# How is the central carbon in 3‐bromopentane‐2,4‐diol chiral?

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?

• The carbons either side bearing the -OH groups are chiral so are potentially non-identical Jan 20, 2021 at 16:49
• The carbon can only be pseudochiral. Its configuration can matter but in that case, it will sit in the plane of symmetry.
– Zhe
Jan 20, 2021 at 17:00
• This issue is addressed here: ursula.chem.yale.edu/~chem220/chem220js/STUDYAIDS/isomers/… Jan 20, 2021 at 17:34
• Also see this; chemistry.stackexchange.com/questions/91539/… Jan 20, 2021 at 18:07

## 1 Answer

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

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.

• So it's chiral in some cases and not in others ? Like Zhe said in the comments ? Jan 20, 2021 at 17:26
• Actually, some of the diastereoisomers are chiral molecules, i.e. without internal mirror image or rotational symmetry. I hope this helps. Jan 20, 2021 at 18:07
• 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. Jan 20, 2021 at 19:00
• Think that should be right now.. Jan 20, 2021 at 19:07