684 PHENOLS Picric acid (2, 4, 6-trinitrophenol) is a stronger acid by far than 2, 4, 6-trimethylphenol All three nitro groups participate in resonance stabilization of the picrate anion O O (b) Stabilization of a phenoxide anion is most effective when electron-withdrawing groups are present at the ortho and para positions, because it is these carbons that bear most of the nega rge in phenoxide 2, 6-Dichlorophenol is therefore expected to be(and is)a stronger acid than 3, 5-dichloro- OH 2, 6-Dichlorophenol, more acidic 3. 5-Dichlorophenol, less acidic (K2=1.6×10-7,pK=68) NO2 (K2=4.1×10-9,pK=84) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE WebsitePicric acid (2,4,6-trinitrophenol) is a stronger acid by far than 2,4,6-trimethylphenol. All three nitro groups participate in resonance stabilization of the picrate anion. (b) Stabilization of a phenoxide anion is most effective when electron-withdrawing groups are present at the ortho and para positions, because it is these carbons that bear most of the nega￾tive charge in phenoxide anion. 2,6-Dichlorophenol is therefore expected to be (and is) a stronger acid than 3,5-dichloro￾phenol. (c) The same principle is at work here as in part (b). A nitro group para to the phenol oxygen is directly conjugated to it and stabilizes the anion better than one at the meta position. OH NO2 4-Nitrophenol, stronger acid (Ka  1.0 108 , pKa  7.2) OH NO2 3-Nitrophenol, weaker acid (Ka  4.1 109 , pKa  8.4) OH Cl Cl 3,5-Dichlorophenol, less acidic (Ka  6.5 109 , pKa  8.2) OH Cl Cl 2,6-Dichlorophenol, more acidic (Ka  1.6 107 , pKa  6.8) O O O O N N O O O O O O N O N N O O O O O O N O N N O O O O O N O O N N O O O O O N O O 684 PHENOLS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website