which among the following compounds the species is least basic
Which among the following compounds the species is least basic?
Answer:
Since the exact list of compounds from LectureNotes was not provided, let’s explore the general principles used to identify the least basic species among common nitrogen-containing compounds:
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Resonance Delocalization:
• If the nitrogen’s lone pair is delocalized through resonance, the electron pair is less freely available to accept a proton.
• Example: In an amide (\text{RCONH}_2), the lone pair on nitrogen is delocalized onto the carbonyl group, significantly reducing the basicity. -
Aromaticity Involvement:
• When a nitrogen is part of an aromatic ring (e.g., pyrrole), the lone pair often contributes to the ring’s aromatic system. Removing or donating that lone pair can disrupt aromaticity, so these species tend to be very weak bases.
• Example: Pyrrole is considerably less basic than pyridine because pyrrole’s nitrogen lone pair is part of the aromatic sextet. -
Inductive Effects:
• Electron-withdrawing substituents nearby pull electron density away from the nitrogen, lowering basicity.
• Electron-donating substituents can increase basicity by pushing electron density toward the nitrogen. -
Hybridization:
• Nitrogens with more s-character (like sp-hybridized in triple bonds) hold the lone pair closer to the nucleus, decreasing its availability for bonding to a proton.
In many typical comparison questions, you may see a rank like this (from more basic to less basic):
• Aliphatic amines (e.g., \text{CH}_3\text{NH}_2) tend to be more basic.
• Aniline (nitrogen attached directly to a benzene ring) is less basic than aliphatic amines due to partial resonance with the ring.
• Pyridine (nitrogen in an aromatic ring but the lone pair is not part of the aromatic sextet) is comparatively less basic than aniline, yet often more basic than pyrrole.
• Pyrrole (lone pair involved in the aromatic sextet) is typically one of the least basic nitrogen-containing aromatic species.
• Amides (significant resonance with the carbonyl) can be even less basic than pyrrole because the nitrogen’s lone pair is strongly delocalized with the carbonyl group.
Without the specific set of options provided in LectureNotes, amide-type compounds or pyrrole are frequently considered among the least basic in typical comparisons.
Below is a summary table highlighting these ideas:
| Compound/Functional Group | Reason for Lower Basicity | General Basicity Trend |
|---|---|---|
| Aliphatic amine (e.g., R–NH₂) | Lone pair not significantly delocalized | Higher basicity |
| Aniline (Ar–NH₂) | Partial resonance with aromatic ring | Less basic than aliphatic amine |
| Pyridine (aromatic ring, N not in ring π) | Lone pair not in aromatic sextet but nitrogen is sp² | Moderate basicity |
| Pyrrole (N in the ring π system) | Lone pair participates in aromatic sextet | Very low basicity |
| Amides (R–CONH₂) | Strong resonance delocalization with carbonyl group | Among the lowest basicity |
In short, any compound in which the nitrogen’s lone pair is heavily involved in resonance or aromaticity will show minimal basic character.
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