As written...#H_3stackrel(1)C-CHBrCHBrstackrel(4)CH_3# (I had trouble representing this on the editor...the chiral carbons, #stackrel2C# and #stackrel3C# are not labelled but their substitution is evident).
Anyway, the beast has two chiral centres... In principle it has #2^n# isomers where #n=2#, i.e. #n="the number of chiral carbons"#; POTENTIALLY 4 diastereomers....
Now #"RR"# is the mirror image of #"SS"#...and these are non-superposable, and I presume that only the ONE of these enantiomers is biochemically active....and the industrial synthesis would produce this isomer. But the #"RS"# disastereomer is THE SAME as the #"SR"# disastereomer.....given the symmetry of the molecule. With me? You will see this relationship if you make a model.
