Which of the following is the best leaving group in a nucleophilic substitution reaction?

• A. ${\mathrm{OH}}_{-}$
• B. ${\mathrm{Br}}_{-}$
• C. ${\mathrm{Cl}}_{-}$
• D. ${\mathrm{TsO}}_{-}$

Answer: (D)

Verified step by step guidance

1. Understand that the quality of a leaving group in nucleophilic substitution reactions depends on its ability to stabilize the negative charge after departure. A good leaving group is typically a weak base and can stabilize the negative charge well.
2. Compare the leaving groups given: hydroxide ion (\( \mathrm{OH}^- \)), bromide ion (\( \mathrm{Br}^- \)), chloride ion (\( \mathrm{Cl}^- \)), and tosylate ion (\( \mathrm{TsO}^- \)).
3. Recall that hydroxide (\( \mathrm{OH}^- \)) is a strong base and generally a poor leaving group because it does not stabilize the negative charge well, making it less likely to leave.
4. Halide ions like bromide (\( \mathrm{Br}^- \)) and chloride (\( \mathrm{Cl}^- \)) are better leaving groups than hydroxide because they are weaker bases and more stable after leaving, with bromide being better than chloride due to its larger size and better charge distribution.
5. Tosylate ion (\( \mathrm{TsO}^- \)) is an excellent leaving group because it is resonance stabilized and very weakly basic, making it the best leaving group among the options for nucleophilic substitution reactions.