QUESTION 150 (5 marks)
The repeat unit of PLA is shown. PLA and polypropene are both used in manufactured products.
(a) Draw the structural formula of the hydroxycarboxylic acid monomer from which this PLA repeat unit can be formed by condensation. [2 marks]
(b) Identify the PLA linkage that water can hydrolyse, and explain the effect on chain length. [2 marks]
(c) Explain why polypropene does not undergo the same backbone hydrolysis. [1 marks]
Practice marking scheme
Answer
The monomer is lactic acid, . Ester links in PLA can hydrolyse and split the backbone into shorter chains. Polypropene has a C–C backbone without ester links.
Working
The PLA repeat contains an alcohol-derived O and an acid-derived carbonyl. Restoring H to the O and OH to the carbonyl gives CH₃CH(OH)COOH. Hydrolysis breaks ester C–O connections, reducing chain length. Polypropene’s carbon–carbon backbone lacks this hydrolysable functional group. This structural comparison does not imply that PLA rapidly degrades under every environmental condition.
Marking criteria
- Draws the CH₃–CH backbone bearing OH. [1 mark]
- Draws the same central carbon attached to COOH, giving lactic acid. [1 mark]
- Identifies the ester linkage as the hydrolysis site. [1 mark]
- Explains chain cleavage into shorter molecules. [1 mark]
- Explains that polypropene lacks backbone ester links and has C–C bonds instead. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.