Stereoselective preparation of 1-phenylmethol from (-)-menthone as a template for molecularly imprinting and its subsequent adsorption from liquid solutions

Abstract

Extraction and purification of target compound or analytes from the matrices is always a goal to achieve in different industries and fields of study. Therefore, there is always a need to purify the desired product to get rid of other unwanted products or contaminants. However, to get 100% purity, proper selection of good extraction method and the sorbent with high selectivity is required. Molecularly imprinted polymers (MIPs) as sorbent have demonstrated its usefulness for recognition and separation of the target molecules by improving the quality and selectivity of analytical methods. The aim of the study was to synthesize 1-phenylmenthol by reducing chiral menthone with Grignard reagent, then use it as a template for the preparation of MIPs by precipitation polymerisation. Menthol was oxidized to menthone by the Jones reagent and then treated with phenyl magnesium bromide (PhMgBr) to give 1-phenylmenthol which was then purified by column chromatography. The major product 1-phenylmenthol was used as a template to prepare MIPs by precipitation polymerization. During polymerization methacrylic acid (MAA) was used as a functional monomer, ethylene glycol di-methacrylate (EGDMA) as cross-linker, azobisisobutyronitrile (AIBN) as an initiator in methanol as a solvent. The non-imprinted polymer was synthesized with the same reagents but without a template. Imprinted polymers were washed to elute or remove the 1-phenylmenthol, with a mixture of methanol/acetic acid (90:10, v/v), all wash out solution was collected and analysed for the presence of 1-phenylmenthol using UV-vis spectroscopy for the total of 10 cycles. The four elution were considered enough to wash out the template. The product (1-phenylmenthol) was characterized by Fourier transform infra-red (FIRT), carbon and proton MNR spectroscopy. The results from 13C NMR confirmed the presence of the two quaternary carbons at 148.6 ppm and 78.4 ppm, while 1H NMR confirmed the presence of OH peak at 2.2 ppm. FTIR showed a broad O-H peak at 3592 cm-1, strong C-H at 2924 cm-1 and 3052 cm-1 from alkanes and benzene, respectively. The polymers (unwashed MIPs, washed MIPs, and washed NIPs) were characterized by Brunauer-Emmett-Teller (BET), Fourier Transform Infra-red (FTIR), thermogravimetric analyser (TGA), and ultraviolet (UV) spectroscopy. The BET results showed higher surface area (17.85 m² g-1), pore size (85.27 Å), and pore volume (0.038 cm3 g-1) for the MIP as compared to NIP with the surface area of (10.05 m² g-1), pore size (70.69 Å) and pore volume (0.018 cm3 g-1). TGA confirmed the stability of washed MIP and NIPs up to 300℃ as compared to unwashed MIP which was gradually iv decreasing in weight from 100℃. FTIR results shows the presence of template by broad O-H peak at a higher wavenumber of 3573 cm-1 and strong C-H stretching peaks from alkane groups on unwashed MIPs as compared to washed MIPs and NIPs which did not have the template. A series of parameters such as the effect of concentration, the effect of polymer mass, effect of sample pH, and effect of the contact time were investigated. The high enrichment factor was achieved at samples pH 7 as the optimum pH. In the effect of concentration, the optimum concentration were 11 mg L-1 and 7 mg L-1 for MIP and NIP, respectively. The optimum mass of the polymer was determined to be 50 and 80 mg for MIP and NIP, respectively. The optimum time for the adsorption of the 1-phenylmenthol was at 180 min. (-)-Menthone and 1-phenylmenthol were successfully synthesised and purified using column chromatography with silica gel. Unlike other essential oils (menthol and menthone), 1-phenylmenthol was found to have these properties as they were never reported before: it has no minty odour, colour is yellowish and not soluble in water. Molecularly imprinted polymer was successful synthesized using precipitation polymerization. The method can be applied to real samples based on high enrichment factors obtained.