504 CARBOXYLIC ACIDS 19.4(b) Propanoic acid is similar to acetic acid in its acidity. A hydroxyl group at C-2 is electron withdrawing and stabilizes the carboxylate ion of lactic acid by a combination of inductive nd field effects CHCH—C Lactic acid is more acidic than propanoic acid. The measured ionization constants are CHCHCOH CH..COH OH (pKa3.8) (pK4.9) (c) A carbonyl group is more strongly electron-withdrawing than a carbon-carbon double bond Pyruvic acid is a stronger acid than acrylic acid. CH,CCO,,HH,C=CHCO,H Acrylic acid K5.1×10-4 K5.5×10-5 (pKa3.3) (pK24.3) (d) Viewing the two compounds as substituted derivatives of acetic acid, RCH, CO, H, we judge CHS to be strongly electron-withdrawing and acid-strengthening, whereas an ethyl group has only a small effect CH3SCH, CO,H CHaCH,CH,CO, H Methanesulfonylacetic acid Butanoic acid pk24) (pK24.7 19.5 The compound can only be a carboxylic acid; no other class containing only carbon, hydrogen, and oxygen is more acidic. A reasonable choice is HC=CCO, H; C-2 is sp-hybridized and therefore rather electron-withdrawing and acid-strengthening. This is borne out by its measured ionization constant Ka, which is 1.4X10(pK. 1.8) 19.6 For carbonic acid, the" true Ki" is given by [HJH True K H,CO3J The"observed K"is given by the expression 4.3×10-7 [HIIHCO3 J CO, Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website__ __ __ 504 CARBOXYLIC ACIDS 19.4 (b) Propanoic acid is similar to acetic acid in its acidity. A hydroxyl group at C-2 is electron￾withdrawing and stabilizes the carboxylate ion of lactic acid by a combination of inductive and field effects. Lactic acid is more acidic than propanoic acid. The measured ionization constants are (c) A carbonyl group is more strongly electron-withdrawing than a carbon–carbon double bond. Pyruvic acid is a stronger acid than acrylic acid. (d) Viewing the two compounds as substituted derivatives of acetic acid, RCH2CO2H, we judge to be strongly electron-withdrawing and acid-strengthening, whereas an ethyl group has only a small effect. 19.5 The compound can only be a carboxylic acid; no other class containing only carbon, hydrogen, and oxygen is more acidic. A reasonable choice is HC>CCO2H; C-2 is sp-hybridized and therefore rather electron-withdrawing and acid-strengthening. This is borne out by its measured ionization constant Ka, which is 1.4  102 (pKa 1.8). 19.6 For carbonic acid, the “true K1” is given by True K1  [H [H ][ 2 H C C O O 3] 3 ] The “observed K” is given by the expression 4.3  107 [H][HCO3 ] [CO2] Butanoic acid Ka 1.5  105 (pKa 4.7) CH3CH2CH2CO2H Methanesulfonylacetic acid Ka 4.3  103 (pKa 2.4) CH3SCH2CO2H O O CH3S O O Pyruvic acid Ka 5.1  104 (pKa 3.3) CH3CCO2H O Acrylic acid Ka 5.5  105 (pKa 4.3) H2C CHCO2H Propanoic acid Ka 1.3  105 (pKa 4.9) CH3CH2CO2H Lactic acid Ka 1.4  104 (pKa 3.8) CH3CHCO2H OH Hydroxyl group stabilizes negative charge by attracting electrons. CH3CH C O OH O Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website