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The three title compounds were obtained by reactions which mimic, with more extreme conditions, the in vivo metabolism of barbiturates. 1-(2-Cyclohex-2-enylpropionyl)-3-methylurea, C(11)H(18)N(2)O(2), (I), and 2-ethylpentanamide, C(8)H(17)NO, (III), both crystallize with two unique molecules in the asymmetric unit; in the case of (III), one unique molecule exhibits whole-molecule disorder. 2-Ethyl-5-methylhexanamide, C(9)H(19)NO, (II), crystallizes as a fully ordered molecule with Z' = 1. In the crystal structures, three different hydrogen-bonding motifs are observed: in (I) a combination of R(2)(2)(4) and R(2)(2)(8) motifs, and in (II) and (III) a combination of R(4)(2)(8) and R(2)(2)(8) motifs. In all three structures, one-dimensional ribbons are formed by N-H···O hydrogen-bonding interactions.

The room-temperature crystal structure of 5-butyl-5-ethylbarbituric acid (generally known as butobarbitone), C10H16N2O3, was reported in space group C2/c [Bideau (1971). C. R. Acad. Sci. Paris Ser. C, 272, 757-760]. A redetermination at 120 K using synchrotron radiation shows the space group at this temperature to be P2(1)/n and not C2/c. There are two crystallographically independent molecules in the asymmetric unit, but no solvent. Reported issues concerning possible disorder of the molecule are addressed; the butyl substituent of one of the molecules adopts an unusual conformation in being not fully extended. A subsequent re-collection at room temperature shows that the space group is indeed C2/c (A2/a with the axes selected in this report), and so the crystal structure undergoes a phase change upon cooling to 120 K.