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Acetic acid is a monobasic acid as it disassociates into CH3COO$^-$ and H$^+$. Why don't the hydrogen atoms of CH3COO$^-$ disassociate too?

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The $\mathrm{p}K_\mathrm{a}$ of the acidic hydrogen in $\mathrm{CH_3COOH}$ is 4.76. The $\mathrm{p}K_\mathrm{a}$ of the remaining hydrogens would be of the order of 50. These atoms are not labile.

When discussing strengths of acids, we consider the conjugate base. The more stable it is, the stronger the acid is. $\ce{CH3COOH}$ is an acid because is conjugate base $\ce{CH3COO-}$ is resonance-stabilized. This means that the negative charge is delocalized on the whole $\ce{CO2-}$ group, making the anion more stable than, say, ethanolate $\ce{CH3CH2O-}$. If one of the remaining hydrogen is dissociated, this would give a dianion: $\ce{H2CCO2^2-}$, which would be extremely reactive (unstable). So such a species is simply not observed.

Perhaps you had this question because you were thinking of the acidity of the alpha protons (the one you are thinking of) in esters (like $\ce{CH3COOR}$), that have a $\mathrm{p}K_\mathrm{a}$ around 25. Malonic diesters can even be deprotonated in aqueous medium. The alpha protons are "acidic" because the ester function has electron-withdrawing properties. So are ketones, aldehydes, nitriles, sulfones, etc. The carboxylate ($\ce{COO-}$) moiety already has excess electrons (negatively charged), so it is not electron-withdrawing.

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    $\begingroup$ 50?! I would have thought lower than that. I've seen sources where LDA has been used to form the acetic acid dianion. $\endgroup$ – SendersReagent Apr 12 '16 at 17:16
  • $\begingroup$ @SendersReagent Indeed $\endgroup$ – julien Apr 12 '16 at 18:12
  • $\begingroup$ \ce{} is a better choice for chemistry formatting than \mathrm{}: meta.chemistry.stackexchange.com/questions/86/… $\endgroup$ – orthocresol Apr 12 '16 at 19:42

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