C-C bond in an alkane molecule is split into two alkyl free radicals by homolytic bond fission. This means the original C-C bonding electron pair is split between the two alkyl radicals formed. 

  • The weakest bonds will break first in the initiation step, so the C-C bond (bond enthalpy 348 kJmol^-1) will tend to break first as the C-H bond is stronger (bond enthalpy is 412 kJmol^-1).

  • The red dots represent the unpaired electron on the free radical.

  • Free radicals are highly reactive species with an unpaired electron and tend to form a new bond as soon as is possible, in this case by ...

    • abstracting a hydrogen from another molecule e.g. steps (2) to (6) or pairing up with another radical e.g. steps (7) and (8).

• Steps (2) to (6) are chain propagation steps, because as well as a product, a free radical is also produced to continue the chain reaction and lead to, in this case, a variety of other products.

• Steps (7) and (8) are two possible chain termination steps which remove the highly reactive alkyl free radicals. The unpaired electrons from the two radicals 'pair up' to form a new bond.

  • You can also have ethyl radical chain termination by simultaneous alkene and alkane formation (not shown in the mechanism 31 diagrams).

    • e.g. 2CH₃CH₂^. ==> CH₂=CH₂ + CH₃CH₃ 

    • or even isomerise on combination to form methylpropane (CH₃)₂CHCH₃ as well as the expected butane.

• FURTHER COMMENTS

  • Not all possible steps are shown, and not necessarily in that order all the time, but those that are shown illustrate the possibilities of how a wide variety of products can be formed.

  • You can write similar mechanisms for any other alkane from butane upwards in carbon chain length.

APPENDIX

  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)