Support Article: Chain Reactions

This article has been written to support the “Artemis” by Andy Weir — Blame it on the Moon post.

An important part of the plot in the novel Artemis is the production of chloroform gas from Jazz’s industrial sabotage of the Sanchez aluminium smelter. Here we present an account of how chloroform might be produced by a 4-step chain reaction from the starting materials of gaseous methane and chlorine.

In the Sanchez plant we have chlorine, methane and heat-energy from Jazz’s tampering with the temperature regulation mechanism. It’s quite likely that some chlorine radicals would be generated (see below)  to start the initiation phase for a chain reaction. Only relatively few chlorine radicals are needed. In the discussion that follows we need to distinguish between chlorine molecules (Cl‑Cl) and chlorine radicals (⋅Cl) and chlorine atoms (Cl).

The energy to form chlorine radicals (⋅Cl) is often provided using UV-light, but heating in the range 400-500°C would work just as well. You might remember from chemistry class [1] that a bond is formed by shared electrons. So when the bond between the chlorine atoms, in a chlorine molecule (Cl‑Cl) is broken, one electron goes with each of the chlorine atoms, denoted below as ⋅Cl, with the dot indicating a free unpaired electron. Because of the unpaired electron they’re called “radicals” and they’re highly reactive.

The reactive chlorine radicals, generated above, start propagation of a chain reaction by abstracting a hydrogen from methane, forming HCl but leaving behind a ⋅CH3 methyl radical. The ⋅CH3 methyl radical abstracts a chlorine from another chlorine molecule (Cl‑Cl) regenerating a chlorine radical ⋅Cl which starts the process all over again. Hence the name propagation. During propagation, you can see from the equations below that the chlorine radicals keep being regenerated from the available chlorine gas (Cl‑Cl).

Moreover, propagation continues through the 4-steps shown below. Additonally, chain reactions are notoriously difficult to control, leading to numerous by-products. In this case we would get a mixture or chloromethane, dichloromethane, chloroform and carbon tetrachloride, all in roughly similar proportions (see below).

The chain reaction comes to an end when all the radicals are consumed by termination reactions (shown below) which leads to further by-products (two of which are shown),

Termination occurs when two radicals combine with each other, thus depleting the chain reaction of radicals needed to continue the propagation reactions.

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[1] Since I’m a former chemistry teacher, I’ve heard people say that they don’t remember anything from chemistry class so often it’s not funny. Don’t worry, I’ll take you through this as gently as possible.