Dolbier HW Solutions 601

Dolbier HW Solutions 601 - d 1.3 ppm, the other tert-butyl...

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24.26 Electrophilic attack of bromine on 2,4,6-tribromophenol leads to a cationic intermediate. Loss of the hydroxyl proton from this intermediate generates the observed product. 24.27 A good way to approach this problem is to assume that bromine attacks the aromatic ring of the phenol in the usual way, that is, para to the hydroxyl group. This cation cannot yield the product of electrophilic aromatic substitution by loss of a proton from the ring but can lose a proton from oxygen to give a cyclohexadienone derivative. This cyclohexadienone is the compound C 18 H 29 BrO, and the peaks at 1655 and 1630 cm 2 1 in the infrared are consistent with C ? O and C ? C stretching vibrations. The compound s symmetry is consistent with the observed 1 H NMR spectrum; two equivalent tert -butyl groups at C-2 and C-6 appear as an 18-proton singlet at
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Unformatted text preview: d 1.3 ppm, the other tert-butyl group is a 9-proton singlet at d 1.2 ppm, and the 2 equivalent vinyl protons of the ring appear as a singlet at d 6.9 ppm. 24.28 Because the starting material is an acetal and the reaction conditions lead to hydrolysis with the production of 1,2-ethanediol, a reasonable reaction course is O O O Compound A O O Compound B HOCH 2 CH 2 OH 1,2-Ethanediol H 2 O, H 1 1 4-Bromo-2,4,6-tri-tert-butyl-2,5-cyclohexadienone Br C(CH 3 ) 3 O (CH 3 ) 3 C C(CH 3 ) 3 2 H 1 Br C(CH 3 ) 3 O (CH 3 ) 3 C C(CH 3 ) 3 1 H 2,4,6-Tri-tert-butylphenol OH (CH 3 ) 3 C C(CH 3 ) 3 C(CH 3 ) 3 Br C(CH 3 ) 3 OH (CH 3 ) 3 C C(CH 3 ) 3 1 Br 2 O H Br Br Br Br 1 2,4,4,6-Tetrabromo-cyclohexadienone Br Br O Br Br 1 Br 2 1 Br 2 OH Br Br Br Br 1 2,4,6-Tribromophenol OH Br Br Br PHENOLS 695...
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