MCAT® ExamChemical and Physical Foundations of Biological SystemsHard
An organic chemist is preparing to synthesize a Grignard reagent. Which of the following conditions is most crucial to ensure a successful reaction and prevent undesirable side reactions?
- AHeating the reaction mixture to reflux in an open flask.
- BExcluding all moisture and protic solvents.
- CUsing an aqueous solvent.
- DUsing a strong oxidizing agent as a catalyst.
Show answer & explanationAnswer & explanation
Correct answer: B. Excluding all moisture and protic solvents.
Grignard reagents are extremely strong nucleophiles and bases. They react violently with even weak protic solvents (like water, alcohols, amines) to form hydrocarbons, thereby destroying the Grignard reagent. Therefore, anhydrous conditions and the exclusion of all protic solvents are paramount for successful Grignard synthesis and subsequent reactions.
Why the other options are wrong
- A. Heating is often used, but an open flask exposes the reaction to atmospheric moisture, which would destroy the Grignard reagent.
- C. Aqueous solvents will immediately quench the Grignard reagent, as water is a protic solvent.
- D. Grignard reagents are powerful nucleophiles and bases; strong oxidizing agents are generally incompatible and would lead to decomposition, not catalysis.
Grignard Reagent Synthesis
Grignard reagents (R-MgX) are organometallic compounds formed from an alkyl or aryl halide (R-X) and magnesium metal, typically in an anhydrous ethereal solvent.
- R-MgX is a very strong nucleophile and base.
- Extremely sensitive to protic solvents (water, alcohols, carboxylic acids).
- Requires anhydrous conditions to prevent quenching.
Memory trick: Grignard's Great, but Get it Dry, or it Will Die!