Organic chemistry study guide · Reaction Mechanism MCAT Tutor
Grignard reaction mechanism: formation, carbonyl addition, and what destroys your reagent
Grignard reagents (R–MgX) are powerful nucleophiles and bases made from alkyl halides and magnesium in dry ether. They add to polar π bonds — especially carbonyls — to build C–C bonds. Any O–H, N–H, or acidic C–H destroys the reagent before it reaches the carbonyl, so dryness and protection matter on paper and in the lab.
Written for undergraduate orgo, pre-med, and MCAT organic chemistry review — United States exam-style framing.
Short lesson
Formation and basic character
R–X + Mg → R–MgX in anhydrous ether or THF. The carbon bears partial negative character; the reagent is strongly basic and nucleophilic.
Protic solvents, water, alcohols, carboxylic acids, and terminal alkynes quench Grignards — exams love “which substrate can be used in one pot with RMgBr?”
Addition to carbonyls
Aldehydes and ketones: nucleophilic addition gives an alkoxide; workup (H₃O⁺) protonates to a secondary or tertiary alcohol depending on the carbonyl.
Mechanism arrows: R⁻ equivalent attacks C=O, π breaks to O⁻; after workup, you have a new C–C bond.
Formaldehyde gives primary alcohols; other aldehydes give secondary; ketones give tertiary.
Esters, nitriles, and epoxides
Esters react with two equivalents of Grignard to give tertiary alcohols (after addition–elimination twice) — know the product class, not every intermediate on timed exams.
Epoxides: attack at less hindered carbon, ring opens; after workup, homoallylic alcohols from vinyl or aryl Grignards are common drill.
Nitriles give ketones after addition and hydrolysis workup in advanced courses.
Common traps on exams
Using Grignard in the presence of alcohol
If the molecule has an OH, the Grignard is consumed. Either protect the alcohol or use a different organometallic strategy.
Wrong alcohol class from carbonyl type
Ketone + Grignard → tertiary alcohol. Students sometimes draw secondary when the new C–R bond replaces the C=O on a ketone.
Forgetting two equivalents with esters
One equivalent is not enough for full conversion to the tertiary alcohol from an ester — product prediction uses two R groups from the reagent.
Epoxide regiochemistry
Grignard opens at the less substituted epoxide carbon; don't swap with acid-catalyzed opening patterns.
Practice-style prompts
Try these on paper, then check product and mechanism in OrgoMech.
- Design a Grignard synthesis of 2-phenylethanol from benzene (any inorganic reagents allowed).
- What is the product of CH₃MgBr + cyclohexanone followed by H₃O⁺?
- Why does CH₃CH₂OH solvent ruin a Grignard reaction? Write the quenching step in words.
- Ethylene oxide + phenylmagnesium bromide, then H₃O⁺ — draw the alcohol product.
- Methyl propanoate + excess CH₃MgBr, then acid — classify the alcohol (primary/secondary/tertiary).
Student questions
Common questions undergraduates ask about this topic before exams.
- What is a Grignard reagent?
- An organomagnesium halide (R–MgX) formed from alkyl or aryl halides and magnesium in dry ether or THF. It acts as a strong nucleophile and base.
- Why must Grignard reactions be dry?
- Water, alcohols, and other acidic protons protonate the carbanion-like carbon and destroy the reagent before it adds to the carbonyl.
- What alcohol do you get from formaldehyde + Grignard?
- Primary alcohol after acidic workup. Other aldehydes give secondary alcohols; ketones give tertiary alcohols.
- How many equivalents of Grignard react with an ester?
- Two equivalents are needed to reach the tertiary alcohol after two additions; the intermediate ketone is often not isolated on exams.
- Which carbon of an epoxide does Grignard attack?
- The less substituted carbon — opposite regiochemistry from acid-catalyzed epoxide opening in many cases.
- Can Grignards react with nitriles?
- In advanced courses, yes — addition followed by hydrolysis can give ketones. Know your course’s coverage.
- What is a typical exam trap with Grignard and alcohols?
- A molecule with both a carbonyl and an OH — the Grignard quenches at the OH unless the alcohol is protected or the synthesis order avoids the conflict.
Further reading (external)
Open textbooks and reference sites for the same topics covered above. We link out to established science education publishers — not affiliate pages.
- Grignard reagents (LibreTexts) — LibreTexts
- OpenStax — Grignard reagents — OpenStax
- PubChem — NIH NLM
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Drill this topic in OrgoMech
Predict the major product for your structure, then order the curved-arrow mechanism — the same skills tested on homework, finals, and the MCAT orgo section. Predict the major product, then order the curved arrows. Organic chemistry practice for reactions, mechanisms, and the days left before your final.