[ektabh]

Hi,
 I am preparing for my DAT exam and I had a few questions... First, I  wanted to ask how many total possible stereoisomers are there for 1,2-dimethylcyclopropane? I thought since the structure has 2 chiral centers, it should have 4 possible stereoisomers... but the answer is 3..

the question also  said to count pairs of enantiomer as two different isomers.

2. Plane polarized light in R stereocenters is in which direction? I thought clockwise, but I am wrong..

3. Racemic mixtures and meso compounds do not rotate plane-polarized light, right??
4.CH3CH=CHCH3  +HBR ---> how come the products are racemic, is it because you are forming a new chiral center???

[Mitch]

There are 4 isomers like you thought but in this case R,S and S,R are the same thing.

[movies]

There are 4 isomers like you thought but in this case R,S and S,R are the same thing.

Yes, the equation to figure out the number of stereoisomers only holds if none of the stereoisomers is a meso compound.  In this case, the compound with the two methyl groups syn to one another is meso.

Remember that the definition of stereoisomer is a compound that differs from another only by the stereochemistry, but that difference can be anything.  Enantiomers are a special case of stereoisomers where to two compounds are also non-superimposable mirror images of one another.  Diastereomers are then stereoisomers that are not mirror images of one another.