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Why is a $\mathrm pK_\mathrm a$ value for carbonyls never mentioned anywhere in textbooks I've use while other functional groups have these values?

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  • $\begingroup$ Are you talking about aldehydes or enolisable ketones? $\endgroup$ – Jan Oct 25 '15 at 2:11
  • $\begingroup$ I guess ketones, although information about both would be nice. $\endgroup$ – notorious Oct 25 '15 at 2:11
  • $\begingroup$ I can't believe they're not mentioned, but look for enols instead ;) $\endgroup$ – Jan Oct 25 '15 at 2:12
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    $\begingroup$ Just to clarify, are you talking about the $\mathrm{pK_a}$ of the $\alpha$ hydrogens or the $\mathrm{pK_a}$ of hydrogens attached directly to the carbonyl carbon? $\endgroup$ – bon Oct 28 '15 at 16:10
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    $\begingroup$ Directly attached to the carbonyl carbon $\endgroup$ – notorious Oct 28 '15 at 23:58
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As per comments you are asking for the $\mathrm{p}K_\mathrm{a}$ value of a hydrogen directly attached to a carbonyl carbon. This means, you are asking only for non-enolisable aldehydes as ketones have no hydrogens on the carbonyl carbon. Aldehydes that have a hydrogen in α-position will be deprotonated there generating an enolate ($\mathrm{p}K_\mathrm{a} \approx 20$) — more acidic than a carbonyl hydrogen. So all we have left is 2,2-dimethylpropanal (or pivaloylaldehyde). Its $\mathrm{p}K_\mathrm{a}$ is not given on SciFinder, but should be somewhere between $45$ (an alkane) and $25$ (an alkyne), probably just slightly higher than an alkene ($40$).

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