Monday, November 8, 2010

Barbeque Chemistry

Hello there! It's me again with a pretty unusual subject for this rather cold period of the year: barbeque chemistry. Why? Because it reminds us of the hot summer weekends when we attend outdoor barbeques and because I have read an interesting science paper regarding barbeque fuel tablets.

When we go out and make a camp fire we might use the traditional method of matches and wood or if we are more lazy, we could use those camp fire lighters such as liquid petrol or solid fuels for rapidly starting up a fire. Some are based on phosphorus while others rely on an organic compound, hexamethylentetramine, also called hexamine or urotropine and this is the compound I am going to talk in the next few lines.

Urotropine has a spatial structure, having the form of adamantane, the simplest diamondoid with the only difference that the four methine groups are replaced by nitrogen atoms. Therefore, the structure is the following.


Industrially it is synthesized from methane oxidized in air at 400 to 600 oC on nitrogen oxides so as to obtain formaldehyde. Four molecules of formaldehyde react afterward with six molecules of ammonia and urotropine is the result. The conditions under which the reaction occurs are temperatures high enough so as the formaldehyde and the ammonia to be in gaseous form.


The possible mechanism for this reaction might involve three steps. The first one would be an addition of NH3 to the formaldehyde. Therefore, NH2-CH2-OH is obtained. After this, the compound eliminates water and the aldimine CH2=NH is formed. Through trimerisation a cyclic compound is formed and further reacted with 3 molecules of formaldehyde and one molecule of NH3 will finally make urotropine.


The urotropine synthesized this way is the major product in the solid fuel tablets. They have the advantage of burning on any type of weather with a rather hot flame at about 760 oC. Moreover, these tables do not give off fumes due to the fact that the only gases released are CO2, N2 and vapors of water: (CH2)6N4 + 9 O2 → 6 CO2 + 2 N2 + 6 H2O.

Image from: http://www.ebit.de/

It is important to mention that urotropine is used also to treat urinary infections due to its activation by the acids in urine but also as a precursor to many explosives such as HMTD, a compound suspected to have been used in the London underground terrorist attacks in 2005.

All in all, it's highly useful but it can turn out to lead to dangerous products so it is important to be used with care.

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