Basicity of Nitrogen Heterocycles

  • #1
baldbrain
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Homework Statement


Select the correct order of basicity of the following compounds:
IMG_20180630_184900.JPG

The Attempt at a Solution


Obviously, (i) is more basic than (iv) because of the 2 acidic phenyl rings in (iv). Also, I know that amides are much less basic than amines. So, that leaves me with either (B) or (C). Further, due to the sp2 hybrid N atom in pyrrole, it is weakly basic.
But how do I decide it's basicity relative to the rest?
Also, though it's clear from (B) & (C) that (iii) will be less basic than both (i) & (iv), how do I decide that the phenyl rings in (iv) won't decrease the basicity to such an extent that (iii) will get more basic?
 

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Answers and Replies

  • #2
TeethWhitener
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Here's a good introduction to amine basicity:
https://www.masterorganicchemistry.com/2017/04/26/5-factors-that-affect-basicity-of-amines/
(i) is more basic than (iv) because of the 2 acidic phenyl rings in (iv). Also, I know that amides are much less basic than amines. So, that leaves me with either (B) or (C)
Yes, yes, yes.
Further, due to the sp2 hybrid N atom in pyrrole, it is weakly basic.
None of these molecules are pyrrole. There's piperidine (i), pyridine (ii), pyrrolidone (iii), and diphenylamine (iv).
Also, though it's clear from (B) & (C) that (iii) will be less basic than both (i) & (iv), how do I decide that the phenyl rings in (iv) won't decrease the basicity to such an extent that (iii) will get more basic?
The carbonyl group is a really good pi acceptor (and also a half-decent proton acceptor), so in general amides really really don't want to be protonated.
 
  • #3
baldbrain
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None of these molecules are pyrrole.
Sorry! Confused pyridine for pyrrole.
 
  • #4
baldbrain
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Ok, so from the link, the answer would be (C)
 
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  • #5
TeethWhitener
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Ok, so from the link, the answer would be (C)
Yes, the big giveaway (or shortcut, now that I'm looking closely at the possible answers) is that piperidine (the aliphatic amine) will be a much stronger base than the rest of them.
 
  • #6
baldbrain
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Yes, the big giveaway (or shortcut, now that I'm looking closely at the possible answers) is that piperidine (the aliphatic amine) will be a much stronger base than the rest of them.
Right:wink:
 

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