Question

In the following groups: OAc (1) \(-\) OMe \((2)\) \(-\mathrm{OSO}_{2} \mathrm{Me}(3)\) \(-\mathrm{OSO}_{2} \mathrm{CF}_{3}(4)\) the order of the leaving group ability is (a) \(1>2>3>4\) (b) \(4>3>1>2\) (c) \(3>2>1>4\) (d) \(2>3>4>1\).

Short answer

The order of the leaving group ability is (b) 4 > 3 > 1 > 2.

Step-by-step solution

Determine Leaving Group Ability

The leaving group ability depends on the stability of the leaving group after it detaches from the molecule. A better leaving group is usually the more stable one that is weakly basic, often due to the ability to accommodate a negative charge.

Analyze Each Group

Examine each group: 1. OAc forms acetate ion, a moderately stable leaving group. 2. OMe forms a methoxide ion, which is less stable due to its strong basicity. 3. OSO2Me forms mesylate ion, which is a very good leaving group due to resonance stabilization. 4. OSO2CF3 forms triflate ion, one of the best leaving groups due to the strong electron-withdrawing effect of CF3.

Rank the Leaving Groups Based on Analysis

Rank the groups based on their stability and thus their leaving group ability: - OSO2CF3 (triflate) is the best leaving group, followed by OSO2Me (mesylate), and then OAc (acetate). OMe (methoxide) is the worst as it is a strong base. - Therefore, the order is OSO2CF3 > OSO2Me > OAc > OMe.

Match the Order with Provided Options

The correct order from Step 3 is 4 > 3 > 1 > 2. The option that corresponds to this order is (b): 4 > 3 > 1 > 2.

Key concepts

Stability of Leaving Groups

When evaluating the leaving group ability in chemical reactions, we must consider the stability of the ion or molecule that leaves. Stability often determines how easily a leaving group departs. A stable leaving group is one that can handle a negative charge efficiently once it detaches. This is important because the more stable the leaving group, the less energy is required for its removal, leading to a more favorable reaction. Key factors for stability include:

• Resonance: The ability of a leaving group to delocalize its charge through resonance enhances its stability considerably.
• Inductive effects: Electron-withdrawing groups can stabilize the negative charge by delocalizing or drawing away electrons.
• Conjugate acidity: A leaving group's stability is often correlated with the acidity of the corresponding acid. A weaker acid produces a weaker conjugate base, which is a poorer leaving group, and vice versa.

Thus, a strong leaving group tends to be a weak base.

Basicity and Acid-Base Reactions

Basicity plays a crucial role in determining the leaving group ability because it directly affects the stability of the group. A basic ion, such as a hydroxide ion ( ext{-OH}), is less stable as a leaving group due to its tendency to attract protons and form a more stable structure. In the context of acid-base reactions:

• Stronger bases are typically poorer leaving groups because they are less stable as ions.
• Conversely, weaker bases are better leaving groups as they are more stable in ionic form.

For example, the methoxide ion ( ext{-OMe}) is a poor leaving group because its strong basicity implies less stability as a free ion, whereas triflate ion ( ext{-OSO}_{2} ext{CF}_{3}) is an excellent leaving group due to its weak base nature and highly stabilized structure.

Electron-Withdrawing Effects

Electron-withdrawing effects greatly influence the stability of leaving groups. These effects occur when surrounding atoms or groups pull electrons away from the leaving group, stabilizing any negative charge that remains after it detaches. The role of electron-withdrawing groups is pivotal:

• They increase the stability of the leaving group by reducing its electron density.
• This occurs because the electron-withdrawing group can delocalize or lower the energy of the negative charge.

A classic example is the triflate ion ( ext{-OSO}_{2} ext{CF}_{3}), where the ext{CF}_{3} group is a strong electron-withdrawing group. It stabilizes the otherwise unstable negative charge, making triflate a superior leaving group.

SN1 and SN2 Reactions

In the arena of substitution reactions, namely SN1 and SN2 reactions, the ability of the leaving group is crucial. **SN1 Reactions:**

• These are unimolecular nucleophilic substitution reactions.
• They involve the leaving group departing first, leading to the formation of a carbocation intermediate.
• The stability of this intermediate largely determines the reaction rate, and a stable leaving group enhances the formation of the carbocation because it departs more readily.

**SN2 Reactions:**

• These are bimolecular nucleophilic substitution reactions.
• They involve the simultaneous bond-making to the nucleophile and bond-breaking from the leaving group.
• A strong leaving group promotes this simultaneous transition, as it readily departs when the nucleophile approaches.

Thus, in both SN1 and SN2 reactions, a good leaving group can significantly enhance the reaction rate by either favoring the formation of an intermediate (SN1) or assisting the transition state’s completion (SN2).