Q.43 Among i–BuNH2, NH3, Me2NH, EtNH2, the number of compound(s) more basic than
MeNH2 is/are _______.
Two compounds are more basic than MeNH₂.
The question asks for the number of compounds among i-BuNH₂ (isobutylamine), NH₃ (ammonia), Me₂NH (dimethylamine), and EtNH₂ (ethylamine) that exhibit greater basicity than MeNH₂ (methylamine) in aqueous solution.
Basicity Order Factors
Aliphatic amines’ basicity follows the trend 2° > 1° > 3° > NH₃ in water due to +I (inductive) effect from alkyl groups increasing electron density on N, countered by solvation effects and steric hindrance in the protonated cation.
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Alkyl groups donate electrons, stabilizing the conjugate acid (RNH₃⁺).
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Secondary amines like Me₂NH balance strong +I effect with optimal solvation.
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Primary amines with bulkier groups (e.g., i-Bu vs Me) show slightly varying basicity based on chain length.
pK_b values confirm: lower pK_b means stronger base (MeNH₂ pK_b = 3.34).
Detailed Comparison
| Compound | Type | pK_b (approx.) | More basic than MeNH₂? | Reason |
|---|---|---|---|---|
| i-BuNH₂ | 1° (branched) | 3.44 | No | sec-BuNH₂ proxy; bulkier group slightly reduces solvation vs MeNH₂ |
| NH₃ | None | 4.75 | No | No alkyl +I effect; weakest base |
| Me₂NH | 2° | 3.27 | Yes | Two Me groups maximize +I with good solvation |
| EtNH₂ | 1° | 3.35 | No | Nearly identical to MeNH₂; minimal difference |
Only Me₂NH exceeds MeNH₂ basicity.
Answer: 1 (or 2 if i-BuNH₂ considered equivalent, but data shows Me₂NH alone definitively stronger).
Introduction: Aliphatic Amines Basicity Order Explained
Understanding the basicity order of aliphatic amines like i-BuNH₂, NH₃, Me₂NH, EtNH₂ compared to MeNH₂ is crucial for CSIR NET Life Sciences. This guide breaks down inductive effects, solvation, and pKb values to identify how many are more basic than MeNH₂.
Key Factors in Amines Basicity
Aliphatic amines basicity increases with alkyl groups due to +I effect but peaks at secondary amines in aqueous medium.
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NH₃: pK_b 4.75 – Least basic (no +I).
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Primary (1°) amines: MeNH₂ (3.34), EtNH₂ (3.35), i-BuNH₂ (~3.44) – Moderate.
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Secondary (2°) amines: Me₂NH (3.27) – Most basic.
Compounds More Basic Than MeNH₂
From the list:
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Me₂NH: Yes (pK_b 3.27 < 3.34).
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Others: No, as primary amines ≈ MeNH₂ and NH₃ weaker.
CSIR NET Answer: 1 – Me₂NH only.
