I understand that 5,5-disubstituated derivates of barbituric acid only show lactam-lactim tautomerism opposed to the lactim-lactam and keto-enol tautomerism of barbituric acid, hence the 5-mono and 5,5-disubstituated derivates are less acidic.

Primidone differs from phenobarbital in a C2 oxo group, so it's lactam groups are not acidic and it is not soluble even in alkali-bases. Why aren't the lactam groups acidic exactly? The oxo groups should still present quite a bit of electron withdrawing effect, shouldn't they?

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  • $\begingroup$ Amides are not acidic and that is all these are $\endgroup$ – Waylander Dec 11 '18 at 21:27
  • $\begingroup$ There's a carbonyl group missing (i.e., a barbituate is made from urea and this is made from diaminomethane). Compare the pKas of a ketone versus a $\beta-$keto ketone. $\endgroup$ – Zhe Dec 11 '18 at 21:27

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