# leaving groups

Rank the following electrophiles in order of decreasing reactivity with NaN_{3} in DMF. (1 = most reactive)

This problem is similar to problem 536. The only difference among these compound is their leaving group, so the compound with the best leaving group will undergo substitution reactions most rapidly.

Good leaving groups are stable. Larger ions tend to be more stable than smaller atoms (due to a smaller charge:size ratio. See problem 288), so when going down the periodic table, stability increases. I^{-} is more stable than Br^{-}, which is more stable than Cl^{-}, etc. So I^{-} is the best leaving group, and 2-iodobutane will react the fastest with a nucleophile.

Rank the following electrophiles in order of decreasing reactivity with NaN_{3} in DMF. (1 = most reactive)

The only differences among these three molecules is their leaving groups, so whichever has the best leaving group will react fastest with the nucleophile (azide, N_{3}^{-}). Stable molecules make good leaving groups, so the best leaving group of the three is SO_{3}R^{-}, which has has three resonance forms (see problem 535). So the third compound will react the fastest with NaN_{3}.