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I'm working on a science project which involves the production of methane. I would like to be able to use the methane as fuel in (e.g.) cars (using octane) or gas tanks (using butane and propane) etc. I have found that there are a variety of coupling reactions out there, but a lot of them seem to involve 'aryl Grignard reagents' which are not relevant to me.

So, I'm looking for a relatively simple method to lengthen an alkane. Preferably the reaction should occur below 100 degrees Celsius.

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    $\begingroup$ Alkanes are not very reactive. I'd say there is no simple method to do anything at all with an alkane. $\endgroup$ – Ivan Neretin Dec 26 '15 at 7:34
  • $\begingroup$ I agree. Thankfully, cars don't run on noble gas fuel. That would be a real problem. :D $\endgroup$ – Shirayuki Rin Dec 27 '15 at 3:05
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    $\begingroup$ If you produce methane, can't you modify that step to generate longer alkanes? (en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_process) $\endgroup$ – Gyro Gearloose Dec 28 '15 at 17:03
  • $\begingroup$ I'd love to. Thing is, I'm using methanogens. $\endgroup$ – Shirayuki Rin Dec 29 '15 at 12:23
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The procedure to convert methane to longer hydrocarbons at low temperature is one of the so-called Holy Grails in catalysis (http://pubs.acs.org/doi/abs/10.1021/ar00051a001). The person who is able to invent a process wherein this is possible (at an efficient rate) would be a very good candidate for a Nobel Prize.

Why is it then so difficult to convert methane to longer hydrocarbons? Methane is a highly stable molecule. In order to form a longer hydrocarbon, methane first has to be broken down to smaller CH$_{x}$ fragments. The first dehydrogenation step of methane (to CH$_{3}$) has a very high activation energy and even using a good catalyst, this is not feasible at an appreciable rate under 100 degrees celcius.

Industrially, this process is done by first creating synthesis gas (CO + H$_{2}$) from methane by partial oxidation using pure oxygen (which is cryogenically separated from air). Thereafter, the synthesis gas is compressed to roughly 20 atm of pressure and converted to very long hydrocarbons (waxes) in the Fischer-Tropsch process. In turn, these waxes are cracked to form diesel fuels and lubricants.

I am afraid that there is no easy way of converting methane to longer hydrocarbons. Though please keep trying because basically the whole world would benefit if you succeed! :-)

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    $\begingroup$ But make sure the methane come from $\ce{CO2}$ and $\ce{H2}$ in some way, because otherwise it would be terrible for the environment ;) $\endgroup$ – Jan Dec 26 '15 at 11:26
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    $\begingroup$ I fully agree :) And let us generate the H$_{2}$ also in a sustainable manner. $\endgroup$ – Ivo Filot Dec 26 '15 at 11:40
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    $\begingroup$ (Hint: Use $\ce{H2}$, that automatically generates $\ce{H2}$ ;)) $\endgroup$ – Jan Dec 26 '15 at 11:42
  • $\begingroup$ You sir, are technically correct. The best kind of correct. Thanks for sharing. ;) $\endgroup$ – Ivo Filot Dec 26 '15 at 14:58
  • $\begingroup$ Thanks @ifilot! By the way, do you think that halogenation of the methane to create $\ce{CH3Cl}$ and using a coupling reaction (or something like it) to get ethane and $\ce{Cl2}$ would work? $\endgroup$ – Shirayuki Rin Dec 27 '15 at 3:01
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You might want to look into the Heck reaction

As ifilot said, you would win a Nobel Prize for this kind of chemistry, and as it happens, a Nobel Prize was awarded for the discovery of this reaction!

This can be used to join together any two carbon chains where one ends in a halogen and the other ends in a hydrogen bonded to a carbon which is part of a double or triple bond.

Here it is in its generalised format (credit Wikipedia): enter image description here

As you can see, any R group which it attached to a halogen (X) will bond to another R group which ends in a double or triple bond.

The exact mechanism is a little involved so I won't bother talking about it.

So now you need halogenated alkanes and alkenes to proceed.

  • Look into the halogenation of alkanes. Even though alkanes are usually unreactive, they can be halogenated in the presence of ultraviolet light. Check out this link for more: http://www.chemguide.co.uk/organicprops/alkanes/halogenation.html

  • Look into cracking alkanes, which produces alkenes as a result.

    EDIT: There are other reactions that can join carbon chains together such as reactions involving Grignard reagents, the Wittig and the Diels-Alder reaction.

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