Answer
The iodide is a better leaving group.
Work Step by Step
1. Identify the possibles leaving groups:
- $Iodide $($I^-$)
- $Water$ ($H_2O$)
2. Determine which one is better.
- In this section, we've learned that some halides, like iodide, are good leaving groups (page 219).
- And we have also learned that $H_2O$ is a good leaving group too (page 218).
But, if we look at the image in page 218, we will see that, the $I^-$ is more stable than $H_2O$. Therefore, iodide is a better leaving group.
** You can also compare the strength of the conjugate acids, so:
- $H_3O^+$ and $HI$.
$HI$ is stronger, therefore, $I^-$ is a better leaving group
