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Question

The decreasing order of pKa values of the labelled nitrogen atoms in the above compound is :

330352_b81668b487fd42aca59fd6ce5b4204ea.png

A
3 > 2 > 1 > 4 > 5
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B
4 > 3 >2 > 1 > 5
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C
3 > 4 > 2 > 1 > 5
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D
5 > 1 > 2 > 4 > 3
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Solution

The correct option is C 3 > 4 > 2 > 1 > 5
We have to draw conjugate acids to check basic character:
Conjugate acids for (3) and (4) are shown in the above structure.
Basic character is least of (5) as it is between two R showing group (Electron withdrawing group) which decreases basic strength. After that least basic is one as it has one C=O group adjacent to it (R).
(3) is most basic as electronegativity of N is less than that of oxygen and it will easily let hold of its lone pair to go into resonance, while O being more electronegative will not allow it's lone pair easily. So, a conjugate base of (3) is more stable than (4). Therefore (3) is more basic than (4)
(2) is a primary amine whose lone pair is in resonance. it is more basic than (1) and (5)
Decreasing pKa order: (3)>(4)>(2)>(1)>(5)

985494_330352_ans_f23177fbd1554a898e208a9260bcd21d.png

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