Zerewitinoff determination

Zerewitinoff determination is a chemical analysis method used to quantify the presence of active hydrogen atoms in a compound. This technique is particularly useful in the study of organic compounds, allowing chemists to determine the number of hydrogen atoms in a molecule that can react with methyl magnesium iodide, a Grignard reagent. The Zerewitinoff determination is named after the Russian chemist Mikhail Zerewitinoff, who developed the method in the early 20th century.

Principle

The principle behind the Zerewitinoff determination involves the reaction of the test compound with methyl magnesium iodide to form methane gas. The reaction can be represented as follows:

\[ \text{R-H} + \text{CH}_3\text{MgI} \rightarrow \text{R-MgI} + \text{CH}_4 \]

Where R-H represents the compound containing the active hydrogen atom. The methane gas produced in the reaction is then measured, usually by gas chromatography, and the amount of methane corresponds directly to the number of active hydrogen atoms in the original compound.

Procedure

The procedure for Zerewitinoff determination involves several steps:

1. Preparation of the Grignard reagent, methyl magnesium iodide.
2. Reaction of the test compound with the Grignard reagent in an appropriate solvent, typically an ether.
3. Quantitative measurement of the methane produced in the reaction. This is often done using gas chromatography.

Care must be taken to ensure that the reaction conditions are strictly controlled, as the presence of moisture or other reactive substances can interfere with the accuracy of the determination.

Applications

Zerewitinoff determination is widely used in organic chemistry for:

• Determining the number of active hydrogen atoms in a compound.
• Characterizing compounds with active hydrogen, such as alcohols, amines, and acids.
• Investigating the structure of organic molecules by identifying functional groups containing active hydrogen.

Limitations

While the Zerewitinoff determination is a valuable tool in organic chemistry, it has some limitations:

• It cannot distinguish between different types of active hydrogen atoms (e.g., those in hydroxyl vs. amino groups).
• The method requires the use of highly reactive Grignard reagents, which can pose safety risks and may react with unintended sites in the molecule.
• The presence of moisture or other impurities can interfere with the reaction, leading to inaccurate results.

See Also

• Grignard reaction
• Gas chromatography
• Organic chemistry
• Functional group