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

  • 2
    $\begingroup$ Please use caution and discuss this with someone who is knowledgeable in this area before undertaking this experiment on your own. $\endgroup$
    – jonsca
    Nov 23 '13 at 21:01

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.

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:

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


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.