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I read that alkanes are 'colourless and odourless'. Heptane is an alkane. Why does heptane smell like heptane? Impurities?

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    $\begingroup$ Alkanes as molecules cannot interact with most receptors, as receptors usually bind to polar or charged groups and to lesser extent to double bonds. This makes light alkanes in small concentrations indeed odorless and colorless. However, alkanes may dissolve in lipid membranes, changing their properties. In addition, yes, alkanes usually are impure. Likely, the most common impurities are alkenes, that also can produce further impurities on contact with air under UV. || not an answer, as I'm not an expert on odor theory. $\endgroup$ – permeakra Jan 16 '15 at 17:29
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Some higher-molecular weight alkanes definitely have an odor, but they aren't as intense as most of the compound types you'll find stinking up an organic lab.

According to the following paper, there are apparently at least two olfactory receptors in rats that respond to alkanes; one accepts alkanes with areas around 3.5 nm$^2$, and another responds to hydrocarbons with areas of about 5 nm$^2$. The authors note that response to cycloalkanes is higher than for normal alkanes, so the shape matters too.

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  • $\begingroup$ Just wanted to add that gasoline in cars is mainly composed of alkanes from 6 to 12 or 14 Carbons. If I'm wrong please correct me. $\endgroup$ – M.A.R. Jan 16 '15 at 17:08
  • $\begingroup$ gasoline is never pure, it usually has alkene impurities, and sometimes special antidetonation additive. $\endgroup$ – permeakra Jan 16 '15 at 17:21
  • $\begingroup$ @permeakra, yes, the smell of gasoline comes from many stinky non-alkanes, so I'll retract that as an example. Nevertheless, I have no problem smelling pure heptanes or hexanes, do you? $\endgroup$ – Fred Senese Jan 16 '15 at 17:49
  • $\begingroup$ @FredSenese When I'm not breathing in vapors directly from the bottle, I do have such problem. Comparing to such solvents as pyridine, ethyl acetate and acetone, hexane (I have not worked with heptane, though) virtually has no odor. On the other hand, even $\ce{CO2}$ in high concentrations produces some feeling. $\endgroup$ – permeakra Jan 16 '15 at 18:29

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