Which of the following amines has lowest \(pK_b\) value?
Show Hint
- \(C_6H_5-N(CH_3)_2\)
- \(C_6H_5-NH(CH_3)\)
- \(C_6H_5-NH_2\)
- [structure (D) - image pending]
The Correct Option is A
Solution and Explanation
A lower \(pK_b\) means a stronger base. Among aromatic amines, replacing the N-H hydrogens with electron-donating alkyl (\(-CH_3\)) groups increases the electron density on nitrogen, making the lone pair more available and the amine more basic.
Step 1:
Compare the substitution on nitrogen: aniline \(C_6H_5NH_2\) has no methyl, N-methylaniline \(C_6H_5NH(CH_3)\) has one methyl, and N,N-dimethylaniline \(C_6H_5N(CH_3)_2\) has two methyl groups donating electron density to N.
Step 2:
More alkyl groups means greater +I (electron donating) effect and stronger base. So basic strength order is \(C_6H_5N(CH_3)_2 > C_6H_5NH(CH_3) > C_6H_5NH_2\). The strongest base has the lowest \(pK_b\).
Answer: Option (1) — \(C_6H_5-N(CH_3)_2\) is the most basic and so has the lowest \(pK_b\) value.
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(i) (CH3 )2CHNH2 (ii) CH3 (CH2 )2NH2 (iii) CH3NHCH(CH3 )2
(iv) (CH3 )3CNH2 (v) C6H5NHCH3 (vi) (CH3CH2 )2NCH3 (vii) m–BrC6H4NH2
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Write IUPAC names of the following compounds and classify them into primary, secondary and tertiary amines.
(i) (CH3 )2CHNH2 (ii) CH3 (CH2 )2NH2 (iii) CH3NHCH(CH3 )2
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(i) Methylamine and dimethylamine
(ii) Secondary and tertiary amines
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Arrange the following:
(i) In decreasing order of the pKb values: C2H5NH2 , C6H5NHCH3 , (C2H5 )2NH and C6H5NH2
(ii) In increasing order of basic strength: C6H5NH2 , C6H5N(CH3 )2 , (C2H5 )2NH and CH3NH2
(iii) In increasing order of basic strength: (a) Aniline, p-nitroaniline and p-toluidine Exercises Rationalised 2023-24 279 Amines (b) C6H5NH2 , C6H5NHCH3 , C6H5CH2NH2 .
(iv) In decreasing order of basic strength in gas phase: C2H5NH2 , (C2H5 )2NH, (C2H5 )3N and NH3
(v) In increasing order of boiling point: C2H5OH, (CH3 )2NH, C2H5NH2
(vi) In increasing order of solubility in water: C6H5NH2 , (C2H5 )2NH, C2H5NH2 .
How will you convert:
(i) Ethanoic acid into methanamine
(ii) Hexanenitrile into 1-aminopentane
(iii) Methanol to ethanoic acid
(iv) Ethanamine into methanamine
(v) Ethanoic acid into propanoic acid
(vi) Methanamine into ethanamine
(vii) Nitromethane into dimethylamine
(viii) Propanoic acid into ethanoic acid?
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