Which of the following compounds is an E isomer?

Question

Quizplus · Accepted Answer

To determine if a compound is E or Z, we need to look at the substituents attached to each carbon of the double bond. In this case, the double bond is between carbons 9 and 10. Looking at compound A, we see that the substituents attached to carbon 9 are a methyl group (assigned priority 2) and a hydrogen atom (priority 4). The substituents attached to carbon 10 are a hydrogen atom (priority 4) and a phenyl group (priority 1). If we assign the highest priority group to be on the opposite side (i.e. "trans") of the double bond, we can see that the methyl and phenyl groups are on the same side, meaning the configuration is Z. The same analysis can be done for compounds C and D, which also have Z configurations.

However, looking at compound B, we see that the substituents attached to carbon 9 are a methyl group (priority 2) and a phenyl group (priority 1). The substituents attached to carbon 10 are a hydrogen atom (priority 4) and a chlorine atom (priority 3). If we assign the highest priority group to be on the opposite side of the double bond, we can see that the methyl and chlorine groups are on opposite sides, meaning the configuration is E. Therefore, the best choice only is B.

Which of the Following Compounds Is an E Isomer