Question

In the following groups: -OAc (I), \(\quad\)-OMe (II) \(-\mathrm{OSO}_{2} \mathrm{Me}\) (III), \(\quad-\mathrm{OSO}_{2} \mathrm{CF}_{3}\) (IV) the order of the leaving group ability is (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) \(\mathrm{IV}>\mathrm{III}>\mathrm{I}>\mathrm{II}\) (c) \(\mathrm{III}>\mathrm{II}>\mathrm{I}>\mathrm{IV}\) (d) II \(>\mathrm{III}>\mathrm{IV}>\mathrm{I}\).

Short answer

The order is (b): IV > III > I > II.

Step-by-step solution

Define Leaving Group Ability

The ability of a group to be a good leaving group is determined by how easily the group can accept electrons as it departs. A more stable leaving group in solution signifies a better leaving group. Generally, weak bases and those groups that stabilize the negative charge are better leaving groups.

Analyze Leaving Groups

For each group, examine the structural features and the stability of the leaving group: - **-OAc (I)**: Acetate anion is moderately stabilized by resonance. - **-OMe (II)**: Methoxy is a poor leaving group since methoxide is a strong base. - **-OSO₂Me (III)**: This group forms the mesylate anion, a good leaving group due to resonance stabilization and the presence of the weaker base. - **-OSO₂CF₃ (IV)**: This triflate group forms the triflate anion, which is an excellent leaving group, highly stabilized by resonance and inductive effects due to the strongly electron-withdrawing CF₃ group.

Rank Leaving Groups

Order the groups based on their stability and ability to leave: 1. **-OSO₂CF₃ (IV)** because it forms the highly stable triflate anion. 2. **-OSO₂Me (III)** forms the mesylate, also a good leaving group but less stable than triflate. 3. **-OAc (I)** because acetate anion is moderately stable due to resonance, but not as stable as IV or III. 4. **-OMe (II)** because the methoxide anion is a strong base and thus a poor leaving group.

Choose the Correct Answer

Based on the ranking, the correct order of leaving group ability is **IV > III > I > II**. This corresponds to option (b).

Key concepts

Leaving Group Ability

In organic chemistry, the concept of leaving group ability is important when understanding reaction mechanisms, especially in nucleophilic substitution reactions. A leaving group is an atom or group that detaches from the parent molecule during a chemical reaction. Its ability to leave effectively depends largely on its stability upon departure. - Stable leaving groups can better accommodate the negative charge they receive when they leave. This often means they are weak bases. - A weak base is less likely to recombine with the parent molecule, making it a more effective leaving group. Consider these properties while evaluating leaving group ability:

• Structural stability in solvation: The more stable the ion in a solvent, the better the leaving group.


• The tendency to stabilize negative charge: Groups that can delocalize or reduce the intensity of the charge make for better leaving groups.

When examining common groups like -OAc (acetate), -OMe (methoxy), -OSO₂Me (mesylate), and -OSO₂CF₃ (triflate), it's evident that the triflate and mesylate are good leaving groups due to their ability to stabilize the negative charge, whereas methoxy is not due to its strong basic nature.

Resonance Stabilization

Resonance stabilization is a key factor influencing the leaving group ability in organic molecules. It involves the delocalization of electrons across a molecule, enhancing the stability of the ion or group resulting from the leaving group. Resonance allows for:

• Spread of electronic charge over multiple atoms, which reduces the energy and instability associated with any single charged atom.


• Increased stability of ions like acetate and triflate, which have resonance structures that help distribute the negative charge.

For instance, the acetate anion (-OAc) is stabilized moderately by resonance, which enables it to be a potential leaving group. However, the trifluoromethanesulfonate anion (-OSO₂CF₃) is significantly more stable due to extensive resonance delocalization, making it one of the best leaving groups. This concept is crucial in the ranking of the groups because a resonance-stabilized leaving group will facilitate the forward direction of the reaction without the anion reverting to the original molecule.

Inductive Effects

Inductive effects refer to the transmission of charge through a chain of atoms, which can influence a molecule's stability and reactivity. In the context of leaving groups, inductive effects can significantly affect how good a group is at departing. Key Points about Inductive Effects include:

• Electron-withdrawing groups imbue stability to a compound through the pull on electron density, often leading to stronger inductive effects.


• In groups like triflate (-OSO₂CF₃), the electron-withdrawing nature of the CF₃ (trifluoromethyl) group exerts a strong inductive effect, greatly stabilizing the anion after the group leaves.


• This increased stability through electron-withdrawal means such groups can leave more readily compared to groups like methoxy (-OMe), which not only lack this stabilization but are also strong donors due to opposite inductive effects.

Understanding inductive effects thus contributes to predicting reaction outcomes, as a leaving group able to stabilize itself via inductive withdrawal will support reaction processes favorably, reinforcing why triflate (IV) is superior to other groups like methoxide (II).