Chapter_21 - P1 PCX JWDD052-21 JWDD052-Solomons-v3 Printer...

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P1: PCX Printer: Hamilton JWDD052-21 JWDD052-Solomons-v3 May 31, 2007 14:58 21 PHENOLS AND ARYL HALIDES: NUCLEOPHILIC AROMATIC SUBSTITUTION SOLUTIONS TO PROBLEMS 21.1 The electron-releasing group (i.e., CH 3 ) changes the charge distribution in the molecule so as to make the hydroxyl oxygen less positive, causing the proton to be held more strongly; it also destabilizes the phenoxide anion by intensifying its negative charge. These effects make the substituted phenol less acidic than phenol itself. OH + + CH 3 H 2 O H 3 O + Electron-releasing CH 3 destabilizes the anion more than the acid. p K a is larger than for phenol. O CH 3 δ + 21.2 An electron-withdrawing group such as chlorine changes the charge distribution in the molecule so as to make the hydroxyl oxygen more positive, causing the proton to be held less strongly; it also can stabilize the phenoxide ion by dispersing its negative charge. These effects make the substituted phenol more acidic than phenol itself. OH ++ Cl H 2 O H 3 O + Electron-withdrawing chlorine stabilizes the anion by dispersing the negative charge. p K a is smaller than for phenol. O Cl δ Nitro groups are very powerful electron-withdrawing groups by their inductive and reso- nance effects. Resonance structures ( B–D ) below place a positive charge on the hydroxyl 473
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P1: PCX Printer: Hamilton JWDD052-21 JWDD052-Solomons-v3 May 31, 2007 14:58 474 PHENOLS AND ARYL HALIDES: NUCLEOPHILIC AROMATIC SUBSTITUTION oxygen. This effect makes the hydroxyl oxygen dramatically more positive, causing the proton to be held much less strongly. These contributions explain why 2,4,6-trinitrophenol (picric acid) is so exceptionally acidic. O O N + N N + H O O O O O O O O O O O A O N N N H O O O D + + + + O N N N H O O O C + + + + B O N N N + H O O O + + + O O O + 21.3 Dissolve the mixture in a solvent such as CH 2 Cl 2 (one that is immiscible with water). Using a separatory funnel, extract this solution with an aqueous solution of sodium bicarbonate. This extraction will remove the benzoic acid from the CH 2 Cl 2 solution and transfer it (as sodium benzoate) to the aqueous bicarbonate solution. AcidiFcation of this aqueous extract will cause benzoic acid to precipitate; it can then be separated by Fltration and puriFed by recrystallization. The CH 2 Cl 2 solution can now be extracted with an aqueous solution of sodium hydroxide. This will remove the 4-methylphenol (as its sodium salt). AcidiFcation of the aqueous extract will cause the formation of 4-methylphenol as a water-insoluble layer. The 4-methylphenol can then be extracted into ether, the ether removed, and the 4-methylphenol puriFed by distillation. The CH 2 Cl 2 solution will now contain only toluene (and CH 2 Cl 2 ). These can be separated easily by fractional distillation.
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Chapter_21 - P1 PCX JWDD052-21 JWDD052-Solomons-v3 Printer...

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