Depolymerization of Polylactic Acid: Saponification to Lactic Acid Reference rea

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Depolymerization of Polylactic Acid: Saponification to Lactic Acid Reference reading assignment: saponification, for example, see Klein 21.11 Chemistry lessons: basic hydrolysis (saponification) Green lessons: design for degradation, atom economy, catalysis, safer solvents and reagents Introduction Polylactic acid (PLA) is a corn-derived biobased polymer that is a promising alternative to fossil- fuel based plastics. PLA is used in various applications, such as medical implants and sutures, compostable packing materials, cups, bags, and utensils. After its use, PLA can be recycled via hydrolysis to produce lactic acid monomer that can be repolymerized and reused in another application. 2n mol NaOH n Na o C-C-o Na 2n mol HCI 50% EtOH 50% EtOH Polylactic acid (a polyester) can be depolymerized to lactic acid by saponification. Saponification, the basic hydrolysis of an ester, yields a carboxylate salt that produces carboxylic acid upon acidic workup. In this experiment, saponification of the polylactic acid polyester gives sodium lactate: lactic acid is formed after the acidic quench. Saponification of a fat is the traditional method for soap making: interestingly, an additional application of lactic acid is a green cleaner to remove soap scum. A general mechanism for saponification is OR O-R O-H acid-base reaction O-H HO-R Several green chemistry principles are used in this lab. There is little waste 1) produced in this reaction: the byproducts are salt and water. The atom economy (82) is high. Safer chemicals and solvents are used (#3 and 4) and the product is biodegradable. The reaction only needs to be heated for an hour and can be accelerated in the microwave (a6, energy efficiency). The polylactic acid is a renewable feedstock (#7) and is also biodegradable 10, design for degradation) Safety Sodium hydroxide is corrosive. Use caution and wear appropriate PPE, including goggles and gloves Notebook (4 points) As part of your normal prelab, prepare your notebook for a synthesis reaction (e.g. with a table of reagents). You will need to calculate how many lactic acid monomers are in a 5g sample of PLA. (Hint: what is the molar mass' of the repeated monomeric unit in PLA?) Use the answer to calculate how much NaOH you will need for the saponification and how much HCl you will need to product lactic acid from sodium lactate.

Explanation / Answer

4) a) we Know, pKa = -logKa where Ka = dissociation contant of an acid. This shows that as the value of pKa increases, dissociation constant of an acid decreases and consequently dissociation of the acid also decreases. Since pKa of lactic acid (3.85) is lower than that of methanol (pKa = 16), dissociation tendency of lactic acid is much higher compared to methanol. Thus in solution, lactic acid will be dissociated to lactate and hence saponification always produces sodium lactate and not lactic acid.

B) we konw, the Henderson equation, pH = pKa + log ([A-]/[HA]). Assume almost all the lactic acid ionizes to sodium lactate, lowest the pH of the solution at which all the lactic acid remain in solution as sodium lactate would be 3.85 since pKa of lactic acid is 3.85. Hence to get the product sodium lactate as lactic acid , the pH of the medium should be < 3.85. Hence it is necessary to acidify the solution to pH <3.85.

C) On acidification, sodium lactate will be converted to lactic acid and hence the organic product after acidification produced is lactic acid.

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