Isomerism – What is the difference between a meso compound and a racemic mixture?
A closer look at the terms would already give you a hint.
A racemic mixture is not chemically bonded together hence we are able to separate the two equal portion (50/50) enantiomers. If we then pass plane-polarized light through just one of the enantiomers, there will be a optical activity.
A meso compound is one that has at least two chiral carbons and a internal plane of symmetry.
Hence the meso compound does NOT have enantiomers. Its mirror images are actually identical molecules. So while each chiral carbon will cause a rotation of plane-polarized light, the other chiral carbon will rotate it back, thus resulting in no net optical activity.
eg. in the tartaric molecule above, the top chiral center may rotate the light 5 degrees clockwise and the bottom chiral center may rotate it 5 degrees anticlockwise.
ps: Adding this on to emphasis that the term “meso compound” is actually quite misleading, as it is in fact one stereoISOMER of a compound. Which means the molecule itself might have other stereoisomers (see D-tartaric and L-tartaric acid below) that are optically active!
In conclusion, meso compound and racemic mixture both do not rotate plane-polarized light but for different reasons. Don’t let isomerism confuse you anymore! Come join our classes at Bukit Timah Shopping Centre to hear the detailed explanation!