Why does heptane have an odour

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




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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