Answer by Diptarka Hait:
The boiling point of alkanes () flies up fairly fast. See the attached image (from Wikipedia):
Admittedly, this is only for straight chain (n) alkanes, and branched alkanes have less surface area, and so boil more easily. However, it suffices to say that anything with more than 6 carbons is probably not going to hang out as a gas (2,2 dimethyl butane has a boiling point of ~ 50 degrees Celsius).
That leaves us with hexanes and lower homologues as potential candidates. And prima facie they are Greenhouse gases too.
We first need to understand what makes a Greenhouse gas. Objects above absolute zero emit radiation (look upfor a deeper explanation), as they are made up of charged particles who move about as a result of thermal energy. This motion causes electromagnetic interactions that ultimately cause electromagnetic radiation.
The Sun is a very hot object, with a surface temperature of . If we approximate it as a perfect blackbody, we find that it emits maximum energy at -right in the middle of the visible part of the spectrum. This matches fairly well with our observation that a lot of the solar radiation comes from the visible part of the spectrum, and so we are somewhat confident that the blackbody approximation is not total lunacy.
The law I am using is , which holds well at these regimes. Using this with the surface temperature of the earth (lets say 300K, but anything thereabout will work)-I find that the earth emits terrestrial radiation at -in the IR region of the spectrum. Anything that absorbs IR radiation, and re-emits it-thus preventing it from flying out of earth is a potential Greenhouse gas (GHG) candidate.–
Fortunately the atmosphere is mainly and , two gasses that are forbidden from absorbing IR radiation on account of their symmetries (they have a zero transition dipole moment, which is necessary to absorb IR radiation). The monoatomic inert gasses cannot absorb IR either (as they do not have vibrational degrees of freedome, necessary to absorb IR). However, and absorbs IR, and both are known GHGs.
Methane () also absorbs IR (despite what it's highly symmetric structure might suggest, it has IR active modes). I attached an image of the sprectum from NIST.
Two peaks are prominent : ~ 3000 from C-H bond stretching and ~1500 from C-H bond bending. Both are nothing special to methane, and every alkane possesses similar peaks (though at slightly different wavelengths). Thus ethane, propane, hexane etc can all absorb terrestrial radiation and are indeed GHGs.
That being said, no one is freaking out over them as:
1. The other gaseous alkanes have no major source of production. I have not yet read about widespread propane or ethane produce bacteria, compared with the ubiquitousness of methanotrophs. Anything that gets out essentially comes from us (whether from industry or someone like me forgetting to cover the hexane containing TLC development chamber-fume hoods are not 100% efficient!).
2. Other alkanes have far shorter atmospheric lifetime. The hydroxy free radical, oxidizes most of them down soon. Methane however, is relatively resistant on account of the instability of the methyl radical, which makes it the slowest to oxidize. The ethyl radical (a primary carbon centre radical and thus second least stable type of radical) is about 16kJ/mol stabler-that corresponds to an approximate lifetime that is times smaller than that of methyl at 298K. Methane lasts for about 7 years into the atmosphere, ethane lasts 3 days. Propane etc lasts even less, as then secondary carbon centres are available to make radicals from-which are stabler than primary radicals.
Take the numbers with a pinch of salt, as the exact oxidation mechanism details can vary somewhat, and the atmosphere can be colder/warmer than 298K at specific locations. However, the basic point is that other alkanes have too small an atmospheric lifetime to be serious GHGs, even if one ignored their tiny production rates.