Q4 · CSIR NET Chemistry, December 2014
Paper: CSIR NET December 2014 · Subject: Organic Chemistry · Chapter: General Organic Chemistry · Topic: Acidity Basicity · Marks: 2 · Difficulty: Hard
The increasing order of $pK_a$ values of the circled hydrogens in the following compounds is 
(I) 
(II) 
(III)
(I) (II) (III)(a)I $<$ II $<$ III
(b)I $<$ III $<$ II
(c)II $<$ I $<$ III
(d)II $<$ III $<$ I
Answer
Answer: C ✓ checked by 4AB · confidence medium
Explanation
The N–H of the N-cyanoguanidinium salt (II) is strongly acidic. The CH₂ flanked by two carbonyls in I has pKa ≈ 10. The C-2 H of the 1,3-dimethylimidazolium ion (III) has pKa ≈ 21–23. II < I < III (c).
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