 Isomers may differ radically in “physical” properties such as melting points and boiling points as well as patterns of chemical reactivity. This is the case with dimethyl ether and ethyl alcohol, which have the compositional formula C2H6O in common, but are represented by two distinct structural formulas: (CH3)2O and C2H5OH. These formulas identify different functional groups, which govern patterns of chemical reactivity. The notion of a structural formula was developed to accommodate other isomers that are even more similar. This was the case with a subgroup of stereoisomers called optical isomers, which are alike in many of their physical properties such as melting points and boiling points and (when first discovered) seemed to be alike in chemical reactivity too. Pasteur famously separated enantiomers (stereoisomers of one another) of tartaric acid by preparing a solution of the sodium ammonium salt and allowing relatively large crystals to form by slow evaporation. Using tweezers, he assembled the crystals into two piles, members of the one having shapes which are mirror images of the shapes of those in the other pile. Optical isomers are so called because they have the distinguishing feature of rotating the plane of plane polarized light in opposite directions, a phenomenon first observed in quartz crystals at the beginning of the 19th century. These isomers are represented by three-dimensional structural formulas which are mirror images of one another as we show in Figure 2.