3
$\begingroup$

My teacher told us the middle carbon in 3‐bromopentane‐2,4‐diol is chiral:

$$\ce{CH3-CH(OH)-\overset{\star}{C}H(Br)-CH(OH)-CH3}$$

Since there are two identical groups around it, I would guess it should not be chiral.

It was a pre-recorded class and I was not able ask this question. Has my teacher made a mistake?

Improve this question
$\endgroup$
4

1 Answer 1

Reset to default
10
$\begingroup$

Carbons #2 and #4 are chiral centres themselves. There are four possible diastereoisomers:

Diastereomers of 3-bromopentane-2,4-diol

If C2 and C4 have different configurations, which is the case in molecules A and D, then C3 is termed a pseudoasymmetric centre and labelled with a small 'r' or 's'. These diastereomers are meso compounds: although they contain chiral centres, they are not chiral, because they have an internal plane of symmetry.

On the other hand, in molecules B and C, C2 and C4 have the same configuration and C3 is not a chiral centre (and not a pseudoasymmetric centre either). These diastereomers are chiral, though: the chirality comes from C2 and C4.

Improve this answer
$\endgroup$
4
  • $\begingroup$ So it's chiral in some cases and not in others ? Like Zhe said in the comments ? $\endgroup$
    – Glowingbluejuicebox
    Jan 20, 2021 at 17:26
  • $\begingroup$ Actually, some of the diastereoisomers are chiral molecules, i.e. without internal mirror image or rotational symmetry. I hope this helps. $\endgroup$
    – z1273
    Jan 20, 2021 at 18:07
  • 2
    $\begingroup$ Rotational symmetry isn't related to chirality. To be more precise, the lack of improper rotations $S_n$ is associated with chirality; but proper rotations $C_n$ have no relation to chirality. PS sorry about the edits. I need to double check this. $\endgroup$
    – orthocresol
    Jan 20, 2021 at 19:00
  • $\begingroup$ Think that should be right now.. $\endgroup$
    – orthocresol
    Jan 20, 2021 at 19:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.