Reactions · Grignard 1900
Grignard addition: phenylmagnesium bromide + benzaldehyde → diphenylmethanol
Overview
PhMgBr + Ph–CHO → [Ph2CH–OMgBr] →(H3O+) Ph2CH–OH
Textbook Grignard addition. Phenylmagnesium bromide (PhMgBr) attacks benzaldehyde's carbonyl carbon; aqueous acidic workup releases diphenylmethanol (benzhydrol). PhMgBr is bought as a 1 M or 3 M solution in anhydrous Et2O or THF — solid Grignards are not weighed. Reaction is run under N2 with rigorous water exclusion.
Conditions: Cool a flame-dried, N2-flushed flask to 0 °C. Charge anhydrous Et2O (~3 mL per mmol of benzaldehyde). Add benzaldehyde (1.0 equiv) by syringe; stir 5 min. Add PhMgBr (1.1 equiv, 1 M or 3 M solution in Et2O) dropwise over 10 min — exotherm; control addition rate to keep T < 5 °C. Warm to RT, stir 1–2 h. Workup — Quench cautiously with sat. NH4Cl (or 10 % HCl); extract with Et2O; wash organic with sat. NaHCO3 + brine; dry MgSO4. Concentrate. Recrystallise from hexane or hexane/EtOAc. White crystals; mp 65–67 °C.
- Expected yield
- 75–92 %
- Expected duration
- 1–2 h at RT after 0 °C addition
Reagents
| Reagent | Role | Equiv | MW | ρ |
|---|---|---|---|---|
|
Benzaldehyde
100-52-7 · C7H6O
Benzaldehyde — limiting reagent. Freshly distilled or wash with 5 % NaHCO3 then water to remove benzoic acid (white film on the bottle indicates oxidation). Anhydrous before use — Grignards quench instantly on water. |
reactant | 1.0 | 106.12 | 1.050 |
|
Phenylmagnesium Bromide
100-58-3 · C6H5BrMg
Phenylmagnesium bromide — purchased as 1 M or 3 M solution in anhydrous Et2O or THF; never weighed as a solid. Slight excess (1.1 equiv) compensates for trace H2O in the substrate or solvent that consumes the Grignard. |
reactant | 1.1 | 181.32 | — |
|
60-29-7
60-29-7 · C4H10O
Diethyl ether — anhydrous, sodium-dried. THF (CAS 109-99-9) is the modern alternative for less reactive Grignards but Et2O remains the textbook choice for PhMgBr addition to aromatic aldehydes. |
solvent | — | 74.12 | 0.710 |
|
91-01-0
91-01-0 · C13H12O
Diphenylmethanol (benzhydrol) — white crystalline solid, mp 65–67 °C; recrystallises from hexane. Mild aromatic odour. Used as a starting material for benzhydrol-derived pharmaceuticals (diphenhydramine, etc.). |
product | 1.0 | 184.24 | — |
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