Reactions of alkane

Cracking reactions
"Cracking" breaks larger molecules into smaller ones. This can be done with a thermic or catalytic method. The thermal cracking process follows a homolytic mechanism, that is, bonds break symmetrically and thus pairs of free radicals are formed. The catalytic cracking process involves the presence of acid catalysts (usually solid acids such as silica-alumina and zeolites) which promote a heterolytic (asymmetric) breakage of bonds yielding pairs of ions of opposite charges, usually a carbocation and the very unstable hydride anion. Carbon-localized free radicals and cations are both highly unstable and undergo processes of chain rearrangement, C-C scission in position beta (i.e., cracking) and intra-and intermolecular hydrogen transfer or hydride transfer . In both types of processes, the corresponding reactive intermediates (radicals, ions) are permanently regenerated, and thus they proceed by a self-propagating chain mechanism. The chain of reactions is eventually terminated by radical or ion recombination.
Here is an example of cracking with butane CH3-CH2-CH2-CH3
• 1st possibility (48%): breaking is done on the CH3-CH2 bond.
CH3* / *CH2-CH2-CH3
after a certain number of steps, we will obtain an alkane and an alkene: CH4 + CH2=CH-CH3
• 2nd possibility (38%): breaking is done on the CH2-CH2 bond.
CH3-CH2* / *CH2-CH3
after a certain number of steps, we will obtain an alkane and an alkene from different types: CH3-CH3 + CH2=CH2
• 3rd possibility (14%): breaking of a C-H bond
after a certain number of steps, we will obtain an alkene and hydrogen gas: CH2=CH-CH2-CH3 + H2
Halogenation reaction
R + X2 → RX + HX
These are the steps when methane is chlorinated. This a highly exothermic reaction that can lead to an explosion.
1. Initiation step: splitting of a chlorine molecule to form two chlorine atoms. A chlorine atom has an unpaired electron and acts as a free radical.
Cl2 → Cl* / *Cl
energy provided by UV.
2. Propagation (two steps): a hydrogen atom is pulled off from methane then the methyl pulls a Cl from Cl2
CH4 + Cl* → CH3* + HCl
CH3* + Cl2 → CH3Cl + Cl*
This results in the desired product plus another Chlorine radical. This radical will then go on to take part in another propagation reaction causing a chain reaction. If there is an excess of Chlorine, other products like CH2Cl2 may be formed.
3. Termination step: recombination of two free radicals
• Cl* + Cl* → Cl2, or
• CH3* + Cl* → CH3Cl, or
• CH3* + CH3* → C2H6.
The last possibilty in the termination step will result in an impurity in the final mixture; notably this results in an organic molecule with a longer carbon chain than the reactants.
Combustion
R + O2 → CO2 + H2O + H2
Is a very exothermic reaction. If the quantity of O2 is insufficient, it will form a poison called carbon monoxide (CO). Here is an example with methane:
CH4 + 2 O2 → CO2 + 2 H2O
with less O2:
2 CH4 + 3 O2 → 2 CO + 4 H2O
with even less O2:
CH4 + O2 → C + 2 H2O