Is Chirality Special?

From WikidChem

No, chirality is not that special. Except for some simple achiral (meso) molecules, most molecules are chiral.

The "LET'S DECODE AT NOON BUT KEEP IT MUM" example helps show why (in two dimensions).

If the message is reflected over the x axis, most of the words are not superimposible. Only "MUM" looks the same (is achiral). If the message is reflected over the y axis, only "DECODE" is superimposible. (Note that "superposition" allows rotation - changing an even number of signs - but not reflection or inversion - changing an odd number - JMM)

When the signs of x and y are changed to rotate the message, "NOON" looks the same, but it is still chiral because it only has rotational symmetry. Only "MUM" and "DECODE" are achiral.

Just as most words are not superimposible on their mirror images, most molecules are not superimposible. Thus, most molecules are chiral.

The notion that chirality (or handedness) is not necessarily a "special" feature of a select few organic molecules is supported by the fact that many familiar compounds are chiral. (Chiral centers are most easily identified as a tetrahedral carbon with 4 different R groups, but as we saw with the twice double bonded carbon chain C=C=C, there are other ways in which molecules can be chiral). Most sugars are chiral, as well as many of the acids we have discussed this semester (tartaric, lactic, malic, etc). Most drugs on the market today consist of chiral compounds, and determining which enantiomer is an active drug and which is not (and efficiently separating the two) is an important process in the pharmaceutical industry. Case in point--one marketed drug had an enantiomer which was actually harmful for pregnant women!