Answer
The best leaving group, in this case, is the mesylate.
Work Step by Step
1. Identify the possibles leaving groups:
- $EtO^-$ ($CH_3CH_2O^-$)
- $Mesylate$ ($CH_3O_3S^-$)
2. Determine which one is better.
- In this section, we've learned that sulfonate ions, like the mesylate, are good leaving groups (page 219).
- And we have also learned that, the $CH_3CH_2O^-$ is a bad leaving group (page 218).
Therefore, this mesylate is better.
