[Dingo F]

I was reading an article about the oxidation of hydrocarbons (specifically jet fuels) and came across the following mechanism and would appreciate an explanation.



 1. What does the `R●` mean? Is it just a free radical of some description?
 2. Where does the `RH` come from in the kerosene?
 3. Although oxygen is required in both reactions, are there any other requirements. Temperature, pressure, light for example?
 4. What is the propagation stage?
 5. Why does only `R●` feature in the propagation stage and not`HO2●`?
 6. What is the chain branching stage?
 7. Why does `ROO●`go to alchohols and ketones. Does it react with
    alkanes and alkenes in the fuel? Why is this undesirable in a fuel?

 

[Borek]

RH basically means any hydrogen sticking out of the molecule.

The dot typically means an unpaired electron, so yes, R· is a radical.

Initiation, propagation, branching and termination are rather trivial - just look where the radicals appear, how they are used in the process, how does the process end.

[Enthalpy]

"R" denotes the Rest of the molecule, so RH is some molecule with a hydrogen atom. Quite general.

The reaction mechanisms rely on intermediates that need much energy, like 1eV or more while the room temperature is 26meV. This explains why reactions take microseconds or centuries while in a liquid, a molecule experiences a collision per picosecond. Even a flame relies on improbable intermediates like radicals.

Chains help many designed or observed reactions a lot. Once an improbable intermediate exists for some reason, it reacts easily with the other species, and in a chain, its reaction step leaves an other improbable and reactive intermediate. Until a different "termination" step suppresses the reactive intermediate(s), the chain continues ("propagates") the reaction hundreds or millions of times. A simpler example:
H° + Cl₂ HCl + Cl°
Cl° + H₂ HCl + H°
both are very efficient are room temperature, because H° and Cl° are reactive enough to break Cl₂ and H₂ with very little help. But H° or Cl° must first be created ("initiation") by a difficult process, and when they disappear in some sterile process ("termination"), for instance because two radicals recombine as a molecule, the chain stops.

In a flame or detonation, heat or pressure create new reactive species often enough, typically radicals, so that the reaction diverges despite the termination steps.

Expect a years-long reaction not to have diverged. It relies on external influences for its initiation steps. It could be the temperature, to which most reactions are extremely sensitive. It can be ozone or nitrogen oxides, catalysts (which can create new propagation steps too), impurities or bacteria, UV or visible light...

Slow oxidation reactions are sometimes desired, for instance when a siccative oil serves as a varnish, or to produce vinegar. They are unwanted in a fuel as they oxidise it partly before you burn it, so it has lost heating power in extreme cases; a fuel can also become more corrosive, get thicker or precipitate solids, develop a bad smell, form detonating peroxides in some cases, and whatever you don't want.

Additives can slow the oxidation. They can introduce new termination steps, like Halon gas quenches a fire, and scavenge the active intermediates. Many antioxidants operate this way.