PHENOLS 677 4.2 Intramolecular hydrogen bonding between the hydroxyl group and the ester carbonyl can occur when these groups are ortho to each other. Methyl salicylate Intramolecular hydrogen bonds form at the expense of intermolecular ones, and intramolecularly hydrogen-bonded phenols have lower boiling points than isomers in which only intermolecular hydrogen-bonding is possible 24.3(b) A cyano group withdraws electrons from the ring by resonance. A p-cyano substituent is con- jugated directly with the negatively charged oxygen and stabilizes the anion more than does an m-cyano substituent p-Cyanophenol is slightly more acidic than m-cyanophenol, the Ka values being 1.0 X 10 and 2. X 10, respectively (c) The electron-withdrawing inductive effect of the fluorine substituent will be more pronounced at the ortho position than at the para. o-Fluorophenol(a=1.9 x 10)is a stronger acid han p-fluorophenol(K=1.3 10) 24.4 The text points out that the reaction proceeds by the addition-elimination mechanism of nucleophilic aromatic substitution Under the strongly basic conditions of the reaction, p-toluenesulfonic acid is first converted to its H,C HOH alfonic acid Hydroxide Toluenesulfonate ion Water Nucleophilic addition of hydroxide ion gives a cyclohexadienyl anion intermediate. OH C SO,+ OH H,C p-Toluenesulfonate ion Hydroxide Cyclohexadienyl anion Loss of sulfite ion(SO 2)gives p-cresol gC OH SO Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website24.2 Intramolecular hydrogen bonding between the hydroxyl group and the ester carbonyl can occur when these groups are ortho to each other. Intramolecular hydrogen bonds form at the expense of intermolecular ones, and intramolecularly hydrogen-bonded phenols have lower boiling points than isomers in which only intermolecular hydrogen-bonding is possible. 24.3 (b) A cyano group withdraws electrons from the ring by resonance. A p-cyano substituent is con￾jugated directly with the negatively charged oxygen and stabilizes the anion more than does an m-cyano substituent. p-Cyanophenol is slightly more acidic than m-cyanophenol, the Ka values being 1.0 108 and 2.8 109 , respectively. (c) The electron-withdrawing inductive effect of the fluorine substituent will be more pronounced at the ortho position than at the para. o-Fluorophenol (Ka  1.9 109 ) is a stronger acid than p-fluorophenol (Ka  1.3 1010). 24.4 The text points out that the reaction proceeds by the addition–elimination mechanism of nucleophilic aromatic substitution. Under the strongly basic conditions of the reaction, p-toluenesulfonic acid is first converted to its anion. Nucleophilic addition of hydroxide ion gives a cyclohexadienyl anion intermediate. Loss of sulfite ion (SO3 2) gives p-cresol.  p-Cresol H3C OH SO3 2 Cyclohexadienyl anion H3C SO3 OH  OH p-Toluenesulfonate ion Hydroxide H3C SO3 Cyclohexadienyl anion H3C SO3 OH H3C SO   O O H p-Toluenesulfonic acid H3C SO O O p-Toluenesulfonate ion OH Hydroxide ion HOH Water O CN O C N OCH3 O O C H Methyl salicylate PHENOLS 677 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website