Safety concerns for radical halogenations of methane in sunlight?
Let's have a look at some fundamental data and the chemistry involved.


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?

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 ($\lambda$ &lt; 490 nm) to initiate the self-propagating reaction:

$$\ce{Cl2 -&gt;[h\nu] 2 Cl*}$$

Then the chain reaction starts:

$$\ce{Cl* +\  CH4 -&gt; HCl + CH3*}$$
$$\ce{CH3* +\  Cl2 -&gt; CH3Cl + Cl*}$$

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

$$\ce{2 Cl*  -&gt; Cl2}$$

$$\ce{2 CH3*  -&gt;  C2H6}$$
$$\ce{ CH3* +Cl* -&gt;  CH3Cl}$$

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.


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!