Rank the following in order of decreasing leaving group ability, putting the best first. $$\begin{array} { l c c c }
\mathrm { Cl } ^ { - } & \mathrm { Br } ^ { - } & \mathrm { I } ^ { - } & \mathrm { H } _ { 2 } \mathrm { O } \
\text { I } & \text { II } & \text { III } & \text { IV }
\end{array}$$

Question

Rank the following in order of decreasing leaving group ability, putting the best first. $$\begin{array} { l c c c } 
\mathrm { Cl } ^ { - } & \mathrm { Br } ^ { - } & \mathrm { I } ^ { - } & \mathrm { H } _ { 2 } \mathrm { O } \
\text { I } & \text { II } & \text { III } & \text { IV }
\end{array}$$

Quizplus · Accepted Answer

The leaving group ability of an anion increases with its size and polarizability. Iodide is the largest and most polarizable of the given anions, so it is the best leaving group. Bromide is smaller and less polarizable than iodide, so it is a less good leaving group. Chloride is even smaller and less polarizable than bromide, so it is an even less good leaving group. Water is the smallest and least polarizable of the given anions, so it is the worst leaving group.

Rank the Following in Order of Decreasing Leaving Group Ability