Synthesis of lactic acid using hydrogen cyanide extracted from cassava (Manihot esculenta) leaves

Abstract

Racemic lactic acid (2-hydroxypropanoic acid) has gained interest in the food and non-food industries and in producing biodegradable and biocompatible lactic acid polymers. Although racemic lactic acid is conveniently synthesised by chemical synthesis via the DL-lactonitrile route, it can also be produced by the fermentation process provided that suitable microorganisms and substrates are used. However, regardless of the sustainability issues associated with the fermentation process, it is the preferred production method since the chemical process relies on fossil fuel resources. In this context, this study aims to extract hydrogen cyanide (HCN) from cassava (Manihot esculenta Crantz) leaves and then use it to chemically produce racemic lactic acid. Cassava leaves were chosen as a natural source of HCN since they release 20 times more HCN than the tubers. HCN is produced by endogenous enzymes (linamarase and hydroxynitrile lyase) hydrolysing the cyanogenic glucosides (linamarin and lotaustralin). Following 120 minutes of maceration at 30 °C, the released HCN was extracted for 45 minutes under vacuum at 35 °C – 45 °C and collected in 400 mL of 5.104 mol/L sodium hydroxide (NaOH) solution (absorbing solution) to give sodium cyanide (NaCN) solution. The extraction process was repeated until saturation of the absorbing solution was achieved. The final concentration of NaCN solution determined by the alkaline picrate method was found to be 4.0421 mol/L. Furthermore, the sodium carbonate (Na2CO3) and residual NaOH content in control and sample sodium cyanide solutions were also determined. The Na2CO3 content was 0.72 % in the control NaCN solution and 2.49 % in the sample NaCN solution. The residual sodium hydroxide content was 2.61 % in the control sodium cyanide solution and 4.20 % in the prepared sodium cyanide solution. 79.241 g of NaCN crystals (0.19 % yield, green NaCN) were obtained from 42.750 kg of fresh cassava leaves. The suggested approach was successful in preparing NaCN, as evidenced by X-Ray Diffraction (XRD), Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM-EDS) results. Control and green NaCN both contained sodium carbonate impurities, as shown by these spectral techniques. Titration tests revealed that the latter was 0.61 % and 2.29 % in control and green NaCN, respectively. In addition, titration studies indicated that the residual NaOH content in control NaCN was 1.63 % and 4.68 % in green NaCN. The high carbonate content can be explained by the reaction between residual sodium hydroxide and atmospheric CO2. Reproducibility and repeatability tests were done to evaluate the reliability of the hydrogen cyanide extraction method. Racemic lactic acid was synthesised using a four-step process. 73 mL of DL-lactonitrile (2- hydroxypropanenitrile) (81.1 % yield, 59.7 % pure) was prepared by reacting 75 mL of acetaldehyde with hydrogen cyanide generated in-situ from green sodium cyanide (62.190 g in 150 mL of Milli-Q water) in the presence of 37 % hydrochloric acid (100 mL). 35 mL of crude racemic lactic acid (84.1 % yield, 14.9 % pure) was prepared by hydrolysing 40 mL of DLlactonitrile with 8 mol/L hydrochloric acid (40 mL). Crude racemic lactic acid underwent a two-step purification process in the presence of concentrated sulphuric acid (5 mL), used as the catalyst. 35 mL of crude lactic acid was first esterified with excess methanol (50 mL) to produce 32 mL of methyl DL-lactate (methyl 2-hydroxypropanoate) (71.6 % yield, 46.1 % pure). The ester was then hydrolysed with excess water (20 mL) to give 22 mL of purified racemic lactic acid (88.0 % yield, 56.0 % pure). The identity of the synthesised products was confirmed by comparing them against control samples using 1H Quantitative Nuclear Magnetic Resonance (1H QNMR) and ATR-FTIR. Their purity was determined by 1H QNMR, using dimethylformamide as the internal standard. The overall yield of synthesised racemic lactic acid was 43.0 %.