PHENOLS 685 (d) A cyano group is strongly electron-withdrawing, and so 4-cyanophenol is a stronger acid than phenol CN Phenol. less acidic (K2=1.1×10-8,pK2=80)(K2=1×10-10,pK2=10) There is resonance stabilization of the 4-cyanophenoxide anion. (e) The 5-nitro group in 2, 5-dinitrophenol is meta to the hydroxyl group and so does not stabilize the resulting anion as much as does an ortho or a para nitro group ON NO ON 2. 6-Dinitrophenol, more acidi 0-4,pK2=3.7) 24.15 (a) The rate-determining step of ester hydrolysis in basic solution is formation of the tetrahedral ntermediat ArOccH +Ho ArOCCH ecause this intermediate is negatively charged, there will be a small effect favoring its for mation when the aryl group bears an electron-withdrawing substituent. Furthermore, this in termediate can either return to starting materials or proceed to products HO ArO→CCH Aro ChcoH Aro CH, co The proportion of the tetrahedral intermediate that goes on to products increases as the leav g group Aro becomes less basic. This is strongly affected by substituents; electron withdrawing groups stabilize ArO. The prediction is that m-nitrophenyl acetate undergoes hydrolysis in basic solution faster than phenol. Indeed, this is observed to be the case; m-nitrophenyl acetate reacts some ten times faster than does phenyl acetate at 25C. OCCH HO 0+ CH- COH han phenoxide because it is less basic) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website(d) A cyano group is strongly electron-withdrawing, and so 4-cyanophenol is a stronger acid than phenol. There is resonance stabilization of the 4-cyanophenoxide anion. (e) The 5-nitro group in 2,5-dinitrophenol is meta to the hydroxyl group and so does not stabilize the resulting anion as much as does an ortho or a para nitro group. 24.15 (a) The rate-determining step of ester hydrolysis in basic solution is formation of the tetrahedral intermediate. Because this intermediate is negatively charged, there will be a small effect favoring its for￾mation when the aryl group bears an electron-withdrawing substituent. Furthermore, this in￾termediate can either return to starting materials or proceed to products. The proportion of the tetrahedral intermediate that goes on to products increases as the leav￾ing group ArO becomes less basic. This is strongly affected by substituents; electron￾withdrawing groups stabilize ArO. The prediction is that m-nitrophenyl acetate undergoes hydrolysis in basic solution faster than phenol. Indeed, this is observed to be the case; m-nitrophenyl acetate reacts some ten times faster than does phenyl acetate at 25°C. HO OCCH3  O2N O m-Nitrophenyl acetate (more reactive)  CH3COH O O O2N m-Nitrophenoxide anion (a better leaving group than phenoxide because it is less basic) ArO  CH3COH O  CH3CO ArO O ArO CCH3 O OH HO ArOCCH3 O OH ArOCCH3  O HO slow OH O2N NO2 2,6-Dinitrophenol, more acidic (Ka  2.0 104 , pKa  3.7) OH NO2 O2N 2,5-Dinitrophenol, less acidic (Ka  6.0 106 , pKa  5.2) O CN O C N OH CN 4-Cyanophenol, more acidic (Ka  1.1 108 , pKa  8.0) OH Phenol, less acidic (Ka  1 1010, pKa  10) PHENOLS 685 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website