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Topic: Why is O=C=O nonpolar?  (Read 11167 times)

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Offline caters

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Why is O=C=O nonpolar?
« on: July 26, 2014, 09:03:21 AM »


I understand that polarity is electronegativity difference and that the more electronegativity difference there is the more polar the bond.

However I have read that O=C=O is nonpolar. This doesn't make sense to me.

C=O. Okay the carbonyl carbon is partially positive and thus carbonyls are polar. O=C=O is like 2 of these bonded together so wouldn't it be that you would have an even more positive carbon and 2 partially negative oxygens causing O=C=O to be polar?

I mean this would explain why more of this dissolves in water than O2 and makes water more acidic than O2 does.

So why is O=C=O nonpolar when 2 oxygens are pulling electron density from THE SAME CARBON?

Offline Ben Bob2

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Re: Why is O=C=O nonpolar?
« Reply #1 on: July 26, 2014, 10:37:31 AM »
Think about the shape of the molecule and consider the rules which govern the polarity of a molecule with regards to its geometry.
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Re: Why is O=C=O nonpolar?
« Reply #2 on: July 26, 2014, 11:46:36 AM »
I was thinking that you had to take the electronegativity differences of the individual bonds into concern more.

Thus you would have O=C=O where the O's are partially negative and the C is partially positive.

This would make it polar and it also makes sense with the fact that quite a bit more CO2 dissolves in water than O2.

Offline kriggy

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Re: Why is O=C=O nonpolar?
« Reply #3 on: July 26, 2014, 01:03:13 PM »
You have to consider both.
What is the shape of CO2?

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Re: Why is O=C=O nonpolar?
« Reply #4 on: July 26, 2014, 01:08:57 PM »
it is linear but how does this have to do with polarity when you have 2 partially negative oxygens and 1 partially positive carbon?

Offline Ben Bob2

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Re: Why is O=C=O nonpolar?
« Reply #5 on: July 26, 2014, 01:38:07 PM »
Correct, it's linear, therefore the partial charges are located symmetrically on the molecule.
Now think about (or just Google) how that affects polarity and you will have your answer ;)
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Re: Why is O=C=O nonpolar?
« Reply #6 on: July 26, 2014, 01:47:43 PM »
I think that since they are double bonds the O's make the C even more positive than it would be if it were methanediol and since they are 2 that that would mean even more positive. I think that CO2 is polar because the O's are pulling a lot of e- density from the C and the C is pulling just a little towards itself and I don't think that at least in this case the shape affects it so much.

The dipoles are positive and not 0 which supports even more that it is polar and more CO2 dissolves in water than O2 because of these reasons:
1) Electronegativity difference between C and O
2) Acid-Base reaction

So I think CO2 is a polar molecule.

Offline Ben Bob2

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Re: Why is O=C=O nonpolar?
« Reply #7 on: July 26, 2014, 02:29:16 PM »
I think that CO2 is polar because the O's are pulling a lot of e- density from the C and the C is pulling just a little towards itself
and I don't think that at least in this case the shape affects it so much.

You are right about part of this: the oxygen atoms are creating a negative charge on the molecule, but since it is symmetric, the charges simply "cancel" each other out, meaning that there are not ends of the molecule with opposite charge, but with like charge:


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The linear geometry with two identical oxygen atoms attached to a center carbon gives rise to a special effect. The molecule is symmetrical. There are partial charges seen from the difference in the electronegativities. The electrostatic potential shows a blue (partially positive) color for carbon and shows a red (partially negative) color for both oxygen atoms. However, the symmetrical nature of the bonds has the overall effect of canceling the dipole, therefore the molecule is non-polar

Read more here:
http://www.elmhurst.edu/~chm/vchembook/212inorganic.html
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Offline Arkcon

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Re: Why is O=C=O nonpolar?
« Reply #8 on: July 26, 2014, 02:31:20 PM »
I was thinking that you had to take the electronegativity differences of the individual bonds into concern more.

The electronegativity difference between the atoms is valid.  However you can't discount molecular symmetry.  So, you're incorrect here, where you say it "concern more."  That's just not a correct statement of chemical facts.

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Thus you would have O=C=O where the O's are partially negative and the C is partially positive.

I've seen this for a number of polar molecules, and yes even for CO2. (Like Ben Bob2: posted) But still, the linear shape shiled the delta+ from each other.  So its not a molecule with a  significant dipole movement.

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This would make it polar and it also makes sense with the fact that quite a bit more CO2 dissolves in water than O2.

Actually, CO2 dissolves so well in water because it reacts with water to form carbonic acid.  http://en.wikipedia.org/wiki/Carbon_dioxide#In_aqueous_solution

/EDIT: Ben Bob2: is a ninja
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

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Re: Why is O=C=O nonpolar?
« Reply #9 on: July 27, 2014, 09:00:32 AM »
2 opposite and equal dipoles means 0 dipole moment.

However it does have a quadrupole moment because of the 2 dipoles.

This is how CO2 is.

and I know that CO2 reacts with water to form carbonic acid. That is how come when you leave water sitting out for a whole day it is significantly more acidic.

Hexamine doesn't really have a dipole moment so to speak or really any significant dipoles in the bonds but it significantly dissolves in water.

equating dipole moment with polarity is thus not always helpful when talking about solubility.

Since the dipoles are vectors the dipole moment is the sum of those vectors.

N is not much more electronegative than C which is not much more electronegative than H which is not much more electronegative than N.

So none of the bonds in hexamine are polar. however I bet hexamine dissolves in water because like CO2 it can do acid base reactions.

The combined opposed dipole moments give the whole molecule a quadrupole moment meaning that if there is a 4-pole electric field with positive at north and south and negative at east and west, the CO2 molecule will tend to turn to a north-south orientation. This is because of the negative oxygen being attracted to the north and south poles and the positive carbon being attracted to the the east and west poles. Larger molecules may have hexapole, octupole or higher moments with progressively less effect.

But in this sense of quadrupole moments CO2 is polar since there is a specific orientation of it in the electric field.

Offline Arkcon

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Re: Why is O=C=O nonpolar?
« Reply #10 on: July 27, 2014, 09:20:27 AM »
caters:, often, conversing with you on the these boards is disappointing, because you make up random facts, that don't fit the topic, and/or are just flat out wrong, and I'd hate to have other students believe them, but I feel like I'd be wasting my time trying to convince you.  I don't want to discourage you from learning however, so I'll try to take this word salad of yours point by point.

2 opposite and equal dipoles means 0 dipole moment.

Wunderbar.  A cogent and apt appellation of chemical concepts.

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However it does have a quadrupole moment because of the 2 dipoles.  This is how CO2 is.

Sorry, but I believe most textbooks would agree that it doesn't have a dipole because the molecule is linear.   Compare water, which has a similar arrangement of atoms, but is a dipole, because the molecule is bent.  Likewise ammonia.  And quadrupole, with regard to molecules, is just a made up term.  Please try to realize this.

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and I know that CO2 reacts with water to form carbonic acid. That is how come when you leave water sitting out for a whole day it is significantly more acidic.

Hexamine doesn't really have a dipole moment so to speak or really any significant dipoles in the bonds but it significantly dissolves in water.

equating dipole moment with polarity is thus not always helpful when talking about solubility.

Valid and clear then.  We'll agree therefore, not to mix these concepts in our discussion.  You will notice, you did just this, at the beginning of the thread:

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C=O. Okay the carbonyl carbon is partially positive and thus carbonyls are polar. O=C=O is like 2 of these bonded together so wouldn't it be that you would have an even more positive carbon and 2 partially negative oxygens causing O=C=O to be polar?

I mean this would explain why more of this dissolves in water than O2 and makes water more acidic than O2 does.

Quote
The combined opposed dipole moments give the whole molecule a quadrupole moment meaning that if there is a 4-pole electric field with positive at north and south and negative at east and west, the CO2 molecule will tend to turn to a north-south orientation. This is because of the negative oxygen being attracted to the north and south poles and the positive carbon being attracted to the the east and west poles. Larger molecules may have hexapole, octupole or higher moments with progressively less effect. 
But in this sense of quadrupole moments CO2 is polar since there is a specific orientation of it in the electric field.

This is completely made up, and has no bearing on the discussion at hand.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

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Re: Why is O=C=O nonpolar?
« Reply #11 on: July 27, 2014, 09:26:55 AM »
Consider each polar bond as a mathematical vector.

The overall polarity of the molecule is given by the sum of the vectors. In CO2 you have two vectors of identical magnitude in opposite direction. The vector sum is 0.

You can extend this to other less obviously non-polar molecules like BF3 or CCl4.
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Re: Why is O=C=O nonpolar?
« Reply #12 on: July 27, 2014, 09:38:59 AM »
The quadrupole moment is not made up.
It is true.

If you have an electrical field with + at N-S and - at E-W(which is standard) CO2 orients itself in the N-S direction meaning that it has a quadrupole moment. If it had 0 quadrupole moment than it would just be moving all over the place in that field in all orientations possible.

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Re: Why is O=C=O nonpolar?
« Reply #13 on: July 27, 2014, 11:39:43 AM »
Carbon dioxide does have a quadrupole moment, but pretty much everything else you wrote about it is wrong. The only practical relevance of the molecular quadrupole moment is how it relates to nonlinear spectroscopic transitions, and even that isn't probably very clear without looking at some pretty complicated transition moment integrals.
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Re: Why is O=C=O nonpolar?
« Reply #14 on: July 27, 2014, 01:55:53 PM »
still it does mean that CO2 is polar in a sense.

Just like how aldehydes have dipole moments and ketones do as well and they are polar because of that CO2 is polar in a sense because of its quadrupole moment.

Why else would CO2-O-C=O be possible other than the fact that CO2 has partially negative oxygens and a quadrupole moment with 0 dipole moment.(in that molecule I wrote the CO2 looks like this [O-C=O  :resonance: O=C-O])

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