# Identifying Meso compounds [closed]

(source: iupac.org)

I can't seem to understand why these compounds are meso compounds..... I don't see any plane of symmetry here. I have trouble identifying meso compounds so can you give me heads up on how to identify them

• Let's start from the obvious. Look at the third compound. Apply a plane which is vertical and perpendicular to the screen. Sep 18 '18 at 11:30
• But the chlorine atoms are projecting out of the plane from different sides while the hydrogen are just going down the plane identically... Wouldn't the chlorine mess the mirror images Sep 18 '18 at 11:34
• Of course the chlorine atoms stick out from different sides. The same can be said about the whole left vs right halves of the molecule. That's how mirror planes work, isn't it? Sep 18 '18 at 11:37
• Yea..........Okay then then how about the second structure. I am pretty sure it doesn't have a plane of symmetry Sep 18 '18 at 11:41
• OK, look at the second structure (I mean the imaginary 3D model, not a picture on the screen). Gently grab it by the horns with both your hands. Now move the left side a little closer to you and the right side farther from you, until the central H eclipses the central OH and you recognize the same vertical plane. Sep 18 '18 at 11:49

Why don't use the definition just above the image?

meso-compound

A term for the achiral member(s) of a set of diastereoisomers which also includes one or more chiral members.

Achiral, i.e. the molecule is superimposable with its mirror image. So, if finding a plane of symmetry, or other symmetry elements in the molecule is difficult for you, a simpler method (and according to the definition) might be drawing a mirror image, i.e. all chiral elements inverted

or unchanged but drawn "right-to-left", i.e. flipped

or flipped vertically

.. (just use any of the mirroring methods) and deciding whether it's the same molecule (stereoisomer), by rotating it, in 3D, in your head. (Some internal rotations around single bonds might be needed as well.)

In the first case, the mirrored structure can be superimposed with the original one by in-plane 180° rotation around an axis perpendicular to the paper plane.

In the second case it can be done by 180° rotation around the in-plane horizontal axis.

In the third case it can be done by 180° rotation around the in-plane vertical axis.

• How did you draw the second structure....... Did you just flip it. And for checking me so conpound, do So have to just make mirror images and check if they are Distereomers Sep 18 '18 at 22:40
• @Abhinav yes, flipping in fact a mirroring (Reflection(mathematics) see 0th section bottom). Make mirror image and check if such expected enantiomer is in fact the same compound (the definition is somewhat confusing). Sep 19 '18 at 5:59
• @Abhinav .. but flipping as an image! Some chemical drawig software flips the structure with preservation of the stereochemistry, i.e. it also inverts the configuration, so the configuration is intentionally preserved. Sep 19 '18 at 6:13

Meso compounds need not only have a plane of symmetry. They can also have a point of symmetry.

The upper left compound has a point of symmetry at the centre of the central C-C bond.

The upper right compound has a plane of symmetry passing through the central H & OH groups.

The lower compound has a plane of symmetry passing through the common side of the two benzene rings.

The ideal way to identify the line of symmetry is to analyse the compound in 3-D. You may use models for that.

• That's not quite enough. The $S_{4}$ point group does not have a plane or point of symmetry, but it has an improper axis of rotation.
– Zhe
Sep 18 '18 at 17:34
• I don’t understand what compound are you talking about. Please reframe. Sep 18 '18 at 17:44
• This isn't a compound, this is a whole class of compounds, in fact anything that's in a $S_{n}$ point group. example
– Zhe
Sep 18 '18 at 17:56