Chapter 26: Problem 91
In the following groups: 1\. \(-\mathrm{OAc}\) 2\. - OMe 3\. \(-\mathrm{OSO}_{2} \mathrm{Me}\) 4\. \(-\mathrm{OSO}_{2} \mathrm{CF}_{3}\) the order of leaving group ability is (a) \(1>2>3>4\) (b) \(4>3>1>2\) (c) \(4>2>1>3\) (d) \(2>3>4>1\)
Short Answer
Expert verified
The correct order is option (b): 4>3>1>2.
Step by step solution
01
Understand Leaving Group Ability
Leaving group ability is a measure of how easily a group can depart from a molecule. A good leaving group is generally stable on its own after departure and usually a weak base.
02
Analyze Each Leaving Group
Evaluate each leaving group's ability. 1. The acetate ion (\( -\mathrm{OAc} \)) is moderately stable, due to resonance stabilization.2. The methoxy group (\( -\mathrm{OMe} \)) is a poor leaving group; it is a strong base and not stable alone.3. The mesylate ion (\( -\mathrm{OSO}_{2} \mathrm{Me} \)) is a good leaving group, due to resonance and inductive effects from the sulfonate group.4. The triflate ion (\( -\mathrm{OSO}_{2} \mathrm{CF}_{3} \)) is an excellent leaving group, the best among these, due to the strong electron-withdrawing effect of the trifluoromethyl group which stabilizes its resulting negative charge.
03
Order Them by Leaving Group Ability
Considering the stability of each leaving group after leaving:- Triflate (\( -\mathrm{OSO}_{2} \mathrm{CF}_{3} \)) is the best due to its strong electron-withdrawing effects.- Mesylate (\( -\mathrm{OSO}_{2} \mathrm{Me} \)) comes next due to good resonance stabilization.- Acetate (\( -\mathrm{OAc} \)) is less effective as a leaving group but better than methoxy due to some resonance stabilization.- Methoxy (\( -\mathrm{OMe} \)) is a poor leaving group because it is a strong base.
04
Match with Given Options
Comparing our determined order of leaving group ability (4 > 3 > 1 > 2) to the provided options, the correct choice is (b) \(4>3>1>2\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Resonance Stabilization
Resonance stabilization is an essential concept in understanding the stability of different ions in organic chemistry. When a molecule or ion can distribute its charge across several structures, it gains stability. This is known as resonance stabilization.
For example, the acetate ion \(-\mathrm{OAc}\) benefits from resonance. It can disperse its negative charge across the oxygen atoms, creating a stabilizing effect.
This effect makes acetate better at leaving compared to a group like methoxy, which lacks significant resonance stabilization. Multiple forms contributing to the overall stability are called resonance structures. These structures do not exist alone but add to the whole molecule's stability by sharing the charge.
For example, the acetate ion \(-\mathrm{OAc}\) benefits from resonance. It can disperse its negative charge across the oxygen atoms, creating a stabilizing effect.
This effect makes acetate better at leaving compared to a group like methoxy, which lacks significant resonance stabilization. Multiple forms contributing to the overall stability are called resonance structures. These structures do not exist alone but add to the whole molecule's stability by sharing the charge.
Electron-Withdrawing Effects
Electron-withdrawing effects involve groups pulling electrons towards themselves, which can stabilize negative charges.
When a leaving group departs, it often leaves behind a negative charge. Groups like the trifluoromethyl (\(\mathrm{CF}_{3}\)) in triflate \(-\mathrm{OSO}_{2} \mathrm{CF}_{3}\) have strong electron-withdrawing effects due to their high electronegativity.
This makes triflate an excellent leaving group since the charge left behind is stabilized, allowing for easy departure from the molecule.
These effects can drastically improve the leaving group capability by helping to manage the negative charge efficiently.
When a leaving group departs, it often leaves behind a negative charge. Groups like the trifluoromethyl (\(\mathrm{CF}_{3}\)) in triflate \(-\mathrm{OSO}_{2} \mathrm{CF}_{3}\) have strong electron-withdrawing effects due to their high electronegativity.
This makes triflate an excellent leaving group since the charge left behind is stabilized, allowing for easy departure from the molecule.
These effects can drastically improve the leaving group capability by helping to manage the negative charge efficiently.
Base Strength
Base strength is often inversely related to leaving group ability in organic chemistry. A strong base holds onto its protons tightly, thus making it less effective as a leaving group. On the other hand, a weak base does not bind protons as tightly and can leave more readily.
The methoxy group \(-\mathrm{OMe}\) serves as a strong base, making it a poor leaving group because it prefers to keep its lone pair of electrons and resists leaving. Conversely, the triflate ion is weakly basic due to its stabilization by electron-withdrawing effects, enabling it to be an excellent leaving group.
The methoxy group \(-\mathrm{OMe}\) serves as a strong base, making it a poor leaving group because it prefers to keep its lone pair of electrons and resists leaving. Conversely, the triflate ion is weakly basic due to its stabilization by electron-withdrawing effects, enabling it to be an excellent leaving group.
Organic Chemistry
In organic chemistry, understanding the behavior of molecules involves examining how atoms and functional groups interact during chemical reactions. Concepts such as leaving group ability, resonance, and electron effects are crucial in predicting reaction outcomes.
Leaving groups are vital to many reaction mechanisms, such as substitution and elimination reactions. The ability to leave the parent molecule smoothly allows for the transformation or formation of new covalent bonds.
Mastering these fundamental principles helps anticipate the reactivity and behavior of organic molecules in various chemical environments.
Leaving groups are vital to many reaction mechanisms, such as substitution and elimination reactions. The ability to leave the parent molecule smoothly allows for the transformation or formation of new covalent bonds.
Mastering these fundamental principles helps anticipate the reactivity and behavior of organic molecules in various chemical environments.
Inductive Effects
Inductive effects are the result of atoms or groups donating or withdrawing electron density through sigma bonds. This effect can influence the acidity, basicity, and stability of molecules.
Substituents that are more electronegative, like in the triflate ion \(-\mathrm{OSO}_{2} \mathrm{CF}_{3}\), tend to withdraw electron density, which stabilizes negative charges left behind as the group leaves. This stabilizing inductive effect enhances the leaving group ability.
Such effects are crucial in organic chemistry for predicting the stability of intermediates and product species, playing into broader concepts like reactivity and mechanism pathways.
Substituents that are more electronegative, like in the triflate ion \(-\mathrm{OSO}_{2} \mathrm{CF}_{3}\), tend to withdraw electron density, which stabilizes negative charges left behind as the group leaves. This stabilizing inductive effect enhances the leaving group ability.
Such effects are crucial in organic chemistry for predicting the stability of intermediates and product species, playing into broader concepts like reactivity and mechanism pathways.
