Abstract
The thermodynamic relationship between FI and FII of ethyl 4-aminobenzoate (benzocaine) has been investigated. Slurry conversion experiments show that the transition temperature below which FI is stable is located between 302 K–303 K (29 °C–30 °C). The polymorphs FI and FII have been characterised by infrared spectroscopy (IR), Raman spectroscopy, transmission powder X-ray diffraction (XRPD) and differential scanning calorimetry (DSC). The isobaric solid state heat capacities have been measured by DSC. The quantitative thermodynamic stability relationship has been determined in a comprehensive thermodynamic analysis of the calorimetric data. The solubility of both polymorphs has been determined in eight pure organic solvents over the temperature range 278 K–323 K by a gravimetric method. The mole fraction solubility of benzocaine decreases in the order: 1,4-dioxane, acetone, ethyl acetate, chloroform, acetonitrile, methanol, n-butanol and toluene. Comparison with the determined activity of solid benzocaine forms shows that negative deviation from Raoult's law ideality is found in dioxane, acetone and ethyl acetate solutions, and positive deviation in solutions of the other investigated solvents.
Highlights
Benzocaine is a para-substituted amino benzoate ester, Fig. 1
Three polymorphs of benzocaine (FI, FII and FIII) are known, all of which have structures reported in the Cambridge Structural database (CSD)
The crystal structure of FII was first solved by Sinha and Pattabhi[3] in the orthorhombic space group P212121 (CSD refcode QQQAXG01)
Summary
Benzocaine is a para-substituted amino benzoate ester, Fig. 1. It is a local anaesthetic, often used topically, and is commonly available over-the-counter in single and compound dosage forms for various mouth and skin discomforts. It is used for anaesthetising the oropharynx for procedures such as bronchoscopy and trans-esophageal echocardiography.[1] generally well tolerated, it has been known to cause methemoglobinemia.[2]. The crystal structure of FII was first solved by Sinha and Pattabhi[3] in the orthorhombic space group P212121 (CSD refcode QQQAXG01).
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