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Choose the Best Answer · Q11

Q.The order of basic strength for methyl-substituted amines in aqueous solution is

a) N(CH3)3 > N(CH3)2H > N(CH3)H2 > NH3
b) N(CH3)H2 > N(CH3)2H > N(CH3)3 > NH3
c) NH3 > N(CH3)H2 > N(CH3)2H > N(CH3)3
d) N(CH3)2H > N(CH3)H2 > N(CH3)3 > NH3
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Step 1. In aqueous solution, basicity is not simply 'more methyl groups = more basic' -- it is the combined result of the alkyl group's +I (electron-donating) effect AND how well the resulting ammonium cation is solvated by water.

Step 2. The unit's own measured pKb values are: CH3NH2 = 3.38, (CH3)2NH = 3.28, (CH3)3N = 4.22, NH3 = 4.74. Since a SMALLER pKb means a STRONGER base, ranking these from strongest to weakest gives (CH3)2NH (3.28) first, then CH3NH2 (3.38), then (CH3)3N (4.22), then NH3 (4.74) last.

Step 3. This exact order -- dimethylamine > methylamine > trimethylamine > ammonia -- is written as N(CH3)2H > N(CH3)H2 > N(CH3)3 > NH3, matching option (d). …

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