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I have question regarding the solution that has been given.

Question:

In the given compound the order of case with which hydrogen atom can be abstracted from carbons I to VI is

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The solution that has been given is:

enter image description here

In the solution they've posted above, how is the 2nd structure more stable than the third structure? The third one clearly has resonance with the pi bond, while the 2nd one has no resonance with the oxygen atom (it can't "accept" oxygen's lone pair as it is not a carbocation, and oxygen does not have an empty d orbital either)

Please can anyone shed light on this issue. Thank you!

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  • $\begingroup$ The order looks fine: C-H BDE of CH3OCH3 = 83 kcal/mol; allylic C-H BDE of propene = 89 kcal/mol. Blanksley and Ellison, Acct. Chem. Res., 2003, 36, 255. $\endgroup$ – user55119 Dec 9 '17 at 20:25
  • $\begingroup$ @user55119 Thank you for digging up references to confirm the order! :) However, I hope you understand the aim of this question was to justify the order using logical reasoning on resonance. Thanks once again! $\endgroup$ – Gaurang Tandon Dec 10 '17 at 1:30
  • $\begingroup$ The non-bonding electrons on oxygen donate electron density to the half filled carbon radical p-orbital. Some what like having a carbocation stabilized by oxygen. Why is propene have a higher BDE? I wouldn't have been surprised if the BDE's were switched. Certainly, Diethyl ether and disopropyl ether undergo autooxidation. Dimethyl ether undoubtedly slower. I presume propene doesn't, or very slowly, autoxidzes. $\endgroup$ – user55119 Dec 10 '17 at 3:22
  • $\begingroup$ @user55119 I am sorry sir but you're confusing me. "Why is propene have a higher BDE? I wouldn't have been surprised if the BDE's were switched" I don't understand what you're trying to say exactly. It seems you're counter-claiming your own data. And what is the role of autooxidation here? And what is the underlying logic behind the data you had presented? Thank you! $\endgroup$ – Gaurang Tandon Dec 10 '17 at 8:40
  • $\begingroup$ I am saying that a priori it is difficult to predict qualitatively whether the C-H bond of dimethyl ether or the allylic C-H bond in propene is weaker. Autooxidation refers to the tendency of diethyl ether to react with O2 by a radical pathway. As to the logic you request, read Ellison's paper. Case closed. $\endgroup$ – user55119 Dec 10 '17 at 18:26

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