Introduction to Stereochemistry

Shown below are two molecules having the formulas CH2ClBr which are drawn as mirror images of each other. Are these molecules superimposable; that is, are these two molecules identical?

You should have found that these two molecules are identical. Now examine the two molecules shown below. These molecules have the formulas CH3CHClBr and again are drawn as mirror images of each other. Are these molecules superimposable; that is, are these two molecules identical?

In this case, these molecules are not identical. There is no way to rotate these molecules that will allow you to line up all four groups around the central carbon into identical positions. The central carbon is referred to as a 'chiral' or 'stereogenic' center. Stereogenic carbon centers require that the carbon be surrounded by four different groups. Note that the entire group attached to the carbon must be considered.