What is the configuration of the following stereoisomer of 3-methyl-2-pentene? (The numbers are for reference in the solution.)\\\quad H_3C \enspace 2 \quad 3\quad CH_3 \\\qquad \diagdown \downarrow \quad \downarrow\diagup \\\qquad\enspace C=C \\\qquad \diagup \qquad \diagdown \\\quad H \qquad CH_2CH_3
Assign the relative priorities of the two groups attached to each carbon. The two atoms directly attached to carbon-2 are C and H. Because C has a higher atomic number (6) than H (1), the CH_3 group is assigned the higher priority, as shown in the example for step 1. Now consider the groups attached to carbon-3. The example accompanying step 2 and rule 2 shows that ethyl has higher priority than methyl. The priority pattern is therefore as follows:
Because groups of like priority are on opposite sides of the double bond, this alkene is the E isomer; its complete name is (E)-3-methyl-2-pentene.