pKa of methylene protons in cycloheptatriene vs cyclopropene


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> The $\mathrm{p}K\_\mathrm{a}$ values of methylene protons in cycloheptatriene vs cyclopropene are found to be 60 and 36, respectively.
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From what I understand - both conjugate bases are non-aromatic (not anti-aromatic) as a result of some sort of Jahn-Teller distortion. I can't pin down the nature of the distortions but they appear to be a pyramidal geometry for the cyclopropenyl anion and either $\mathrm{sp^{2}}$ hybridisation or unequal bond lengths for the 'tropylium' anion.


In either case, if both species and both conjugate bases are non-aromatic what is the cause of the massive $\mathrm{p}K\_\mathrm{a}$ difference? Is it something to do with the $\mathrm{sp^{2.68}}$ hybridisation in the three membered ring?




Not all "antiaromatic" rings are created equal.


Assuming the cyclopropenide anion is formed by deprotonating a methylene proton, forming a cyclopropen-3-ide ion (in reality, a vinylic hydrigen appears to be deprotonated instead, forming cyclopropen-1-ide ion; see Poutnik's comment to the question), it can remove the unfavorable antiaromatic coupling only by fully localizing the negative charge and pyramidalizing the bonding at the selected carbon, giving up any possibility of stabilizing the charge via delocalization.


Not so with the cycloheptatrien-7-ide ion. While delocalizing the charge over the full ring results in an antiaromatic coupling, a nonaromatic and resonance-stabilized alternative is available by forming an allyl-anion-butadiene contributing structure (see below). Thus with the seven-carbon anion, some stabilization is possible while still circumventing antiaromatic coupling. With multiple positions possible for the allyl-anion component, the $pK\_a$ reported here for cycloheptatriene actually beats that for the simple allyl-anion former propene ($36$ versus [$43$](https://owl.oit.umass.edu/departments/OrganicChemistry/appendix/pKaTable.html)).


[![enter image description here](https://i.stack.imgur.com/cDEw3.png)](https://i.stack.imgur.com/cDEw3.png)


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