Draw the stereoisomers of the following amino acids. Indicate pairs of enantiomers and pairs of diastereomers.
Pictured it has leucine and isoleucine as the structures to draw stereoisomers of. I am going to focus of the first part of the problem, labeling R and S of just the second molecule, isoleucine. In the solution there are four options of stereoisomers that have two chiral centers, such as in isoleucine. There are the R R, S S, R S, and S R configuation options. Leucine, with only one chiral center has only one sterioisomer, its enantiomer.
I mostly wanted to do this problem to show my "Idiot's" way to determine R and S. First thing is to be able to decipher which, R or S, is clockwise. R is easy to remember because when you turn a car Right, the steering wheel turns clockwise. If for some reason I forget that I remember S is counterclockwise because if you start an arror from the top tip of the line that makes the S and draw arrow barbs pointing downward midway down the S the arrow curves around in a counter clockwise fashion. That would be much easier to understand if drawn out...
Now, as far as determining configuration. Rather than spatially rearranging to get the lowest priority group in the back before finding the configuration of the first, second, and then third priority groups I use a rule a of enantiomers. An enantiomer is the opposite configuration of another molecule which occurs when two of its groups are switched. That noted, if the lowest priority group isn't in the back you can still determine the configuration, just make sure you switch it. If you have a Hyrodgen as a substituent off of a chiral center and is not in the back and you determine the first, second, and third priority groups turn clockwise, R fashion, then it has the S configuration.