Phenylacetones from Phenacyl Halides

Reactions of some α-chloroacetophenones (RCOCH₂Cl) with Grignard reagents in ratio of 1:1 and 1:2 were studied and compared (cf. Chem. Abs. 53, 17971d).

Ratio of 1:1: To a Grignard reagent prepd. from 17.3g PhBr, 2.7 g Mg, and 100 mL Et₂O was dropped 15.6g α-chloroacetophenone (I) in 100 mL C₆H₆ at -3°C to 0°C, the mixt. stirred at 0°C for 30 min, concd. till the bp of the mixt. increased to 72°C, and then refluxed 4 hrs., cooled, decompd. with ice and HCl, and the sepd. org. solvent layer distd. to give 40% deoxybenzoin, bp 160-170°C/6 mmHg, mp 55-56°C (MeOH); semicarbazone mp 147-148°C. Similarly were carried out the following reactions; I with MeMgI, 45% phenylacetone (101-104°C/23mmHg; semicarbazone mp 189-190°C). α-chloro-p-fluoroacetophenone (III) with MeMgI, 41% p-fluorophenylacetone (120-130°C/30mmHg; semicarbazone mp 199-200°C).

Ratio of 1:2: I (7.7g) in 100 mL C₆H₆ was treated with Grignard reagent prepd. from 17.3g PhBr, 2.7g Mg, and 100 mL Et₂O to give 44% 1,2,2-triphenylethanol, mp 87-94°C (ligroin), and 1.8g triphenylethylene, bp 200-120°C/10 mmHg, mp 67-68°C (EtOH). Similarly were carried out the reaction between III with p-F-PhMgBr. 72% tris(p-fluorophenyl)ethylene (bp 195-205°C/12 mmHg, mp 71-72°C).

These results showed that ketones were obtained when the molar ratio of the reactants was 1:1, and 1,1,2-trisubstituted ethanols were obtained when that ratio was 1:2. Based on these experimental results and on the conception of migratory aptitude of atomic groups, the mechanism of the reaction was also discussed.