Timeline for Why are DCM and chloroform so resistant towards nucleophilic substitution?
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| Sep 3, 2014 at 3:11 | comment | added | Philipp | @Martin Thanks again. You're right, to each his own "tool". Since looking at the foundations of perturbational molecular orbital theory I try to use its concepts for everything. But the problem is, as my professor stated it, you can explain everything with this theory in a very elegant and plausible way, even if your explanation is totally wrong :) | |
| Sep 3, 2014 at 2:11 | comment | added | Martin - マーチン♦ | Thinking about it a little more, the EDA-NOCV may give you exactly what you are looking for. (I do not have access to this program, so I cannot help out here.) I would suspect your analysis would be correct. On the other hand, I have grown quite fond of the simplicity of Bent's rule, which explains the shortening of the bonds. So maybe there is no use overanalyzing it. As for many things, there is no definite truth when it comes to interpretation, we always put some of our own to any analysis. Your analysis is well founded and (from my point of view) a valid approach to this problem. | |
| Sep 3, 2014 at 2:02 | comment | added | Martin - マーチン♦ | @Philipp There are many schemes that decompose energy based on orbital interactions, but they all work in the same way, i.e. using the original AO. I guess you can go ahead and do a full Valence Bond Theory approach, to find out which resonance structure contributes how much to the total structure and then you get an idea of how strong the $\pi$ and $\sigma$ interactions are - then this approach would be (at least semi) quantitative. The unfortunate thing is, that you can only observe the total bonding directly and not the singular contributions. | |
| Sep 2, 2014 at 19:25 | comment | added | Philipp | ...of course this treatment is far from being exact and additional symmetry consideration would have to be made but in principle I think it should be possible to assign energies to the fragment orbitals used here (although I would never go so far as to really do that with any accuracy as MO-theory usually serves only a qualitative purpose). | |
| Sep 2, 2014 at 19:21 | comment | added | Philipp | @Martin Thanks for the comment. Your point is well taken. But as for "... something that is not there. Since you are arguing with orbitals that cannot be assigned energies, magnitudes would have no meaning": Are you sure that there is not a little more reality in there than you give it credit. I mean, the procedure of looking at the interaction of the $n(\ce{e-})$ and the $\sigma^{*}(\ce{C-Cl})$ orbitals is basically the usage of perturbational MO-theory on the problem of what happens if I take a $\ce{{}^{+}CH2-Cl}$ fragment and let it interact with a $\ce{Cl-}$ fragment... | |
| Sep 2, 2014 at 4:01 | comment | added | Martin - マーチン♦ | Your qualitative analysis is sound. A (partial) $\pi$ contribution would strengthen the one bond while weaken the other. But you are interpreting only a concept here, something that is not there. Since you are arguing with orbitals that cannot be assigned energies, magnitudes would have no meaning. The anomeric effect is therefore a concept, that allows us to understand delocalisation in terms of a much simpler concept - it is not the true reason. | |
| Aug 31, 2014 at 22:58 | history | edited | Philipp | CC BY-SA 3.0 |
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| Aug 31, 2014 at 17:41 | history | edited | Philipp | CC BY-SA 3.0 |
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| Aug 31, 2014 at 17:31 | history | edited | Philipp | CC BY-SA 3.0 |
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| Aug 31, 2014 at 14:24 | history | answered | Philipp | CC BY-SA 3.0 |