Questions
1–2 questions in major theory papers
Difficulty
Medium
Importance
Core curriculum requirement for Pharmacognosy
Overview
Tannins and volatile oils are critical secondary metabolites with significant pharmacological activity, often appearing in B.Pharm pharmacognosy examinations. Mastery of this topic requires distinguishing between the chemical structures of tannins and the extraction methods and biological sources of essential oils.
Classification of Tannins
Tannins are complex polyphenolic compounds characterized by their ability to precipitate proteins. They are primarily classified into two main groups based on their structural backbone and hydrolysis products, which is a frequent question in university viva exams.
- Hydrolyzable tannins: Yield gallic or ellagic acid upon hydrolysis
- Condensed tannins (Proanthocyanidins): Formed by polymerization of flavan-3-ols
- Pseudo-tannins: Lower molecular weight compounds like gallic acid
- Goldbeater's skin test: Used for identification of tannins
- Precipitation with alkaloids, gelatin, and heavy metal salts
Sources and Chemistry of Volatile Oils
Volatile oils, or essential oils, are odoriferous principles of plant origin that evaporate when exposed to air. They are primarily composed of terpenes, terpenoids, and aromatic compounds derived from the mevalonic acid or shikimic acid pathways.
- Major constituents: Monoterpenes (C10H16) and Sesquiterpenes (C15H24)
- Distillation methods: Steam distillation, hydrodistillation, and enfleurage
- Peppermint: Source Mentha piperita, constituent Menthol
- Clove: Source Syzygium aromaticum, constituent Eugenol
- Cinnamon: Source Cinnamomum zeylanicum, constituent Cinnamaldehyde
Pharmacological Uses and Evaluation
Understanding the therapeutic applications is essential for both theory papers and practical assessments. Tannins are largely utilized for their astringent and anti-inflammatory properties, while volatile oils serve as carminatives, flavorings, and antimicrobial agents.
- Astringent action: Used in treatment of diarrhea and minor cuts
- Carminative effect: Relieves flatulence (e.g., Fennel, Caraway)
- Antiseptic property: Thymol and Eugenol are potent antibacterial agents
- Counter-irritant effect: Used in topical rubefacient formulations
- Sensory evaluation: Odor, taste, and refractive index measurement
Formula Sheet
Isoprene unit: C5H8
Monoterpene: (C5H8)2
Sesquiterpene: (C5H8)3
Gallic acid structure: 3,4,5-trihydroxybenzoic acid
Exam Tip
Always draw the chemical structure of a basic terpene unit (Isoprene rule, C5H8) when describing volatile oils, as it adds significant value to your answer.
Common Mistakes
- Confusing hydrolyzable tannins with condensed tannins regarding their hydrolysis products.
- Forgetting to mention the specific plant source (biological name) alongside the chemical constituent.
- Misidentifying volatile oils as fixed oils; failing to highlight the steam distillation requirement for the former.
More Revision Notes
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