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Effect of Molecular Structure Change of BPT-Esters on Crystal Structure and Solubility

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journal contribution
posted on 01.07.2009, 00:00 by Takayuki Hara, Kosuke Adachi, Midori Takimoto-Kamimura, Mitsutaka Kitamura
Methyl 2-(4-(2-metylpropoxy)-phenyl)-4-methyl-thiazole-5-carboxylate (Me-est(H)), propyl 2-(4-(2-metylpropoxy)-phenyl)-4-methyl-thiazole-5-carboxylate (Pr-est(H)), and 2-methyllpropyl 2-(4-(2-metylpropoxy)-phenyl)-4-methyl-thiazole-5-carboxylate (i-But-est(H)) were prepared, and the relationship between the molecular structure, and crystal structure and solubility was investigated. The crystallization of Me-, Pr-, and i-But-est(H) from methanol (MeOH), ethanol (EtOH), and acetonitrile (MeCN) solutions by a rapid cooling method resulted in only one crystal form for each ester. Crystallographic analysis using a single crystal indicated that the crystal was constructed by stacking the sheet of molecules, the phenyl ring, the thiazole ring, and the ester group along the same plane in all esters. The hydrogen bonding by the carbonyl group was observed in the case of Pr- and i-But-est(H), whereas no hydrogen bonding was observed in the Me-est(H) crystal. From solubility measurements in MeOH, EtOH, MeCN, and c-hexane (c-Hxn), it was found that each ester has the lowest solubility in MeOH and the highest solubility in c-Hxn. These results indicate that the decrease of polarity of the solvents make the solubility increase. However, MeCN shows irregular behavior suggesting the presence of interactions between the ester and MeCN. In the esters, i-But-est(H) showed the lowest solubility in every solvent. This may correspond to the highest fusion enthalpy and the crystal structure, in which a dimer of molecules is formed by strong hydrogen bonding. In comparison with the previous work, it was indicated that the presence of nitrile group creates the additional possibilities for hydrogen bonds and therefore creates the possibility of other crystal forms.