Problem 134 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

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

Step 1: Understand Leaving Groups

Leaving groups are atoms or groups of atoms that detach from a substrate during a chemical reaction. A good leaving group is one that can stabilize the negative charge after leaving.

Step 2: Analyze Each Group's Ability

Compare the ability of each group by considering how well they can stabilize a negative charge once they leave. A good leaving group typically makes a stable, weakly basic molecule once it leaves.

Step 3: Evaluate Each Leaving Group

- : OAc: Acetate ion that can leave because it's resonance stabilized. - II: OMe: Methoxy group, generally a poor leaving group because it's not very stable when alone. - III: OSO2Me: Mesylate, a good leaving group due to resonance stability and electronegativity. - IV: OSO2CF3: Triflate, one of the best leaving groups due to high electronegativity and resonance stabilization.

Step 4: Determine Order of Leaving Group Ability

Based on the stability analysis of the leaving anions, the order of leaving group ability is: IV (triflate) > III (mesylate) > I (acetate) > II (methoxy).

Step 5: Select the Correct Option

Given the order of leaving group ability, the correct option is (b) IV > III > I > II.

Key Concepts

Resonance Stability

Resonance stability plays a crucial role in determining the ability of a molecule to act as a leaving group. When a group leaves a molecule, it ideally wants to disperse its negative charge over a wider area. This is where resonance comes into play.

• Resonance allows for the distribution of negative charge across multiple atoms.
• This distribution stabilizes the molecule, making the leaving group more favorable.

For example, in the acetate ion (\(\text{OAc}\)), resonance stability is achieved as the negative charge can be delocalized between the two oxygen atoms.
In our example, both mesylate and triflate possess strong resonance stabilization, making them excellent leaving groups.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons towards itself. In the context of leaving groups, electronegativity can enhance a molecule's ability to stabilize a negative charge after it departs.

• A more electronegative atom can better stabilize the negative charge.
• This often results in a more stable, less reactive leaving group.

Triflate (\(\text{OSO}_2\text{CF}_3\)), with its highly electronegative fluorine atoms, exemplifies this property, making it an outstanding leaving group due to its lower basicity and increased ability to stabilize the negative charge.

Negative Charge Stabilization

Negative charge stabilization is a key factor in evaluating potential leaving groups. The better a group can stabilize a negative charge, the more likely it can act effectively as a leaving group.

• This stabilization can occur through resonance, electronegativity, or the overall structure of the molecule.
• A stable negative charge means the leaving group does not react undesirably with the surrounding environment.

For instance, mesylate (\(\text{OSO}_2\text{Me}\)), through both resonance and its structure, can effectively stabilize the negative charge, making it a proficient leaving group.

Mesylate

Mesylate(\(\text{OSO}_2\text{Me}\))is considered a good leaving group due to its ability to stabilize a negative charge. It achieves this through two main properties:

• Resonance stability: Mesylate ions can distribute the negative charge through resonance, leading to an overall stable ion.
• Electronegativity: With oxygen atoms present, mesylate benefits from additional electronegative elements that assist in stabilizing the charge.

As a result, its structure makes it an attractive and effective leaving group in many chemical reactions.

Triflate

Triflate (\(\text{OSO}_2\text{CF}_3\))represents one of the most effective leaving groups available in organic chemistry. Its exceptional properties stem from:

• High electronegativity: The presence of fluorine atoms, which are highly electronegative, enhances triflate's ability to stabilize a negative charge.
• Resonance capability: Similar to mesylate, triflate can disperse its negative charge over multiple atoms, making it highly stable.

These unique features make triflate a powerful leaving group, often used in challenging chemical reactions where a reliable leaving group is essential.