Chemistry 9701 · AS & A Level · Organic synthesis

Organic synthesis — practice question

Lactic acid, 2-hydroxypropanoic acid, $\text{CH}_3\text{CH(OH)CO}_2\text{H}$, is found naturally in sour milk and in our muscles after strenuous exercise. It is chiral and exhibits stereoisomerism.
(a)[3]

Draw complete displayed structures of the two optical isomers of lactic acid. Use an asterisk (*) to show the chiral carbon atom in the lactic acid molecule.

(b)[6]

Lactic acid can be made from ethanol by the route below. $\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{step 1}} \text{CH}_3\text{CHO} \xrightarrow{\text{step 2}} \text{CH}_3\text{CH(OH)CN} \xrightarrow{\text{step 3}} \text{CH}_3\text{CH(OH)CO}_2\text{H}$ State the reagent(s) and the essential condition(s) for each stage.

(c(i))

What kind of chemical compound is the enzyme lactic acid dehydrogenase?

(c(ii))

How would you detect a small quantity of pyruvic acid in a sample of lactic acid? State the reagent(s) you would use and what you would observe in the test.

(c(iii))

How would you detect a small quantity of lactic acid in a sample of pyruvic acid? State the reagent(s) you would use and what you would observe in the test.

(c(iv))

Which chemical reagent would you use to convert pyruvic acid into lactic acid? $\text{CH}_3\text{COCO}_2\text{H} \rightarrow \text{CH}_3\text{CH(OH)CO}_2\text{H}$

Worked solution & mark scheme

This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: the correct three-dimensional arrangement of the specified molecule with the chiral centre

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