JEE Advanced 2017 · Paper 2 · Chemistry Q7 · Dronacharya Exams

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ChemistrySingle correct+3 / -1Question 25 of 54

The order of basicity among the following compounds is

I: acetamidine, CH₃C(=NH)NH₂. II: 4,5-dihydroimidazole (2-imidazoline). III: imidazole. IV: guanidine, (H₂N)₂C=NH.

A

II > I > IV > III

B

IV > II > III > I

C

IV > I > II > III

D

I > IV > III > II

Answer and worked solution

Official answer: C

Rank the protonated forms by how well they spread the positive charge:

IV, guanidine. The guanidinium ion spreads the charge over three equivalent N by resonance. Strongest base.
I, acetamidine. The amidinium ion spreads it over two N.
II, 2-imidazoline. A cyclic amidine, but with weaker resonance (ring strain and geometry) than the acyclic one.
III, imidazole. Its basic N is $sp^2$ and the ring is aromatic, so it is much weaker ( $\mathrm p K_a\approx7$ , against about 11-13 for amidines).

Order: IV > I > II > III.

(C)

Topics:Acidity and basicity Electronic effects (inductive, resonance, hyperconjugation)Aromaticity

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