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Topic: Why is O=C=O nonpolar?  (Read 11117 times)

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Offline Corribus

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Re: Why is O=C=O nonpolar?
« Reply #15 on: July 27, 2014, 02:13:03 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.
No, it doesn't. In chemistry the conventional meaning of "polar" means a dipole moment ≠ 0. Most molecules that don't have a permanent dipole will have some higher order charge distribution like octupole, quadrupole, etc. Calling these "polar" as well would pretty much make every molecule inclusive in the definition, so it would be useless as a general term. Carbon dioxide does not have a permanent dipole moment; that is a fact. Therefore it is considered by chemists to be a nonpolar molecule, irrespective of the polarity of any single constituent bond. This is the accepted definition of "nonpolar".

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])
Quadrupolarity has nothing to do with it, first of all. Second, just because molecules are nonpolar does not mean they are nonreactive. You are confusing a lot of different concepts. The polarity of a molecule refers to the overall dipole moment. Individual bonds may be polarized in a nonpolar molecule. But this has already been explained to you. Furthermore, molecules (and their constituent electrons) are not frozen in time. Though carbon dioxide has no permanent dipole moment on average, vibrations can give rise to temporary dipole moments (one C=O bond lengthening, one shortening, say) which are often a gateway toward chemical reactions. Instantaneous fluctuations in electron density and nearby permanent dipoles also give rise to momentary charge asymmetries, even in nonpolar molecules - and these fluctuations are the basis of various intermolecular interactions like London and van der Waals forces. This is why a nonpolar molecule like hexane is a liquid at room temperature instead of a gas.

What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline caters

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Re: Why is O=C=O nonpolar?
« Reply #16 on: July 27, 2014, 02:15:14 PM »
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.

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.

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:

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.

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.

by "because of the 2 dipoles" I mean that the 2 bonds in CO2 are dipoles because of the electronegativity difference between C and O.

and I didn't think that I was equating dipole moment to polarity in the first post. I knew that CO2 had 2 equal and opposite dipoles in the bonds and thus no dipole moment and that a carbonyl has a dipole moment as well as a dipole in the bonds.
« Last Edit: July 27, 2014, 02:29:35 PM by caters »

Offline snorkack

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Re: Why is O=C=O nonpolar?
« Reply #17 on: August 03, 2014, 05:46:30 AM »
Another example of a highly polar molecule with no dipole moment is SO3. The 3 polar S=O bonds are in a plane and form axisymmetric triangle, so dipole moments cancel (but higher moments do not).

Another good example is C6F6. Also 6 polar bonds in a plane, form axisymmetric hexagon, dipole moments cancel, higher moments do not.

The polarity of C6F6 turns out to be in opposite direction to C6H6 - which has observable consequences, like the negative F atoms get attracted to positive F atoms, something that is aided not opposed by the attraction of oppositely charged carbon rings.

Offline alexanda

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Re: Why is O=C=O nonpolar?
« Reply #18 on: August 11, 2014, 05:49:02 AM »
polar BOND is different from polar MOLECULE. a molecule may have polar bond but it may not be a polar molecule.

For CO2, it does have polar bond since O is more electronegative than C. However, CO2 has a linear shape and this shape is symmetrical  .Therefore u can see it as 2 forces cancel each other

O=C=O is the arrangement of CO2

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