CHAPTER 19 CARBOXYLIC ACIDS SOLUTIONS TO TEXT PROBLEMS 19.1(b)The four carbon atoms of crotonic acid form a continuous chain. Because there is a double bond between C-2 and C-3, crotonic acid is one of the stereoisomers of 2-butenoic acid. The stereochemistry of the double bond is E. (c)Oxalic acid is a dicarboxylic acid that contains two carbons. It is ethanedioic acid. (d)The name given to C6H5CO2H is benzoic acid. Because it has a methyl group at the para position, the compound shown is p-methylbenzoic acid, or 4-methylbenzoic acid. p-Methylbenzoic acid or 4-methylbenzoic acid (p-toluic acid) H3CCO2H HO2CCO2H Ethanedioic acid (oxalic acid) CC HCO2H H H3C (E)-2-Butenoic acid (crotonic acid) 502 BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website CARBOXYLIC ACIDS503 19.2Ionization of peroxy acids such as peroxyacetic acid yields an anion that cannot be stabilized by res- onance in the same way that acetate can. 19.3Recall from Chapter 4 (text Section 4.6) that an acidbase equilibrium favors formation of the weaker acid and base. Also remember that the weaker acid forms the stronger conjugate base, and vice versa. (b)Theacidbasereactionbetweenaceticacidandtert-butoxideionisrepresentedbytheequation Alcohols are weaker acids than carboxylic acids; the equilibrium lies to the right. (c)Bromide ion is the conjugate base of hydrogen bromide, a strong acid. In this case, the position of equilibrium favors the starting materials, because acetic acid is a weaker acid than hydrogen bromide. (d)Acetylide ion is a rather strong base, and acetylene, with a Kaof 10?26, is a much weaker acid than acetic acid. The position of equilibrium favors the formation of products. (e)Nitrate ion is a very weak base; it is the conjugate base of the strong acid nitric acid. The position of equilibrium lies to the left. (f)Amide ion is a very strong base; it is the conjugate base of ammonia, pKa? 36. The position of equilibrium lies to the right. ?CH3CO2H Acetic acid (stronger acid) H2N? Amide ion (stronger base) ?CH2CO2? Acetate ion (weaker base) NH3 Ammonia (weaker acid) ?CH3CO2H Acetic acid (weaker acid) NO3? Nitrate ion (weaker base) ?CH3CO2 ? Acetate ion (stronger base) HNO3 Nitric acid (stronger acid) ?CH3CO2? Acetate ion (weaker base) Acetylene (weaker acid) HCCH?CH3CO2H Acetic acid (stronger acid) Acetylide ion (stronger base) HC ? C ?CH3CO2H Acetic acid (weaker acid) Br? Bromide ion (weaker base) ?CH3CO2? Acetate ion (stronger base) HBr Hydrogen bromide (stronger acid) Acetic acid (stronger acid) tert-Butoxide (stronger base) tert-Butyl alcohol (weaker acid) CH3CO2H(CH3)3CO? Acetate ion (weaker base) CH3CO2?(CH3)3COH? Delocalization of negative charge into carbonyl group is not possible in peroxyacetate ion. CH3COO O ? _ _ _ BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website _ _ _ 504CARBOXYLIC 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 fi eld 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 carboncarbon 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. 19.5The compound can only be a carboxylic acid; no other class containing only carbon, hydrogen, and oxygen is more acidic. A reasonable choice is HCCCO2H; 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 ? 10?2(pKa1.8). 19.6For 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 ? 10?7? H?HCO3? ? CO2 Butanoic acid Ka 1.5 ? 10?5 (pKa 4.7) CH3CH2CH2CO2H Methanesulfonylacetic acid Ka 4.3 ? 10?3 (pKa 2.4) CH3SCH2CO2H O O CH3S O O Pyruvic acid Ka 5.1 ? 10?4 (pKa 3.3) CH3CCO2H O Acrylic acid Ka 5.5 ? 10?5 (pKa 4.3) H2CCHCO2H Propanoic acid Ka 1.3 ? 10?5 (pKa 4.9) CH3CH2CO2H Lactic acid Ka 1.4 ? 10?4 (pKa 3.8) CH3CHCO2H OH Hydroxyl group stabilizes negative charge by attracting electrons. CH3CHC O OH O? BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website which can be rearranged to H?HCO3? ? (4.3 ? 10?7)CO2 and therefore True K1? (4.3 ? H 1 2C 0? O 7 3 ) CO2 ? ? ? 1.4 ? 10?4 Thus, when corrected for the small degree to which carbon dioxide is hydrated, it can be seen that carbonic acid is actually a stronger acid than acetic acid. Carboxylic acids dissolve in sodium bicar- bonate solution because the equilibrium that leads to carbon dioxide formation is favorable, not be- cause carboxylic acids are stronger acids than carbonic acid. 19.7(b)2-Chloroethanol has been converted to 3-hydrox