Stability study of alpha-bromophenylacetic acid: Does it represent an appropriate model analyte for chiral separations?
Abstract
The stability of alpha-bromophenylacetic acid (BPAA) in 50% aqueous methanol solution has been tested. CE in different running buffers was used to separate BPAA from the decomposition reaction products alpha-hydroxyphenylacetic (mandelic) acid and alpha-methoxyphenylacetic acid.
Suitable CE separation of all three compounds and other product, bromide, was achieved in 60 mmol/L formate buffer (pH 3.0) at -30 kV in 50 mu m (i.d.) poly(vinyl alcohol)-coated fused silica capillary (30 cm/24.5 cm) with UV detection at 200 nm. The CE method was applied to determine the reaction order of the decomposition of BPAA (0.47 mmol/L) via nucleophilic substitution in 50% aqueous methanol.
The first-order reaction kinetics was confirmed by linear and non-linear regression, giving the rate constants 1.52 x 10(-4) +/- 2.76 x 10(-5)s(-1)and 7.89 x 10(-5) +/- 5.02 x 10(-6)s(-1), respectively. Additionally, the degradation products were identified by CE coupled to mass spectrometric (MS) detection.
The CE-MS experiments carried out in 60 mmol/L formate buffer (pH 3.0) and in 60 mmol/L acetate buffer (pH 5.0) confirmed the results obtained by CE-UV. Furthermore, the stability of BPAA in polar solvents was tested by(1)H NMR experiments.
Our results provide strong evidence of the instability and fast degradation of BPAA in 50% aqueous methanol indicating that BPAA is not suitable as the model analyte for chiral separations.