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Topic: basicity of pyridine, pyrimidine and imidazole  (Read 50938 times)

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Offline pesona3f

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basicity of pyridine, pyrimidine and imidazole
« on: April 20, 2009, 12:52:00 PM »
hello guys, i want to ask bout their basicity regarding the resonance structure and inductive effect, need it very urgently...  :'(

Offline Arkcon

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #1 on: April 20, 2009, 04:04:05 PM »
We recognize that you may be in a hurry, but you'll learn more, and better, if you can start figuring it out for yourself.  Can you use the definitions of your terms (resonance, induction, basicity) and the structures of the molecules in question, to begin to come up with an answer yourself.  Or show us were you're confused, and we can help.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline pesona3f

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #2 on: April 20, 2009, 04:24:33 PM »
i know that pyrimidine is less basic than pyridine due to the inductive effect of nitrogen that cause uneven distribution of electron( correct me if i'm wrong) becoz pyrimidine has 2 N and pyridine has 1 N...

my question, is there any other reason that cause that diferrent in basicity? and why imidazole is more basic than pyridine ? that really confusing me.. what actually make imidazole more basic than pyridine?

Offline pesona3f

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #3 on: April 20, 2009, 04:30:49 PM »


this is resonance structure of imidazole



this is resonance structure for pyridine


is there any chance that resonance structure cause different basicity in both structure??

Offline nj_bartel

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #4 on: April 20, 2009, 05:43:41 PM »
looks good to me

Offline ufalynn88

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #5 on: April 20, 2009, 06:11:27 PM »
well first you should recognize that these are all aromatic structures. and also that in those structures the nitrogen's lone pair has the potential to donate it's pi electrons into the system if it helps to gain aromaticity..now, is aromaticity favorable or not? remember that it is not possible for electrons to be in "2 places at once". that is, if the electrons are in the aromatic system they will NOT favorably be available to act as a base..

Offline pesona3f

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #6 on: April 20, 2009, 06:43:19 PM »
can you explain it a bit detail, i think i start to understand it... but still need the explanation :'( :'(

Offline aldoxime_amine

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #7 on: April 21, 2009, 01:40:57 AM »
Consider pyrrole. The lone pair on the nitrogen atom is "swimming" within the pi-electron ring, because by doing so the ring now satisfies Huckel's rule, and the molecule is aromatic.

As a previous post mentioned, the lone pair cannot be on nitrogen at the same time.

Now when a lewis acid approaches, (a proton, say) he looks for electrons because he is electron deficient. But nitrogen's electrons are not with nitrogen. They are "swimming" in the aromatic ring.

The appropriate term we use is that the lone pairs on nitrogen are "delocalized" within the benzene ring and are not easily available for "donation."

Pyridine on the other hand, has a "localized" lone pair and hence is more basic than pyrrole.
Its lone pair stays on nitrogen all the time, as you have shown in your resonating structures.

Hope this helped.

Offline pesona3f

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #8 on: April 21, 2009, 02:40:04 AM »
thankz for the answer, i know that lone pair in pyrole involve in delocalization and it is unfavourable in term of energy to disturb aromatic ring.. then how about pyridine and pyrimidine ?? both contain nitrogen and its lone pair not involve in aromatic like pyrole. In addition, pyrimidine has 2 N means it has more lone pair and accessable to proton, should not it more basic than pyridine which only have 1 N???

Offline aldoxime_amine

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #9 on: April 21, 2009, 06:50:45 AM »
I seem to agree with you, but unfortunately, pyridine is more basic. I hope someone can clear our doubts.

Offline pesona3f

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #10 on: April 21, 2009, 06:57:35 AM »
ok, i think i can give some idea bout this ( correct me if wrong ) nitrogen in pyrimidine and pyridine are inductively withdrawing group due to electronegatvity of nitrogen, so maybe pyridine with 1 N will have greater electron density in contrast with pyrimidine which need to equally distribute the electron....

by the way, why imidazole is more basic than pyridine? anyone have a clue? i think it related with resonance effect but i cant figure it... i really need the answer for my exam.. :'(

Offline nj_bartel

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #11 on: April 21, 2009, 01:12:25 PM »
You're spot on about why pyrimidine is less basic than pyridine.  As far as imidazole vs pyridine, consider the fact that nitrogen is generally a better resonance electron donor (if it has that option) than it is an inductive electron withdrawer.  Draw some resonance structures and see what you come up with.

Offline pesona3f

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Re: basicity of pyridine, pyrimidine and imidazole
« Reply #12 on: April 21, 2009, 01:47:06 PM »
i get what you want to say, about resonance effect of nitrogen because it has lone pair.
that make me wonder, in imidazole, only one nitrogen is available because the other lone pair in sextet. if it has resonance effect, will it decrease the basicity?

and i found this. imidazole with 2 N is more basic than pyridine because pyridine only have one N to stabilize the positive charge... whats it mean? i really dont get it since N should have inductive effect in pyridine..

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