Alkaloid Reactions with Reagents

I. General Introduction:

Alkaloids are nitrogen-containing organic compounds that are often found in plants and sometimes in animals. They have a wide range of biological activities and are known for their potent effects. To determine the presence of alkaloids in a sample, specific reagents are often used to induce characteristic reactions.

II. Reactions with General Reagents:

• General alkaloid reagents: These are large, complex acids that form precipitates with alkaloids.

• Classification:

• Amorphous precipitate reagents: These produce precipitates that lack a defined crystalline form.

• Examples: Mayer’s reagent (Valse-Mayer), Wagner’s reagent, Dragendorff’s reagent, Marme’s reagent, Bertrand’s reagent, Scheibler’s reagent, Sonnenchein’s reagent, Reineck’s reagent, Cobalthiocyanate reagent, Tannic acid (Tannin).

• Crystalline precipitate reagents: These produce precipitates with a definite crystalline form and melting point.

• Examples: Picric acid (Hager’s reagent).

III. Specific Reagents:

1. Mayer’s reagent (Valse-Mayer):

• Formula: K2[HgI4]

• Reaction: Forms a yellowish-white amorphous precipitate.

• Note:

• Negative with very weak alkaline alkaloids and alkaloids without a heterocyclic nitrogen.

• The precipitate may dissolve in acetic acid, ethanol, methanol, and excess reagent.

2. Wagner’s reagent:

• Formula: KI3 (KI+I2)

• Reaction: Forms a brownish-red amorphous precipitate.

3. Dragendorff’s reagent:

• Formula: K[BiI4]

• Reaction: Forms an orange-red to red amorphous precipitate.

• Note:

• Rapid detection of alkaloids.

• False positive reactions with compounds containing active methylene groups.

• Used as a detection reagent in chromatography.

4. Marme’s reagent:

• Formula: K[CdI3]

• Reaction: Forms a pale yellow-white amorphous precipitate that can crystallize.

• Note:

• The precipitate may dissolve in excess reagent or methanol, ethanol.

5. Bertrand’s reagent:

• Formula: Silicotungstic acid H3[Si(W3O10)4]

• Reaction: Forms a white or off-white amorphous precipitate.

• Note:

• Sensitive, complete precipitation.

• Stable composition, suitable for indirect quantitative analysis.

6. Scheibler’s reagent:

• Formula: Phosphotungstic acid H3[P(W3O10)4]

• Reaction: Forms a white amorphous precipitate.

7. Sonnenchein’s reagent:

• Formula: Phosphomolybdic acid H3[P(Mo3O10)4]

• Reaction: Forms a white amorphous precipitate.

8. Reineck’s reagent:

• Reaction: Forms a pink amorphous precipitate that can crystallize over time, providing a characteristic shape and melting point for identification.

• Note:

• The precipitate may dissolve in acetone, producing a pink color, allowing for colorimetric quantification.

9. Cobalthiocyanate reagent:

• Formula: Co(SCN)2

• Reaction: Forms a blue amorphous precipitate.

• Note:

• The precipitate may dissolve in chloroform, allowing for colorimetric quantification.

10. Tannic acid (Tannin) solution:

• Reaction: Forms a white flocculent precipitate.

• Note:

• The precipitate may dissolve in alcohol, acetic acid, or ammonia.

IV. General Characteristics of Precipitate Reagents:

• Reagents are acids and large molecular weight complexes.

• Reagents are unstable in alkaline environments.

• They only indicate the presence of alkaloids in the material, not the specific type of alkaloid.

V. Conditions for Performing Reactions with General Reagents:

• Aqueous environment.

• Slightly acidic to neutral pH.

• Small sample amounts are typically used (due to the sensitivity of the reactions).

VI. Crystalline Precipitate Reagents:

• Picric acid (Hager’s reagent):

• Reaction: Forms yellow-orange crystals.

• Note: Forms crystals with a defined melting point.

VII. Commonly Used Amorphous Precipitate Reagents:

• Mayer’s (Valse-Mayer)

• Dragendorff’s

• Tannic acid

• Bouchardat

• Bertrand

VIII. Crystalline Precipitate Reagents:

• Picric acid (Hager’s)

IX. Reactions with Specific Reagents:

• Reaction with a group or a specific alkaloid produces a characteristic color.

• These reagents do not react with all alkaloids, except for Wasicky’s reagent, which identifies alkaloids with an indole ring.

X. General Characteristics of Specific Reagents:

• Strong oxidizing agents (in strong acids).

• The color often changes quickly, requiring immediate observation.

• Specific colors are used to qualitatively identify a particular alkaloid or group of alkaloids.

XI. Conditions for Performing Reactions with Specific Reagents:

• A sufficient amount of extracted sample is required, except for caffeine.

• Reactions are typically performed in a dry environment.

• Alkaloids must be pure as the reaction color depends on purity and reaction conditions like temperature and pH.

Note:

• The sensitivity of general reagents is quite high for qualitative analysis.

• The sensitivity of a reagent can vary between different alkaloids.

• The sensitivity of different reagents can vary for the same alkaloid.

XII. Conclusion:

Reactions with reagents are crucial methods for identifying the presence of alkaloids in a sample. Selecting the appropriate reagent depends on the type of alkaloid being investigated and the purpose of the test.