[Compaq]

Why exactly do these gases not react with other chemicals? Why do they have such a long life?

I'm thinking it has to be because of very strong intramolecular forces. Halogens are very electronegative, and I'm assuming the molecules must be very polar, with a big positive charge on the carbon atom(s).
I'd think that one of the halogen atoms in a CFC molecule would make a good leaving group in substitution reactions?

I'm lost here, I'd appreciate any help

[Schrödinger]

The bond strength is the reason for their inertness. Halogens being highly electronegative strengthen the bonds due to bond polarity. They are so strong that such bonds don't break under the energy of ordinary visible light; UV light (upper atmospheric levels) is required to break them.

[Compaq]

So with four halogens pulling on the electrons, the molecule becomes near ionic with very strong bonds that's very difficult to break?

HCFCs are much more reactive, and react on their way up into the stratosphere, and thus won't give as many Cl radicals as CFC gases. It would seems to me that that H atom in tetrachloromethane would sit very loosely, and be easily removed by a nucleophile, and undergo further destruction. Does this, in a nutshell, explain why HCFCs are more reactive than CFCs?

[Schrödinger]

Yes, consider the residue after a proton leaves the HCFC. That particular anion is stabilized very well by the presence of good electron withdrawing groups

[Compaq]

Indeed. And that carbanion is prone to react with electrophiles in the atmosphere, correct?

Thanks a lot for the help, here!

[orgopete]

The bond strength is the reason for their inertness. Halogens being highly electronegative strengthen the bonds due to bond polarity. They are so strong that such bonds don't break under the energy of ordinary visible light; UV light (upper atmospheric levels) is required to break them.

I think this is the simple answer. However, you need to be cautious about the electronegativity argument. Pauling argued that high bond strength could be due to ionic character, therefore to argue that CFC have ionic character and therefore high bond strength is a circular argument. An obvious fallacy of in equating high bond strength to ionic character is that a "highly ionic" molecule like HF does not ionize as readily as a "covalent" molecule like HI.

If you apply the underlying principle that high electronegativity means high homolytic bond strength, that should match well the properties of CFCs in the stratosphere. Although I am not an expert in atmospheric chemistry, I presume that monoatomic or radical chemistry prevails. Therefore, the high homolytic bond strength should result in more stable compounds.

If we compare the CFCs and HCFCs in solution chemistry, then indeed the hydrogen can be removed by bases. The stability of CFCs in solutions should be examined vis a vis the reactions that it can undergo. In this case, we well may argue that a molecule like methane is an a very stable compound as it resists forming ions while a CFCs can react with reducing metals.

Conclusion, Schroedinger was correct that CFCs have strong bonds and therefore more stable. Secondly, do not deduce heterolytic bond stability from homolytic bond strengths. They measure different reactions.

[Compaq]

Brilliant, that's logic. Something like that actually crossed my mind, that CFCs might be stable in the atmosphere, but maybe be easily soluble in water or something. I'm not too far in my education yet, just in my second year of my bachelor's degree. I have no in-depth knowledge of stratospheric chemistry, but radicals do dominate the reactions, I believe. At least when it comes to ozone depletion, which is what I'm focused on in my environmental chemistry course (and writing a term paper on). I don't strive to become an environmental chemist, but maybe rather an organic chemist.

That's why I want to, in my paper, try to explain some theoretical, physical and chemical properties to the species I'm studying, and trying to really understand why, and not just accept that CFCs are very stable molecules that indirectly contribute to ozone depletion. That is my approach, not just focused on environmental problems, but show that I at least have some knowledge of the chemistry, physics and thermodynamics of the stuff about which I'm writing.

Thanks again!


edit: homolytic scission of bonds will restore all atoms to their elementary state, and in highly polar substances, such as CFCs, that require a lot of energy as the electron density is pushed so much toward the halogens.
hetereolytic scission would require much less energy as the halogens almost have the electrons anyway, and we would get a C⁴⁺ along with four halogens each with a negative charge?