Biodegradable polyesters
3 marks · EasyLactic acid, which contains both a carboxylic acid group and an alcohol group in its molecule, can be used to make a biodegradable polyester.
State the two functional groups present in lactic acid that allow it to form a polyester.
Describe how a polyester such as PLA can be formed from lactic acid monomers.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Carboxylic acid group (–COOH) and alcohol group (–OH) · must show Both correct functional group names
(b) Lactic acid monomers react with each other in a condensation reaction. The –OH group of one lactic acid molecule reacts with the –COOH group of another, forming an ester linkage and releasing water. This process repeats to form a long-chain polyester (PLA). · must show Condensation reaction, Ester linkage forms between –OH and –COOH, Water is eliminated, Chain continues forming a polyester
Common mistakes: Missing either functional group; Incomplete description of ester formation
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Marking points
B1 (a) · 1 mark · Both correct functional groups identified: alcohol (–OH) and carboxylic acid (–COOH).
M1 (b) · 1 mark · Describes formation of ester linkage (—COO—) between –OH and –COOH groups, with water being lost.
A1 (b) · 1 mark · Mentions that the process repeats to form a polyester chain (PLA).
Worked solution
a) The functional groups are:
• Carboxylic acid group (–COOH)
• Alcohol group (–OH)
b) PLA is made when lactic acid molecules react in a condensation reaction. The –OH of one molecule joins with –COOH of another, releasing water and forming an ester linkage (–COO–). This repeats, creating a long-chain biodegradable polyester.