Basicity of Nitrogen Heterocycles

  • #1
232
20

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?
 

Attachments

Answers and Replies

  • #2
TeethWhitener
Science Advisor
Gold Member
1,844
1,184
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.
 
  • #4
232
20
Ok, so from the link, the answer would be (C)
 
  • Like
Likes TeethWhitener
  • #5
TeethWhitener
Science Advisor
Gold Member
1,844
1,184
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
232
20
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:
 

Related Threads on Basicity of Nitrogen Heterocycles

Replies
3
Views
3K
  • Last Post
Replies
4
Views
2K
Replies
15
Views
43K
Replies
1
Views
4K
Replies
2
Views
7K
  • Last Post
Replies
2
Views
3K
  • Last Post
Replies
2
Views
2K
Replies
3
Views
23K
  • Last Post
Replies
1
Views
1K
Top