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JEE Advanced levelSingle correct optionDifficulty 3 of 3g1028

Assertion: In aqueous solution, trimethylamine is a weaker base than dimethylamine. Reason: Trimethylamine has more +I methyl groups than dimethylamine, so the lone pair on its nitrogen is less available for protonation.

  1. ABoth the assertion and the reason are true and the reason is the correct explanation of the assertion
  2. BBoth the assertion and the reason are true but the reason is not the correct explanation of the assertion
  3. CThe assertion is true but the reason is false
  4. DThe assertion is false but the reason is true

Answer: C

In water the measured basicity is (pKb about 3.3 and 4.2), so the assertion is true. The reason is false: more +I groups make the lone pair more, not less, available. The actual cause is that the trimethylammonium ion has only one N-H bond for hydrogen-bond solvation, and so is stabilised less than the dimethylammonium ion.
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