Difference between L&D and S&R in naming [duplicate]








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[What is the difference between D and L configuration, and + and −?](/questions/44260/what-is-the-difference-between-d-and-l-configuration-and-and-%e2%88%92)

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What is the difference between L&D and S&R? Can we say S-alanine instead of L-alanine?




The main difference between L, D configuration and *S*, *R* configuration is that the first one is relative configuration while the second one is absolute configuration. 


When you are distinguishing L-alanine from D-alanine, you only know that the $\ce{-NH2}$ group on the chiral carbon of alanine is on the left hand side, while in D it is on the right hand side relative in a Fischer projection. There you can't be specific about other functional groups (like $\ce{-COOH}$ or $\ce{-H}$) position. 


But when you distinguish *S* form of Alanine from the *R* form, it is done by proper arrangement of other two functional groups and a hydrogen atom. If any one of those is switched to some other position, the absolute configuration may change from *S* to *R* or from *R* to *S*. So, the configuration is more rigid in case of *R*-*S* configuration.  

As the L-D configuration doesn't specify other group's position, you can't always say *S*-alanine instead of L-Alanine unless you know total spatial arrangement of all functional groups of alanine.


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