Safety concerns for radical halogenations of methane in sunlight?

Question

I wanted to know about the substitution reaction of alkanes particularly interested in the reaction of methane with fluorine and chlorine.

Answer 1

Let's have a look at some fundamental data and the chemistry involved.

1. Chlorine is an inflammable gas with a boiling point of -35 °C and a nice yellowish-green colour. It is heavier than air.

   • It is highly irritant and toxic. It is a strong oxidant. With water, it partly decomposes to hydrogen chloride and hypochlorous acid. Your mucous membranes are wet. Inhalation of chlorine results in severe corrosive injuries. Inhale a tad longer to face dyspnoea, coughing of blood and asphyxia. Did I mention the pulmonary oedema and fatality?

2. Methane is a flammable gas with a boiling point around -160 °C and a flash point around -180 °C. The lower explosion limit of methane in air is around 5%.

   • Open flames and sparks are not your friend here.

For the mixture of methane and chlorine, red light districts are safe. The bond dissociation energy of chlorine is $243\, \mathrm{kJ \cdot mol^{-1}}$, you will need blue light ($\lambda$ < 490 nm) to initiate the self-propagating reaction:

$$\ce{Cl2 ->[h\nu] 2 Cl*}$$

Then the chain reaction starts:

$$\ce{Cl* +\ CH4 -> HCl + CH3*}$$ $$\ce{CH3* +\ Cl2 -> CH3Cl + Cl*}$$

All that goes on and on; termination steps are radical recombinations:

$$\ce{2 Cl* -> Cl2}$$

$$\ce{2 CH3* -> C2H6}$$ $$\ce{ CH3* +Cl* -> CH3Cl}$$

Apparently hydrogen chloride gas is is produced in the reaction, together with methyl chloride.

• Methyl chloride is a flammable gas (boiling point around -24 °C, flash point around -20 °C, lower explosion limit in air around 8%).
• GHS hazard statements file it with H-351: suspected of causing cancer.

If you perform the reaction with a large excess of chlorine, $\ce{Cl*}$ radicals will attack methyl chloride, generate dichloromethane via the chloromethyl radical $\ce{*CH2Cl}$, etc. Tetrachloromethane ($\ce{CCl4}$) would be the final product then.

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That was the healthy and boring part!

When fluorine talks to chlorine, it goes like in the musical "Annie, Get Your Gun":

Anything you can do I can do better!

Particularly when it comes to running havoc on your tissue!

As far as the photolysis of fluorine in the presence of methane is concerned: People have done that!

Typical conditions were:

• Deposit methane in a fluorine matrix at 15 K
• Co-deposit methane/argon and fluorine/argon samples at 12 K
• Trap fluorine in a methane matrix at 15 K

Yes, these guys had no sense for adventure - but they weren't blown away either!