# What's the pKa (range) of a carbonyl functional group?

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?

• Are you talking about aldehydes or enolisable ketones? – Jan Oct 25 '15 at 2:11
• I guess ketones, although information about both would be nice. – notorious Oct 25 '15 at 2:11
• I can't believe they're not mentioned, but look for enols instead ;) – Jan Oct 25 '15 at 2:12
• 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? – bon Oct 28 '15 at 16:10
• Directly attached to the carbonyl carbon – notorious Oct 28 '15 at 23:58

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 ($40$).