[vineet]

When can we say that electron is delocalised??for example in the structure of pyridine,we say that lone pair of nitrogen is not delocalized whereas the   lone pair of pyrole and aniline is delocalised and involved in resonance.....whenever lone pair will be given how can we decide whether it will be involved in resonance or not??please give me a detailed explanation...

                   

[mjc123]

What is the condition for electrons to participate in delocalisation? What is different about the nitrogen in pyridine compared to the nitrogen in pyrrole?

[vineet]

Sir,that is what i am asking....

[mjc123]

Please read the forum rules, we don't just give out pat answers, we try to help people work out the problem for themselves and (hopefully) understand it all the better.
So. What's the most obvious difference between N in pyridine and N in pyrrole. What are they bonded to?

[vineet]

in pyrrole,N is bonded with hydrogen,whereas it is not the case in pyridine....

[orgopete]

This is where models can be helpful. Compare the non-bonded electrons of pyridine with the electrons in a C-H bond of benzene. Resonance? Compare the non-bonded electrons of pyrrole with the (which?) electrons of benzene. What can you infer from these models?

[vineet]

ok sir..so we can say that in case of pyridine,lone pair of nitrogen is free to be delocalised which is not the case in pyrrole..am i right??
if this is the reason..then why does this not apply in case of aniline..we say that aniline's lone pair is delocalised..

[orgopete]

ok sir..so we can say that in case of pyridine,lone pair of nitrogen is free to be delocalised which is not the case in pyrrole..am i right??

No

[vineet]

so,what is the exact reason?

[orgopete]

See the structures linked from the Prentiss-Hall website.
Pyridine and  furan

These images should explain it better than words. The caption in furan in the second link notes the difference. One pair is delocalized and one is not.

[vineet]

sorry sir,but i am still not able to understand the concept.

[orgopete]

Did you see the caption on furan that read, "These electrons are perpendicular to the p-orbitals"? That should have told you they are different. See caption on furan describing the other electron pairs. The model should be telling you the electrons are different and how or why they are different. If that doesn't explain it, perhaps someone else can do it better than I.

[OrgXemProf]

Key phrase: "orthogonal orbitals"

How might that phrase apply to electron delocalization in pyridine vs. pyrrole? How 'bout delocalization of electrons in aniline?

Potentially helpful reference, URL:

http://www.chtf.stuba.sk/~szolcsanyi/education/files/Chemia%20heterocyklickych%20zlucenin/Prednaska%201/Odporucane%20studijne%20materialy/Pyrid%EDn-vlastnosti%20a%20reaktivita.pdf

[vineet]

yes!!that is exactly what i am telling..i am not able to understand it..and what about aniline and pyridine??