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Author Topic: Dipole Moment  (Read 9501 times)

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ritwik06

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Dipole Moment
« on: May 02, 2009, 07:56:00 AM »

Which has greater dipole moment- pyrrole or pyrrolidine?

My attempt at an answer:
The polarity will be due to the C-N bond and the lone pair on N, for both the cases.

For pyrrole the N is sp2 hybridised. The dipole due to lone pair will be perpendicular to the plane of the ring. The dipoles of the 2 C-N bonds will be at 120 degree to each other in the plane of the ring.
For pyrollidine, all three dipoles- 2 due to C-N bonds and one due to lone pair on N, will make an angle of about 109 degrees with each other
as the N is sp3 hybridised.
Therefore I think, pyrrolidine will have greater dipole moment.

Please correct me if my solving technique or the whole answer is wrong.
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macman104

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Re: Dipole Moment
« Reply #1 on: May 02, 2009, 08:17:39 AM »

Aromaticity.
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ritwik06

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Re: Dipole Moment
« Reply #2 on: May 02, 2009, 06:11:06 PM »

Aromaticity.
Yep! I know pyrrole is aromatic, but why does that matter?
I mean the lone pair on nitrogen will always be perpendicular to the ring, even if it is delocalised. Isnt it? Please elaborate!!!
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macman104

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Re: Dipole Moment
« Reply #3 on: May 02, 2009, 06:30:28 PM »

But if it is delocalized, then the overall contribution to the dipole moment is going to be decreased, because the electrons are spread out over the whole molecule.
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ritwik06

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Re: Dipole Moment
« Reply #4 on: May 02, 2009, 06:39:36 PM »

But if it is delocalized, then the overall contribution to the dipole moment is going to be decreased, because the electrons are spread out over the whole molecule.
I really didnt understand that.Please note the point that I wish to make. Dipole moment is a vector, right? Now in pyrrole, all the carbons and the nitrogen have sp2 hybridisation. So even if the electron pair is delocalised in the ring, wont the lone pair remain in an unhybridised p orbital that would be perpendicular to the ring? The direction will be same, i.e. perpendicular to the ring. So why does the delocalisation matter if the dipole vector due  to lone pair does not change in any of the canonical forms for pyrrole?
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Squirmy

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Re: Dipole Moment
« Reply #5 on: May 02, 2009, 07:02:00 PM »

I think it might be easier to look at the difference between a cyclopentadienyl anion vs a cyclopentyl anion (i.e. take nitrogen out of the question for now).

For the cyclopentadienyl (Cp) anion, you can draw resonance structures where the negative charge shows up on each carbon. So, there isn't one side of the structure that's negative, and one side that's positive (no dipole).

For the cyclopentyl anion there are no resonance structure. The negative charge is localized on one carbon.

Pyrrole/pyrrollidine are isoelectronic with Cp anion/cyclopentyl anion, so you could make similar arguments.
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ritwik06

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Re: Dipole Moment
« Reply #6 on: May 02, 2009, 07:39:20 PM »

The dipole moment in question here is due to "LONE PAIR", so please don't change the question. Please re-read my previous post and then point out the points where I made mistakes. And I found something:
http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/heterocyclen/arom_gessaet_heterocyclen.vlu/Page/vsc/en/ch/12/oc/heterocyclen/fuenfaromat/fuenfring_aromat.vscml.html

It says pyrrole has greater dipole moment than pyrrollidine. How would you justify that?
I used Chem Draw Ultra to calculate the dipoles. It also says that pyrrole has greater dipole moment. Please elaborate upon it. And for the sake that I am a still a class XII student, I would be glad if you exaplain using vectors. Thanks a lot for your replies till now.
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Squirmy

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Re: Dipole Moment
« Reply #7 on: May 02, 2009, 08:52:14 PM »

Interesting link.

My analogy was different from your case b/c the lone pair...wait, I mean LONE PAIR...was on a negatively charged carbon rather than a neutral nitrogen.

This does make a difference. For pyrrole, most of its resonance structures have a negative charge on carbon, positive charge on nitrogen. According to your link, the dipole vector points away from nitrogen.

For pyrrolidine, there are no resonance structures. The only effect on dipole is the inductive effect (electronegativity of the nitrogen). As such, the dipole vector points toward the nitrogen.

If the vectors are pointing in opposite directions, though, I don't see any simple way of predicting the relative sizes of the dipole moments. I had imagined the vectors pointing in the same direction...toward the nitrogen due to electronegativity, but with a smaller dipole moment for the delocalized system.

Btw, I find give/take snacks lame. Just ask for clarification or tell me if you find me unhelpful and I will stop trying.
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ritwik06

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Re: Dipole Moment
« Reply #8 on: May 02, 2009, 11:47:08 PM »

I am sorry, I didn't notice the example thoroughly. I had been trying this question for a long so I was a bit irritated. I apologise.
I have a doubt.
When there is a lone pair on an atom, what is the direction of the dipole? Towards the lone pair? And what is the direction when there is a carbon anion present in conjugation?
Now, to find the total ipole moment of a compound, we sum up vectorially all the dipoles due to polarised bonds and the lone pairs, Am I right?
I just want to get my concepts crystal clear.

And about those snacks, I was unfamiliar to them. I was just trying to see what happens when we give or take snacks. If you didnt like that here you go.. you've got your snacks back :)
regards
Ritwik
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Squirmy

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Re: Dipole Moment
« Reply #9 on: May 03, 2009, 07:46:14 AM »

I am sorry, I didn't notice the example thoroughly. I had been trying this question for a long so I was a bit irritated. I apologise.

No problem :)

Quote
When there is a lone pair on an atom, what is the direction of the dipole? Towards the lone pair?
...

Now, to find the total ipole moment of a compound, we sum up vectorially all the dipoles due to polarised bonds and the lone pairs, Am I right?
I just want to get my concepts crystal clear.

You have it...the total dipole moment is a combination of the two...often they go hand in hand b/c more electronegative atoms also often have the most lone pairs.

Quote
And what is the direction when there is a carbon anion present in conjugation?

Saved for last :) A carbon anion (carbanion) is ionic...it's only the negative portion of an ionic compound. The positive part would have to be a counterion (Li+, Na+, K+, etc). Since it's not a neutral or molecular compound, the analogy fails when looking at the dipole moment.

The point was that in both systems (pyrrole and Cp-) the lone pair is not localized on one atom. It is part of the aromatic pi system and spread out over all 5 atoms.
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