How do you distinguish E and Z isomers for a disubstituted alkene?

• A. Assign CIP priorities; E if high-priority groups are opposite, Z if they are on the same side
• B. Count the number of substituents on each carbon
• C. Use infrared spectroscopy to identify configuration
• D. Compare boiling points of the two alkene isomers

Answer: (A)

The key idea is stereochemical arrangement around a carbon–carbon double bond. To tell E from Z, you assign priorities to the two substituents on each double-bond carbon using the CIP rules. Then you compare where the high-priority groups sit relative to the plane of the double bond: if the high-priority groups are on opposite sides, the isomer is E (from entgegen); if they are on the same side, it is Z (from zusammen). This method works for disubstituted alkenes because you’re looking at which groups of highest priority on each carbon point across the bond. Infrared spectroscopy or boiling points don’t reliably reveal this arrangement, since they reflect other properties rather than the spatial orientation of substituents around the double bond.