Biodegradable polyesters
3 marks · EasyPolylactic acid (PLA) is formed by joining multiple lactic acid units.
Draw the displayed formula of lactic acid.
Show (with a labelled diagram or description) how two lactic acid molecules join to form an ester linkage.
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.
Show answer
(a) Displayed formula: CH3–CH(OH)–COOH · must show All atoms and bonds shown
(b) Diagram shows two lactic acid molecules reacting: • The –OH group of one lactic acid reacts with the –COOH of another, forming an ester linkage (–COO–) and releasing H2O. · must show Ester linkage clearly labelled, Water molecule shown as produced
Common mistakes: Omitting water molecule in esterification; Incorrect connectivity
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Displays lactic acid formula with full bond connectivity (all atoms and bonds visible, OH and COOH clearly shown).
M1 (b) · 1 mark · Shows or describes reaction between –OH and –COOH groups to form an ester bond.
A1 (b) · 1 mark · Ester linkage labelled (–COO–), and water molecule shown as a product.
Worked solution
a) Displayed formula: H OH | | H—C—C—COOH | CH3 b) HO—CH(CH3)—COOH + HO—CH(CH3)—COOH → HO—CH(CH3)—CO—O—CH(CH3)—COOH + H2O The OH from the carboxyl group and an H from the alcohol group combine to give water, the bond formed between C=O and O is the ester linkage.