sm_logo.gif (1738 bytes) Meso Compounds

- Meso compounds are a special case of stereoisomers which occur when a molecule has two chiral centers and each chiral center has identical substituants.
- Consider the stereoismers of 2,3-dibromobutane.

Both C-2 and C-3 are chiral centers, they are sp3 hybridized and they have four different substituants: -H, -CH3, -Br and -CHBrCH3.

Notice that both chiral centers have the same substituants.

- In this case, we have the possiblity of stereoisomers called meso compounds.
- There should be four stereoisomers of 2,3-dibromobutane:

This is a pair of isomers are enantiomers:

(2R,3R)      (2S,3S)

To see this clearly, rotate each molecule so that the Br attached to C-2 (colored cyan so it is easy to spot) is pointed up and the methyl groups attached to each C-2 are pointed towards each other.

These molecules are non-superimposable, mirror images or enantiomers.

What about this pair of molecules?
(2R,3S)      (2S,3R)

Rotate them as described above... they look like enantiomers, but they are actually the same compound!

To prove this, rotate one of the molecules 180o around the y-axis (i.e. the vertical axis).

Both absolute configurations shown above (i.e. 2R,3S and 2S,3R) describe this compound well, yet neither is correct!

The correct designation of this stereoisomer is (meso)-2,3-dibromobutane.

- The meso compound is a diastereomer of both of the compounds shown on the top row.

Copyright 1996 -1999, 2007 by Frank R. Gorga - All rights reserved.

Last Update: 12-Mar-2007