Reaction of Alkaloids with Reagents

Reaction of Alkaloids with Reagents

# I. General Reagents

1. Concept:

General reagents for alkaloids are complex organic compounds with large molecular weights and are often acidic. They are divided into two groups:

• Reagents that form amorphous precipitates: form fluffy precipitates, with no definite shape.

• Reagents that form crystalline precipitates: form crystalline precipitates, with definite shape and melting point.

2. Commonly used general reagents:

| Reagent | Formula | Precipitate properties | Notes |

|—|—|—|—|

| Valse-Mayer (Meyer) | K2[HgI4] | Amorphous precipitate, yellowish-white | Negative with very weak alkaline alkaloids and alkaloids without heterocyclic nitrogen. Soluble in acetic acid, ethanol, methanol, and excess reagent. |

| Wagner | KI3 (KI+I2) | Amorphous precipitate, brownish-red | |

| Dragendorff | K[BiI4] | Amorphous precipitate, orange-red to red | Rapid detection of alkaloids. False positive reaction with active methylene groups. Used as a detection reagent in chromatography. |

| Marme | K[CdI3] | Amorphous precipitate, pale white-yellow, may crystallize | Soluble in excess reagent or methanol, ethanol. |

| Bertrand | Silicotungstic acid H3[Si(W3)10)4] | Amorphous precipitate, white or off-white | Sensitive, complete precipitation. Stable composition, can be determined by indirect titration. |

| Scheibler | H3[P(W3O10)4] | Amorphous precipitate, white | |

| Sonnenchein | Phosphomolybdic acid H3[P(Mo3O10)4] | Amorphous precipitate, white | |

| Reineck | | Amorphous precipitate, pink, crystallizes after a while | Has characteristic shape and melting point, used for identification. Soluble in acetone to give pink color, used for colorimetric determination. |

| Cobalthiocyanate | Co(SCN)2 | Amorphous precipitate, blue | Soluble in CHCl3, used for colorimetric determination. |

| Tannic acid (tannin) | | White fluffy precipitate | Soluble in alcohol, acetic acid or NH4OH. |

3. Sensitivity of the reagent:

• General reagents are quite sensitive for qualitative reactions.

• The sensitivity of the reagent to different alkaloids varies.

• The sensitivity of two different reagents to the same alkaloid can also be different.

4. General characteristics of precipitation reagents:

• Reagents are acidic, often complex compounds with very large molecular weights.

• Reagents are unstable in alkaline environments.

• Reagents only determine the presence of alkaloids in the drug.

5. Conditions for carrying out the reaction:

• Aqueous environment.

• Slightly acidic to neutral pH.

• Small sample size due to the sensitivity of the reaction.

6. Reagents that form crystalline precipitates:

• Reagents that form crystalline precipitates give easily crystallizable precipitates.

• The product is usually colored.

• Crystalline form, characteristic melting point.

• The reaction usually goes through a stage of forming amorphous precipitates and then crystallizing.

• Can be recrystallized, the solvent is usually alcohol.

Example:

• Picric acid (Hager): Forms yellow to orange-red crystalline precipitates with defined melting points.

7. Commonly encountered amorphous precipitation reagents:

• Valse-Mayer

• Dragendorff

• Tannic acid

• Bouchardat

• Bertrand

8. Reagents that form crystalline precipitates:

• Picric acid (Hager)

# II. Specific Reagents

1. Concept:

• Specific reagents react with a specific group or a specific alkaloid, creating a characteristic color.

• Do not color with all alkaloids, except Wasicky reagent to identify alkaloids containing an indole ring.

2. General characteristics:

• Reagents are usually strong oxidizers in concentrated acid.

• The color usually changes quickly, it is necessary to observe immediately.

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

3. Conditions for carrying out the reaction:

• The extracted sample must be large enough, except for caffeine.

• Usually carried out in a dry environment.

• The alkaloid must be pure as the reaction color depends on the purity and reaction conditions such as temperature and pH.

# III. Conclusion

The reaction of alkaloids with reagents is an important method for identifying and quantifying alkaloids in drugs. Choosing the right reagent depends on the analytical purpose and the type of alkaloid to be identified.