It’s the old system – it predates Cahn-Ingold-Prelog. So why does it still get used? Shouldn’t it be consigned to the dustbin of history, along with slide rules, 8-track cassettes, and 5 ¼” floppy disks? The D-L system is literally a remnant of the horse-and-buggy era, dating back to Emil Fischer’s work on carbohydrates in the late 1800s – a time when organic chemists had no way to determine the absolute configuration of stereocenters, which only became possible in 1951 ( thx, Bijvoet). (Maybe someday there will be communes where people only use 1970s and 1980s computer technology?) Well, there are thriving communities in parts of rural America where horse-drawn carriages persist – if you know where to look. Likewise there is a pocket of organic chemistry where D-L system still finds use, and that is specifically in the realm of sugars and amino acids. There are at least 3 good reasons, in the specific case of sugars and amino acids, for using L- and D- : This not a revolt by Amish chemists against the modern evils of the CIP system, by the way. The L-/D- system allows for the configuration of a molecule with multiple chiral centers to be summarized with a single letter (plus its common name, of course – thanks to Noel for the reminder) D-glucose is a heck of a lot faster to write and say than ( 2R,3S,4R,5R)2,3,4,5,6-pentahydroxyhexanal.
Plus, L- and D- refer specifically to absolute configuration, while (as we noted previously) there is no simple relationship between the sign of optical rotation and configuration. It turns out that most naturally occurring sugars are D-, and most naturally occurring amino acids are L.
There is a tremendous amount of information compressed in that statement, and there is no competing system (R/S, +/–) which could replace the L- and D- with a single character.
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