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Two confusing thing about (2⁠R,3⁠S)-2,3-dichloropentane:

  1. The 2 stereocenters have opposite absolute configuration.
  2. But there is no plane of symmetry.

So, is (2⁠R,3⁠S)-2,3-dichloropentane optically inactive?

(This is actually a follow-up question of: Monochlorination of 2-chloropentane and possible enantiomer products)

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    $\begingroup$ ad 1: the two stereo centers are different also constitutionally - structurally. So what should be the reason for optical inactivity? $\endgroup$ – mykhal May 11 '17 at 11:51
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    $\begingroup$ You're misinterpreting the answers for the other question. Both of those compounds are chiral and count towards the 5 mentioned in the other answer. $\endgroup$ – Zhe May 11 '17 at 12:17
  • $\begingroup$ @zhe Ok, now I understand. I had misinconception I had the notion that initial chirality of 2-carbon could change in the cholrination. $\endgroup$ – Mockingbird May 11 '17 at 12:30
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To have an optically inactive meso compound with two chiral centers:

1) the chiral centers must have opposite configuration

2) the chiral centers must have the same set of fragments bonded to them

Among other things the 2 carbon has a methyl group bonded to it versus the 3 carbon having an ethyl group instead. You do not satisfy (2), so you do not have the symmetry required for a meso compound, and the molecule is still chiral and optically active.

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