New methanesulfonic acid salt forms of the anticonvulsant and analgesic active pharmaceutical ingredient carbamazepine and its closely related structural analogue 10,11-dihydrocarbamazepine have been prepared and characterized by single-crystal X-ray diffraction at 120 and 100 K, respectively {namely [(5H-dibenzo[b,f]azepin-5-yl)(hydroxy)methylidene]azanium methanesulfonate, C15H13N2O(+)·CH3SO3(-), and [(10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl)(hydroxy)methylidene]azanium methanesulfonate, C15H15N2O(+)·CH3SO3(-)}. In light of the structural information obtained, the crystal structure of the carbamazepine trifluoroacetic acid monosolvate [dibenzo[b,f]azepine-5-carboxamide-trifluoroacetic acid (1/1), C15H12N2O·CF3COOH] was redetermined at 100 and 270 K, and from this data it was concluded that the protonation state for this solvate species is best described as in an `intermediate state' with the acidic proton located almost at the mid-point between the acid and base.
A second metastable polymorphic form of the antifungal anilinopyrimidine active pyrimethanil was isolated from an attempted co-crystallization experiment with meso-erythriol in dimethyl sulfoxide (DMSO). The origin of the polymorphic behaviour is revealed in that the conformation of each dimer present in the asymmetric unit of the structure is unique and determined by the rotation of the second molecule in the dimer with respect to the first.
Reactions of 2-cyanoacetamide (CA) with both bromine and iodine at low halogen concentration in aqueous acid solution were first order in halogen and CA, consistent with an interpretation involving rate-limiting halogenation of the enol tautomer. There was no acid dependence in the range 0.1-0.5 mol dm Ϫ3 . The observed rate constants for bromination and iodination were very similar, indicating that both probably occurred at the encounter limit. This enabled a value for K E , the equilibrium constant for enolisation, to be determined as 6 × 10 Ϫ10 . There was strong evidence that at low acidities (pH ∼ 4) enolisation is rate-limiting, but we were unable to achieve a fully zero-order kinetic dependence upon halogen. However we did obtain a value for the acid-catalysed enolisation rate constant k e of 2.7 × 10 Ϫ3 dm 3 mol Ϫ1 s Ϫ1 from measurements of rates of deuterium exchange with the solvent. The bromination of malonamic acid (ML) in dilute acid solution at low [Br 2 ] was zero-order in [Br 2 ] and we obtained a value of 1.3 × 10 Ϫ2 dm 3 mol Ϫ1 s Ϫ1 for the acid catalysed enolisation rate constant. There is some evidence which suggests that enolisation of ML takes place via the amide group rather than the carboxylic acid group. The results are compared with earlier studies.
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