Quick Summary
The Zerewitinoff reaction (or Zerevitinov determination) is an analytical chemical method used to determine the quantity of active hydrogen atoms in an organic compound. By reacting the sample with a strong Grignard Reagent (typically methylmagnesium iodide), it quantitatively liberates methane gas, which is then measured volumetrically.
What is the Zerewitinoff Reaction?
Discovered by Russian chemist Fedor Zerevitinov, this reaction is a cornerstone of quantitative organic analysis. It specifically targets hydrogens attached to electronegative heteroatoms (such as oxygen, nitrogen, or sulfur), which are sufficiently acidic to react with a Grignard reagent.
General Chemical Equation
When an organic compound containing an active hydrogen (represented as R-OH, R-NH2, H-C≡C-R, etc.) reacts with methylmagnesium iodide (CH3MgI), the reaction proceeds as follows:
Every mole of active hydrogen yields exactly one mole of methane gas (CH4). By measuring the volume of methane gas evolved under standard temperature and pressure (STP), the number of active hydrogen atoms per molecule can be precisely calculated.
Reaction Mechanism & Steps
- Nucleophilic Attack: The highly nucleophilic methyl group (CH3-) of the Grignard reagent attacks the acidic/active hydrogen atom of the substrate.
- Deprotonation: The active hydrogen is abstracted, breaking the polar bond (e.g., O-H, N-H).
- Gas Evolution: Methane gas escapes the solution, leaving behind a magnesium salt adduct.
Functional Groups with Active Hydrogen
The Zerewitinoff method successfully detects and quantifies hydrogen atoms in the following functional groups:
| Functional Group | Structure | Methane Moles Evolved (per group) |
|---|---|---|
| Alcohols | R-OH | 1 Mole |
| Primary Amines | R-NH2 | 2 Moles (Requires higher temperatures for the 2nd H) |
| Carboxylic Acids | R-COOH | 1 Mole |
| Terminal Alkynes | R-C≡C-H | 1 Mole |
Applications
- Identification of alcohols, amines, and acids.
- Quantitative determination of active hydrogens.
- Useful in structural elucidation of organic compounds.
Frequently Asked Questions (FAQ)
Why is methylmagnesium iodide used in the Zerewitinoff reaction?
Methylmagnesium iodide is preferred because it yields methane (CH4), which is an insoluble, easily isolated gas that allows for highly accurate volumetric measurements.
What are the limitations of the Zerewitinoff determination?
Compounds containing functional groups that can undergo addition reactions with Grignard reagents (like ketones, aldehydes, or esters) can interfere with the results, leading to inaccurate measurements.
Exam Relevance
This reaction is frequently asked in NEET, IIT JEE, and CBSE Class 12 Chemistry exams. Students should focus on mechanism and applications.