SolutionsConcept of Leaving Group
1. Order the halides
Order from best to worst leaving group: I−, F−, Cl− and Br−.
I− > Br− > Cl− > F−: the larger the atom, the more dispersed the charge and the less basic the anion.
2. HO− and H2O
Why is hydroxide a poor leaving group and water a good one? What does this mean for alcohols?
HO− is a strong base (the pKa of water is 15.7) and H2O a very weak one (the pKa of H3O+ is −1.7). Alcohols do not undergo substitution directly, but in acid the OH group is protonated and leaves as water.
3. Sulfonates
Why are sulfonates, RSO3−, excellent leaving groups?
The negative charge is shared by resonance among the three oxygens, so the anion is a very weak base: its conjugate acid, the sulfonic acid, is a strong acid.
4. Which reacts faster?
Which will react faster with NaCN by SN2, CH3CH2Cl or CH3CH2I?
CH3CH2I, because iodide is a better leaving group than chloride.