What Do You Call A Left-Handed Lemon? QI

QI is a high-intelligence television show in Britain, formerly hosted by Stephen Fry and now hosted by Sandi Toksvig.

In this clip, the QI panelists are asked, 'what do you call a left-handed lemon' and subsequently learn that the molecule that produces the distinct smell of a lemon is a chiral molecule meaning that it has a mirror image that cannot be superimposed upon the original molecule and that the mirror image molecule produces the smell of an orange instead. 

Source - wikipedia

Chirality is real and present in any molecule where a central carbon atom has four unique substituent groups off of the same center. Think of the molecules as being similar like your left and right hands. They look identical but can't be rotated in any way to make them overlap identically. Instead, they would have to be 'mirrored' to allow for the overlap.

Interestingly, I read an awful Fantastic Four comic a few years back based on this idea of chirality, but I digress.

Sadly, even though limonene is, indeed, chiral with mirror images of the compound existing, those mirror images aren't really the reason for the difference in the smells of lemons and oranges. According to the American Chemical Society, that's a myth.

Actually, both oranges and lemons contain mostly (R)-limonene. Only 1-4% of the limonene in either fruit is in the (S) configuration. As for what these enantiomers actually smell like, (R)-limonene does have a pleasant, citrusy aroma, but does not smell like oranges. Instead, various other, fragrant molecules found in orange oil carry the odor. At high purity, (S)-limonene carries notes of turpentine and lemon. However, this enantiomer is barely present in lemons and is unlikely to contribute much to their aroma. As with orange odor, a number of different molecules in lemon oil contribute to lemons’ fresh scent.