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

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  • $\begingroup$ Read stereochemistry in any standard textbook of organic chemistry. $\endgroup$ – Chakravarthy Kalyan Feb 17 '18 at 13:01
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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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