# Explain how the functional groups present in lactic acid (2-hydroxypropanoic acid) allow it to be classified into two different homologous series.

> IB DP Chemistry Standard Level (2023 syllabus) — S3.2 Functional groups: classification of organic compounds · Explain · 4 marks

> Lactic acid, found in sour milk and produced in muscles during anaerobic respiration, has the molecular formula C₃H₆O₃ and contains two distinct functional groups.

## Mark scheme (4 marks)

1. Lactic acid contains a hydroxyl (–OH) group, classifying it as an alcohol.
2. Lactic acid contains a carboxyl (–COOH) group, classifying it as a carboxylic acid.
3. A homologous series is a family of compounds sharing the same functional group and general formula, with successive members differing by –CH₂–.
4. Because lactic acid possesses both functional groups simultaneously, it belongs to both homologous series at the same time / it is both an alcohol and a carboxylic acid.

## Key terms

- [functional group](https://www.gradenine.co.uk/glossary/functional-group)
- [homologous series](https://www.gradenine.co.uk/glossary/homologous-series)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-functional-groups-present-57b35b78) · Published by Druglandscape Ltd.