P-Bromoacetophenone

This substance is conveniently obtained by treating p-bromoacetophenone (Section IV, 138) with the calculated quantity of bromine dissolved in glacial acetic acid ... 

Place a solution of 50 g. of p bromoacetophenone (Section IV,138) in 100 ml. of glacial acetic acid in a 500 ml. flask. Add very slowly (about 30 minutes) from a dropping funnel 40 g. (12-5 ml.) of bromine shake the mixture vigorously during the addition and keep the temperature below 20°. p-Bromophenacyl bromide commences to separate as needles after about half of the bromine has been introduced. When the addition is complete, cool the mixture in ice water, filter the crude product at the pump, and wash it with 50 per cent, alcohol imtil colourless (about 100 ml. are required). RecrystaUise from rectified (or methylated) spirit (ca. 400 ml.). The yield of pure p-bromophenac bromide (colourless needles, m.p. 109°) is 50 g. 

Besides acetophenone, this reaction was also applied to p-methoxy-and p-bromoacetophenone, but in all cases yields are low. 

Molar absorptivity. 502 Molecular ion (M+), 410 Molecular mechanics. 130 Molecular model, dopamine, 930 acetaminophen, 29 acetylene, 18 adenine, 67 adrenaline, 323 alanine, 28, 1016 alanylserine, 1028 rr helix, 1039 p-aminobenzoic acid, 25 anti periplanar geometry, 387 a recoline, 79 aspartame, 29 aspirin. 17 ball-and-stick, 61 /3-pleated sheet, 1039 p-bromoacetophenone, 449 bromocyclohexane, 121 butane, 80... 

C Nuclear magnetic resonance spectrum, acetaldehyde, 732 acetophenone, 732 anisole, 672 benzaldehyde, 732 benzoic acid, 771 p-bromoacetophenone, 449 2-butanone, 449, 732 crotonic acid. 771 cyclohexanol, 634 cyclohexanone, 732 ethyl benzoate, 477 methyl acetate, 443 methyl propanoate, 450 methyl propyl ether, 672... 

Method Cl (typical procedure) n-BuLi solution (1.6 M, 7.5 ml, 12.0 mmol) is added under Nj to a stirred solution of thiophene (1.12 g, 13.3 mmol) in dry THF (50 mL) at 0°C. After 50 min at room temperature, finely powdered tellurium (1.50 g, 11.8 mmol) is rapidly added under N2. After 30 min all the Te will have been consumed, and a solution of p-bromophenacyl bromide (1.6 g, 5.75 mmol) in THF (5 mL) is then added drop-wise over 10 min. The solution turns from light yellow to deep red as bis(2-thienyl) ditelluride is formed. After 30 min the solvent is evaporated and the residue partitioned in HjO/ether. The organic phase is dried (CaClj) and evaporated to give a red solid. Chromatography (Si02, CHjClj/hexane, 1 1) gives p-bromoacetophenone (0.82 g (72%) m.p. 49-50°C). 

Similar decomposition is observed in p-bromoacetophenone, o-bromo-, p-bromo, and p,p -dibromobenzophenone, and p-iodobenzophenone44 but not in the fluoro- and chloro-substituted compounds. This order of reactivity follows the bond dissociation energies for aromatic halides which are about 90 kcal/mole for chlorobenzene, 70 kcal/mole for bromobenzene, and 60 kcal/ mole for iodobenzene. The lowest-lying triplet of p-bromoacetophenone is 71.2 kcal45 while that of the substituted benzophenones is slightly lower since benzophenone itself has a lower triplet energy than acetophenone. p,p Dibromobenzophenone was the least reactive of the compounds that photoeliminated halogen atoms. 

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