Explain how the functional groups present in lactic acid (2-hydroxypropanoic acid) allow it to be classified into two different homologous series.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lactic acid, found in sour milk and produced in muscles during anaerobic respiration, has the molecular formula C₃H₆O₃ and contains two distinct functional groups.
Model answer (4 marks)
Lactic acid contains a hydroxyl (–OH) group, so it is an alcohol and belongs to the alcohol homologous series.
Lactic acid also contains a carboxyl (–COOH) group, so it is a carboxylic acid and belongs to the carboxylic‑acid homologous series.
A homologous series is a family of compounds that share the same functional group and general formula, with successive members differing by –CH₂–.
Because lactic acid has both –OH and –COOH groups, it can be classified simultaneously as an alcohol and as a carboxylic acid, i.e. it belongs to two different homologous series.
Lactic acid also contains a carboxyl (–COOH) group, so it is a carboxylic acid and belongs to the carboxylic‑acid homologous series.
A homologous series is a family of compounds that share the same functional group and general formula, with successive members differing by –CH₂–.
Because lactic acid has both –OH and –COOH groups, it can be classified simultaneously as an alcohol and as a carboxylic acid, i.e. it belongs to two different homologous series.
Examiner tips
- State both functional groups explicitly; link each to its series. Explain what a homologous series is. Show that lactic acid satisfies both series. Use correct terminology: alcohol, carboxylic acid, homologous series.
Common mistakes
- Forgetting to mention the carboxyl group or the alcohol group. Confusing the definition of a homologous series. Writing that lactic acid belongs to only one series.
Mark scheme (4 marks)
- Lactic acid contains a hydroxyl (–OH) group, classifying it as an alcohol.
- Lactic acid contains a carboxyl (–COOH) group, classifying it as a carboxylic acid.
- A homologous series is a family of compounds sharing the same functional group and general formula, with successive members differing by –CH₂–.
- 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 in this question
functional group · homologous series
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Functional groups: classification of organic compounds questions
- Outline how the functional groups present in ethanoic acid and ethylamine allow …
- Distinguish between the structures and chemical properties of a primary alcohol …
- Explain how the functional groups present in an amino acid molecule account for …
- Explain why a molecule of glycine (an amino acid with the molecular formula C₂H₅…