# What are some examples of enantiomers?

Suppose you had an identical twin (for all I know you might have one!). When you two shake hands, can your right hand shake his/her left hand? The answer is no, even though your hands should be identical.* I raise this example, because left and right hands are enantiomers; they are "handed" or enantiomeric. Some molecules can exhibit the same sort of isomerism; where one stereoisomer cannot be superimposed on its mirror image. In simple terms, any carbon that has 4 distinct substituents, i.e. $C {R}_{1} {R}_{2} {R}_{3} {R}_{4}$, can exist as a pair of non-superimposable mirror images. That particular carbon is the steroegenic or chiral centre.
Lactic acid, ${H}_{3} C - C \left(O H\right) \left(H\right) \left(C {O}_{2} H\right)$, fits the above description. I can't recall which is the naturally occurring isomer. The amino acid alanine, ${H}_{3} C - C \left(N {H}_{3}^{+}\right) \left(H\right) C {O}_{2}^{-}$, glyceraldehyde, $H O C {H}_{2} C H \left(O H\right) C \left(= O\right) H$, are all examples of molecules that can exist as two optical isomers, depending on disposition of substituents around the chiral carbon (can you identify this centre?). Biology features a great deal of handedness: proteins and sugars all have a particular such handedness or chirality.