Imagine a compound such as 1,3-dibromo-1,2,3-trichloropropane. Since the first and third carbons are attached to four different groups, they are both chiral centers. Assuming that both of these stereocenters are mirror images of one another (as in (1$S$-3$R$)-1,3-dibromo-1,2,3-trichloropropane), would the second carbon also be considered a stereocenter? If so, how would you differentiate the two enantiomers around that carbon using IUPAC nomenclature? Also, would such a carbon be optically active?
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asked May 23, 2016 at 21:41
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1$\begingroup$ Related: IUPAC name for 1,2,3-trichlorocyclopropane? $\endgroup$– user7951May 23, 2016 at 21:44
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$\begingroup$ Thanks! That addresses the nomenclature part, but would such a compound be optically active, as the two substituents on either side of the central carbon are different conformations despite being a meso compound? $\endgroup$– Niels KornerupMay 23, 2016 at 21:51
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$\begingroup$ It's also addressed in the question: "Note that the stereodescriptors “r” and “s” describing a pseudoasymmetric stereogenic unit are invariant on reflection in a mirror" Hence the mirror image is superimposable on itself and the molecule is optically inactive. For example if you look at the trichloropropane example given, the molecule clearly possesses a plane of symmetry $\endgroup$– orthocresolMay 23, 2016 at 23:03
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1 Answer
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Yes, carbon-2 is a stereocenter, and by (arbitrarily) giving the R conformation higher priority than the S, we can assign its stereochemistry using s/r notation:
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Pseudochirality simply means that a molecule has two substituents of a stereocenter are mirror images of each other, and the other substituents are either achiral or also mirror images of each other. Despite having multiple stereocenters, pseudochiral molecules are ahciral (optically inactive) because the mirror images of both the s and r conformations are identical.
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$\begingroup$ Pseudo-pseudo-chirality should also exist, right? If so, how do you designate it? $\endgroup$ May 24, 2016 at 13:07