I read that alkanes are 'colourless and odourless'. Heptane is an alkane. Why does heptane smell like heptane? Impurities?
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3$\begingroup$ 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. $\endgroup$– permeakraCommented Jan 16, 2015 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.
"The odor threshold for hexane is 130 parts per million (ppm), with a faint peculiar odor reported." ---http://www.epa.gov/ttnatw01/hlthef/hexane.html
"[Heptane is a] colorless liquid with a mild gasoline-like odor." ---http://www.cdc.gov/niosh/docs/81-123/pdfs/0312.pdf
"Alkanes have relatively high odor detection threshold (37 to 50 mg/m$^3$ for dodecane) as compared to other VOC groups such as sulphides (0.0003 to 0.16 mg/m$^3$ for dimethylsulphide), phenolics (0.022 to 4 mg/m$^3$ for phenol), and nitrogen heterocycles (0.0004 to 0.0008 mg/m$^3$ for skatole) (O'Neill and Phillips, 1992)" ---http://www.bae.uky.edu/jbicudo/odorVOC_02a.pdf
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.
- "Concanavalin A reveals olfactory receptors which discriminate between alkane odorants on the basis of size." ---http://www.ncbi.nlm.nih.gov/pubmed/2803264
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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.Commented Jan 16, 2015 at 17:08
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$\begingroup$ gasoline is never pure, it usually has alkene impurities, and sometimes special antidetonation additive. $\endgroup$ Commented Jan 16, 2015 at 17:21
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$\begingroup$ @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? $\endgroup$ Commented Jan 16, 2015 at 17:49
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$\begingroup$ @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. $\endgroup$ Commented Jan 16, 2015 at 18:29