TBR says not all enantiomers/diastereomers are optically active. can you give an example?
When is an enantiomer not optically active?
- Thread starter theCLE
- Start date
- Joined
- Mar 18, 2012
- Messages
- 271
- Reaction score
- 2
Has to be experimentally determined as far as I remember. No way to predict activity, degree of activity or direction. All we know if it is +x one way, then its enatiomer will go -x the other way.
- Joined
- Aug 13, 2006
- Messages
- 1,226
- Reaction score
- 14
meso compounds, or when enantiomers are in a racemic mixture. also i am not certain diastereomers are necessarily optically active since I believe they include geometric isomers with alkenes. Chiral atoms are needed for optical activity. I am doing this from memory and not checking my sources so feel free to correct me if I am wrong.
Good luck!
Verónia
- Joined
- Jun 23, 2010
- Messages
- 11,799
- Reaction score
- 2,808
You're correct about meso compounds. The key here is that both optical isomers (chiral centers AKA R and S isomers) and geometric isomers (asymmetry across an alkene double bond AKA E and Z isomers) are classes of configurational isomers and thus can both have enantiomers and diastereomers.
Geometric isomers don't show optical activity at all, so the enantiomer or diastereomer of a particular geometric isomer of a compound will not be optically active.
As far as optical isomers, the only case where an optical isomer doesn't show optical activity that I know of is with meso compounds, as Verónica said. In that case, the enantiomer of a meso compound is itself, because it exhibits an internal plane of symmetry.
