How do E and Z isomers differ in alkenes?

E and Z isomers in alkenes differ in the spatial arrangement of their substituents around the double bond.

In alkenes, the E and Z isomers are types of stereoisomers, which are molecules with the same molecular formula and sequence of bonded atoms, but different three-dimensional orientations. The terms 'E' and 'Z' are derived from the German words 'entgegen' and 'zusammen', meaning 'opposite' and 'together' respectively. These terms are used to describe the relative positions of the highest-priority groups on each carbon of the double bond.

In the E isomer, the two highest-priority groups are on opposite sides of the double bond. This is often remembered as 'E for Enemy', as the groups are away from each other. On the other hand, in the Z isomer, the two highest-priority groups are on the same side of the double bond. This can be remembered as 'Z for Zusammen', as the groups are together on the same side.

The priority of the groups is determined by the Cahn-Ingold-Prelog (CIP) rules, which state that the group with the higher atomic number gets higher priority. If the atomic numbers are the same, then the next atom in the group is considered, and so on. For example, in an alkene with a bromine (Br) and a chlorine (Cl) atom on one carbon of the double bond, and a hydrogen (H) and a methyl (CH3) group on the other carbon, the Br would have higher priority than Cl, and the CH3 group would have higher priority than H.

The difference in the spatial arrangement of the E and Z isomers can lead to different physical and chemical properties. For instance, they may have different boiling points, densities, and reactivity. This is because the spatial arrangement of atoms in a molecule can affect how it interacts with other molecules. Therefore, understanding the difference between E and Z isomers is crucial in the study of organic chemistry.