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Consider 2,3,4-pentanetriol.Is the middle carbon atom a stereocentre?I know how to count the stereoisomers, but is the middle carbon a stereocentre by definition?Let us say we have a 2R and 4S configuration.Now if we change the orientation of the OH group at the 3rd carbon, we do get a different compound,but both the compounds are optically inactive, so it doesn't seem it should be called a stereocentre.If we consider 2R,4R, or 2S,4S if we change the configuration at C3, we get the same compoound.So I feel the carbon at C3 should not be a stereocentre.But it seems that in the first case, we are changing the C3 configuration and getting a new diastereoisomer, so C3 might be a stereocentre after all.Which one is correct?If C3 is a stereocentre, what would be its R,S nomenclature?

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    $\begingroup$ This earlier answer deals with a similar situation and should prove helpful. $\endgroup$ – ron Apr 22 '17 at 17:36
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The carbon atom in question is called pseudo-asymmetric. Check it out on the IUPAC page.

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