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The question is about finding the most stable molecule or species. Now my thought process was that cyclopropenylidene(1) was a carbene so would be unstable even though it is aromatic. Cyclopenta-1,3-diene(4) is anti-aromatic so is very unstable. Between cyclopropenium(2) and cyclohexa-1,3-diene(3) though, I am confused, especially since my answer key has the former. I think (3) should be the most stable out of them all. Cyclopropenium is an especially stable carbocation, but it's still electron deficient. How could it possibly be more stable than (3) which is a neutral conjugated diene? On another note, is it even possible to measure the stability of a cation with respect to a neutral molecule?

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    $\begingroup$ This is a meaningless question. Most stable with respect to what? $\endgroup$
    – Ian Bush
    Feb 7 at 20:04
  • $\begingroup$ Reactivity I guess? Maybe the enthalpy of the species? This is a pretty common type of question in organic courses... $\endgroup$
    – mechanist
    Feb 7 at 20:25
  • $\begingroup$ Unless it states what the criterion for stability is it's not even a bad question - as I say it's meaningless. $\endgroup$
    – Ian Bush
    Feb 7 at 20:27
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    $\begingroup$ Even for a crappy jee/neet question this is quite bad. I suggest just skipping it. I know a lot of times they just ask which is more stable and leave it at that, but sometimes (co-ordination complexes for example) you can somewhat quantitatively go about things in terms of the stability constant. Here I really don't understand what they're going for as @IanBush said. $\endgroup$ Feb 7 at 20:42
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    $\begingroup$ What this question is actually asking is simply "Which one of these is aromatic?", but it got redressed very badly. $\endgroup$ Feb 7 at 21:22

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Yes this question is poorly stated, probably through no fault of the OP. (It looks like something from a textbook, in which case the authors of the book are to blame.) You can really compare only things that match up in mass, atomic composition and charge. They probably want the student to choose the cyclopropenyl cation, choice 2, because it is aromatic and the full symmetry of the ion (as opposed to choice 1 which is not fully symmetric) maximizes the effect of this aromaticity.

The cyclopropenyl cation offers a case where a salt containing the ion is more stable than a compositionally equivalent set of neutral molecules (which actually is a proper comparison); to wit, $\ce{[C3H3]+[SbCl6]^-}$ is more stable than the combination of neutral molecukes $\ce{C3H3Cl + SbCl5}$ ($\ce{C3H3Cl}$ = 3-chlorocyclopropene). When the latter two are combined at -20°C in carbon tetrachloride solvent, the salt spontaneously forms and precipitates[1]. This is considered evidence of the strong stabilization of the cyclopropenyl cation that would theoretically be predicted by the usual aromaticity rules.

Reference

  1. Breslow, R.; Groves, J. T. (1970). "Cyclopropenyl Cation. Synthesis and Characterization". J. Am. Chem. Soc. 92 (4): 984–987. doi:10.1021/ja00707a040.
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    $\begingroup$ Thanks for the answer, and now I get the spirit of the question. Just a little thing I found weird though, isn't the stability of the cyclopropenyl cation a little overstated? Tropylium has stable bromides that dissolve in water with no reaction(though there is an adduct formed). It just seems weird to call that salt 'stable' when it's only stable in a non-polar solvent at -20°C. While hexa-1,3-diene is a liquid at room temperature... $\endgroup$
    – mechanist
    Feb 9 at 9:15

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