CHAPTER 19 CARB○ XYLIC AC|DS 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. H3C、 COH (E)2-Butenoic acid (crotonic acid) (c) Oxalic acid is a dicarboxylic acid that contains two carbons. It is ethanedioic acid HO, CCO,H Ethanedioic acid (oxalic acid (d) The name given to C HSCO, H is benzoic acid. Because it has a methyl group at the para position, the compound shown is p-methylbenzoic acid, or 4-methylbenzoic acid. H,C -COH p-Methylbenzoic acid or 502 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
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) H3C CO2H HO2CCO2H Ethanedioic acid (oxalic acid) C C H CO2H H H 3C (E)-2-Butenoic acid (crotonic acid) 502 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOXYLIC ACIDS 503 19.2 Ionization of peroxy acids such as peroxyacetic acid yields an anion that cannot be stabilized by res not possible is sanborn group is 19.3 Recall from Chapter 4(text Section 4.6) that an acid-base equilibrium favors formation of the weaker acid and base. Also remember that the weaker acid forms the stronger conjugate base, and vIce versa (b) The acid-base reaction between acetic acid and tert-butoxide ion is represented by the equatio CHiCO,H +(CH3)3CO CH3 CO, +(CH3)COH Acetic acid tert-Butoxide Butyl alcohol (stronger acid) (stronger base) (weaker base) Icohols 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 CH CO,H+ CH3CO2 HBI Acetic acid Bromide ion Acetate ion Hydrogen (weaker base) 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 k of 10, is a much weaker acid than acetic acid. The position of equilibrium favors the formation of products CH2COH+HC≡C CH2CO3+HC≡CH (stronger acid) (stronger base) eaker base) (weaker acid (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 CH3CO,H HNO Acetic acid Nitrate ion Acetate ion Nitric acid (weaker acid weaker base) (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. CH,CO,H H,N CHCO Amide ion Ammonia (stronger base) (weaker base (weaker acid Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
CARBOXYLIC ACIDS 503 19.2 Ionization of peroxy acids such as peroxyacetic acid yields an anion that cannot be stabilized by resonance in the same way that acetate can. 19.3 Recall from Chapter 4 (text Section 4.6) that an acid–base equilibrium favors formation of the weaker acid and base. Also remember that the weaker acid forms the stronger conjugate base, and vice versa. (b) The acid–base reaction between acetic acid and tert-butoxide ion is represented by the equation 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 Ka of 1026, 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) CH3CO2H HC CH 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. CH3CO O O __ __ __ 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 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 electronwithdrawing 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
CARBOXYLIC ACIDS 505 which can be rearranged to H']HCO3]=(43×10-CO2l and therefore Tuek,(4.3×10-)CO2 H,CO3I 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. Carboxy lic acids dissolve in sodium bicar bonate solution because the equilibrium that leads to carbon dioxide formation is favorable, not be ause carboxylic acids are stronger acids than carbonic acid 19.7 (b) 2-Chloroethanol has been converted to 3-hydroxypropanoic acid by way of the corresponding HOCHCHCI HOCH.. CN HOCH..H The presence of the hydroxyl group in 2-chloroethanol precludes the preparation of a Grignard agent from this material, and so any attempt at the preparation of 3-hydroxypropanoic acid via the grignard reagent of 2-chloroethanol is certain to fail (c) Grignard reagents can be prepared from tertiary halides and react in the expected manner with carbon dioxide. The procedure shown is entirely satisfactory. 1. CO (CH3)cCI (CH,) CMgCl 2.H2O (CH3)CCO,H r-Butyl chloride terT-Butylmagnesium 2, 2-Dimethylpropanoic Preparation by way of the nitrile will not be feasible. Rather than react with sodium cyanide by substitution, tert-butyl chloride will undergo elimination exclusively. The SN2 reaction limited to pri 19.8 Incorporation of O into benzoic acid proceeds by a mechanism analogous to that of esterification The nucleophile that adds to the protonated form of benzoic acid is O-enriched water(the 0 atom is represented by the shaded letter e in the following equations). OH oH CHOH H CsH-COH C6HSCOH C6HSCOH Benzoic acid Tetrahedral intermediate The three hydroxyl groups of the tetrahedral intermediate are equivalent except that one of them is labeled withO. Any one of these three hydroxyl groups may be lost in the dehydration step; when the hydroxyl group that is lost is unlabeled, anO label is retained in the benzoic acid. CHOH + H C6H5CY hO C HsC Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
which can be rearranged to [H][HCO3 ] (4.3 107 )[CO2] and therefore True K1 (4.3 [H 1 2C 0 O 7 3 ) ] [CO2 ] 1.4 104 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 bicarbonate solution because the equilibrium that leads to carbon dioxide formation is favorable, not because carboxylic acids are stronger acids than carbonic acid. 19.7 (b) 2-Chloroethanol has been converted to 3-hydroxypropanoic acid by way of the corresponding nitrile. The presence of the hydroxyl group in 2-chloroethanol precludes the preparation of a Grignard reagent from this material, and so any attempt at the preparation of 3-hydroxypropanoic acid via the Grignard reagent of 2-chloroethanol is certain to fail. (c) Grignard reagents can be prepared from tertiary halides and react in the expected manner with carbon dioxide. The procedure shown is entirely satisfactory. Preparation by way of the nitrile will not be feasible. Rather than react with sodium cyanide by substitution, tert-butyl chloride will undergo elimination exclusively. The SN2 reaction with cyanide ion is limited to primary and secondary alkyl halides. 19.8 Incorporation of 18O into benzoic acid proceeds by a mechanism analogous to that of esterification. The nucleophile that adds to the protonated form of benzoic acid is 18O-enriched water (the 18O atom is represented by the shaded letter in the following equations). The three hydroxyl groups of the tetrahedral intermediate are equivalent except that one of them is labeled with 18O. Any one of these three hydroxyl groups may be lost in the dehydration step; when the hydroxyl group that is lost is unlabeled, an 18O label is retained in the benzoic acid. H Tetrahedral intermediate C6H5COH OH OH H2O C6H5C OH OH 18O-enriched benzoic acid C6H5C O OH C6H5C O OH H H Benzoic acid C6H5COH O C6H5COH OH C6H5COH OH H H O Tetrahedral intermediate C6H5COH OH OH H2O O tert-Butylmagnesium chloride (CH3)3CMgCl 2,2-Dimethylpropanoic acid (61–70%) (CH3)3CCO2H tert-Butyl chloride (CH3)3CCl Mg diethyl ether 1. CO2 2. H3O 2-Chloroethanol HOCH2CH2Cl 2-Cyanoethanol HOCH2CH2CN NaCN H2O 3-Hydroxypropanoic acid HOCH2CH2CO2H H3O heat (4.3 107 )(99.7) 0.3 CARBOXYLIC ACIDS 505 __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
506 CARBOXYLIC ACIDS 19.9(b) The 16-membered ring of 15-pentadecanolide is formed from 15-hydroxypentadecanoic Disconnect this bond COH 5-Pentadecanolide 15-Hydroxypentadecanoic (c) Vernolepin has two lactone rings, which can be related to two hydroxy acid combinations CHECH OH HOCH CH=CH H H, H,C OH COH Be sure to keep the relative stereochemistry unchanged. Remember, the carbon-oxygen bond of an alcohol remains intact when the alcohol reacts with a carboxylic acid to give an 19.10 Alkyl chlorides and bromides undergo nucleophilic substitution when treated with sodium iodide in acetone(Section8. 1). A reasonable approach is to brominate octadecanoic acid at its a-carbon atom, then replace the bromine substituent with iodine by nucleophilic substitution CHa(CH,)15CH,CO,H CH3(CH)15 CHCO acetone CH3(CH) CHCO2H 2- Bromooctadecanoic acid 2-lodooctadecanoic acid 19.11 (b) The starting material is a derivative of malonic acid. It undergoes efficient thermal decar- boxylation in the manner shown HO H CH,(CH2)CHO CH3(CH,)CH c O, CH3(CH2)CH, COH Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
19.9 (b) The 16-membered ring of 15-pentadecanolide is formed from 15-hydroxypentadecanoic acid. (c) Vernolepin has two lactone rings, which can be related to two hydroxy acid combinations. Be sure to keep the relative stereochemistry unchanged. Remember, the carbon–oxygen bond of an alcohol remains intact when the alcohol reacts with a carboxylic acid to give an ester. 19.10 Alkyl chlorides and bromides undergo nucleophilic substitution when treated with sodium iodide in acetone (Section 8.1). A reasonable approach is to brominate octadecanoic acid at its -carbon atom, then replace the bromine substituent with iodine by nucleophilic substitution. 19.11 (b) The starting material is a derivative of malonic acid. It undergoes efficient thermal decarboxylation in the manner shown. heat CH3(CH2)6CH C O H O C O HO 2-Heptylmalonic acid CO2 Carbon dioxide CH3(CH2)6CH C OH OH CH3(CH2)6CH2COH O Nonanoic acid Br2, PCl3 NaI acetone Octadecanoic acid CH3(CH2)15CH2CO2H 2-Bromooctadecanoic acid CH3(CH2)15CHCO2H Br 2-Iodooctadecanoic acid CH3(CH2)15CHCO2H I CH2 O O O OH O CH2 H2C CH CH2 OH CH2 HOCH2 HO2C H2C CO2H CH OH Disconnect this bond. O OH COH O O 15-Pentadecanolide 15-Hydroxypentadecanoic acid 506 CARBOXYLIC ACIDS __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOXYLIC ACIDS 507 (c) The phenyl and methyl substituents attached to C-2 of malonic acid play no role in the decar ①m OH dioxide 2-Phenylpropanoic acid 19.12 (b) The thermal decarboxylation of B-keto acids resembles that of substituted malonic acids. The structure of 2, 2-dimethylacetoacetic acid and the equation representing its decarboxylation were given in the text. The overall process involves the bonding H O CHaCCH(CH,) H,C H3 2. 2-Dimethylacetoacetic Enol form of 3-methy 3-Methyl-2-butanone 19.13 (a) Lactic acid(2-hydroxypropanoic acid) is a three-carbon carboxylic acid that bears a hydroxyl C-2. CH CHCOH 2-Hydroxypropanoic acid (b The parent name ethanoic acid tells us that the chain that includes the carboxylic acid func tion contains only two carbons. A hydroxyl group and a phenyl substituent are present at C-2. CHCO.H 2-Hydroxy-2-phenylethanoic acid (c) The parent alkane is tetradecane, which has an unbranched chain of 14 carbons. The termi- nal methyl group is transformed to a carboxyl function in tetradecanoic acid. Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(c) The phenyl and methyl substituents attached to C-2 of malonic acid play no role in the decarboxylation process. 19.12 (b) The thermal decarboxylation of -keto acids resembles that of substituted malonic acids. The structure of 2,2-dimethylacetoacetic acid and the equation representing its decarboxylation were given in the text. The overall process involves the bonding changes shown. 19.13 (a) Lactic acid (2-hydroxypropanoic acid) is a three-carbon carboxylic acid that bears a hydroxyl group at C-2. (b) The parent name ethanoic acid tells us that the chain that includes the carboxylic acid function contains only two carbons. A hydroxyl group and a phenyl substituent are present at C-2. (c) The parent alkane is tetradecane, which has an unbranched chain of 14 carbons. The terminal methyl group is transformed to a carboxyl function in tetradecanoic acid. Tetradecanoic acid (myristic acid) CH3(CH2)12COH O 2-Hydroxy-2-phenylethanoic acid (mandelic acid) CHCO2H OH 2-Hydroxypropanoic acid CH3CHCO2H OH 3 21 Enol form of 3-methyl- 2-butanone C CH3 OH CH3C CH3 3-Methyl-2-butanone CH3CCH(CH3)2 O CO2 2,2-Dimethylacetoacetic acid O H C H3C CH3 O C O CH3C C CH3 O C OH O O C H heat CO2 H3C C C OH OH CHCOH O CH3 2-Phenylpropanoic acid Carbon dioxide CARBOXYLIC ACIDS 507 __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
508 CARBOXYLIC ACIDS (d) Undecane is the unbranched alkane with 1l carbon atoms, undecanoic acid is the correspond- carboxylic acid, and undecenoic acid is an 11-carbon carboxylic acid that contains a double bond. Because the carbon chain is numbered beginning with the carboxyl group, 10-undecenoic acid has its double bond at the opposite end of the chain from the carboxyl group H,C=CH(CH2)&CO,H 10-Undecenoic acid (undecylenic acid) (e) Mevalonic acid has a five-carbon chain with hydroxyl groups at C-3 and C-5, along with a methyl group at C-3 HOCH,CH, CCH,CO, H 3. 5-Dihydroxy-3-methylpentanoic acid (f) The constitution represented by the systematic name 2-methyl-2-butenoic acid gives rise to CH CH=CCO.H 2-Methy 1-2-butenoic acid Tiglic acid is the e isomer and the z isomer is known as angelic acid. The higher ranked sub stituents,methyl and carboxyl, are placed on opposite sides of the double bond in tiglic acid and on the same side in angelic acid. H,C H3 (E)-2-Methyl-2-butenoic (Z)-2-Methyl-2-butenoic acid (g) Butanedioic acid is a four-carbon chain in which both terminal carbons are carboxylic acid groups. Malic acid has a hydroxyl group at C-2. HOCCHCHCOH h) Each of the carbon atoms of propane bears a carboxyl group as a substituent in 1, 2, 3-propane- tricarboxylic acid. In citric acid C-2 also bears a hydroxyl group COH HO, CCH, CCH, CO,H 2-Hydroxy-1, 2, 3-propanetricarboxylic acid Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(d) Undecane is the unbranched alkane with 11 carbon atoms, undecanoic acid is the corresponding carboxylic acid, and undecenoic acid is an 11-carbon carboxylic acid that contains a double bond. Because the carbon chain is numbered beginning with the carboxyl group, 10-undecenoic acid has its double bond at the opposite end of the chain from the carboxyl group. (e) Mevalonic acid has a five-carbon chain with hydroxyl groups at C-3 and C-5, along with a methyl group at C-3. ( f) The constitution represented by the systematic name 2-methyl-2-butenoic acid gives rise to two stereoisomers. Tiglic acid is the E isomer, and the Z isomer is known as angelic acid. The higher ranked substituents, methyl and carboxyl, are placed on opposite sides of the double bond in tiglic acid and on the same side in angelic acid. (g) Butanedioic acid is a four-carbon chain in which both terminal carbons are carboxylic acid groups. Malic acid has a hydroxyl group at C-2. (h) Each of the carbon atoms of propane bears a carboxyl group as a substituent in 1,2,3-propanetricarboxylic acid. In citric acid C-2 also bears a hydroxyl group. HO2CCH2CCH2CO2H OH CO2H 2-Hydroxy-1,2,3-propanetricarboxylic acid (citric acid) HO2CCHCH2CO2H OH 2-Hydroxybutanedioic acid (malic acid) C C H H3C CO2H CH3 (E)-2-Methyl-2-butenoic acid (tiglic acid) C C H H3C CH3 CO2H (Z)-2-Methyl-2-butenoic acid (angelic acid) CH3CH CCO2H CH3 2-Methyl-2-butenoic acid HOCH2CH2CCH2CO2H OH CH3 3,5-Dihydroxy-3-methylpentanoic acid (mevalonic acid) 10-Undecenoic acid (undecylenic acid) H2C CH(CH2)8CO2H 508 CARBOXYLIC ACIDS __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOXYLIC ACIDS 50 (i) There is an aryl substituent at C-2 of propanoic acid in ibuprofen. This aryl substituent is a benzene ring bearing an isobutyl group at the para position. CH3CHCO,H CH,CH(CH3) (p-lsobutylphenyl)- propanoic acid O Benzenecarboxylic acid is the systematic name for benzoic acid. Salicylic acid is a derivative of benzoic acid bearing a hydroxyl group at the position ortho to the carboxyl COH o-Hydroxybenzenecarboxy lic acid (salicylic acid) 19.14 (a) The carboxylic acid contains a linear chain of eight carbon atoms. The parent alkane is octane, and so the systematic name of CH3(CH2)CO,H is octanoic acid (b) The compound shown is the potassium salt of octanoic acid. It is potassium octanoate. (c) The presence of a double bond in CH,=CH(CH)5CO, H is indicated by the ending -enoic acid. Numbering of the chain begins with the carboxylic acid, and so the double bond is be- tween C-7 and C-8. The compound is 7-octenoic acid. (d) Stereochemistry is systematically described by the E-z notation. Here, the double bond be- tween C-6 and C-7 in octenoic acid has the Z configuration; the higher ranked substituents are he same side HaC (CH,)CO,H (Z)-6-Octenoic acid (e) A dicarboxylic acid is named as a dioic acid. The carboxyl functions are the terminal carbons of an eight-carbon chain; HO, C(CH,) CO, H is octanedioic acid. It is not necessary to iden- tify the carboxylic acid locations by number because they can only be at the ends of the chain when the -dioic acid name is used (f) Pick the longest continuous chain that includes both carboxyl groups and name the compound a-dioic acid. This chain contains only three carbons and bears a pentyl group as a sub- stituent at C-2. It is not necessary to specify the position of the pentyl group, because it can nly be attached to C-2 CH3(CH,)CHCO,H Malonic acid is an acceptable synonym for propanedioic acid; this compound may also be named pentylmalonic acid. Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(i) There is an aryl substituent at C-2 of propanoic acid in ibuprofen. This aryl substituent is a benzene ring bearing an isobutyl group at the para position. ( j) Benzenecarboxylic acid is the systematic name for benzoic acid. Salicylic acid is a derivative of benzoic acid bearing a hydroxyl group at the position ortho to the carboxyl. 19.14 (a) The carboxylic acid contains a linear chain of eight carbon atoms. The parent alkane is octane, and so the systematic name of CH3(CH2)6CO2H is octanoic acid. (b) The compound shown is the potassium salt of octanoic acid. It is potassium octanoate. (c) The presence of a double bond in CH2?CH(CH2)5CO2H is indicated by the ending -enoic acid. Numbering of the chain begins with the carboxylic acid, and so the double bond is between C-7 and C-8. The compound is 7-octenoic acid. (d) Stereochemistry is systematically described by the E–Z notation. Here, the double bond between C-6 and C-7 in octenoic acid has the Z configuration; the higher ranked substituents are on the same side. (e) A dicarboxylic acid is named as a dioic acid. The carboxyl functions are the terminal carbons of an eight-carbon chain; HO2C(CH2)6CO2H is octanedioic acid. It is not necessary to identify the carboxylic acid locations by number because they can only be at the ends of the chain when the -dioic acid name is used. ( f ) Pick the longest continuous chain that includes both carboxyl groups and name the compound as a -dioic acid. This chain contains only three carbons and bears a pentyl group as a substituent at C-2. It is not necessary to specify the position of the pentyl group, because it can only be attached to C-2. Malonic acid is an acceptable synonym for propanedioic acid; this compound may also be named pentylmalonic acid. Pentylpropanedioic acid CH3(CH2)4CHCO2H CO2H 2 1 3 C C H H3C H (CH2)4CO2H (Z)-6-Octenoic acid o-Hydroxybenzenecarboxylic acid (salicylic acid) OH CO2H 2-( p-Isobutylphenyl)- propanoic acid CH3CHCO2H CH2CH(CH3)2 CARBOXYLIC ACIDS 509 __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
510 CARBOXYLIC ACIDS (g) A carboxylic acid function is attached as a substituent on a seven-membered ring. The com- olic acid CO,H (h) The aromatic ring is named as a substituent attached to the eight-carbon carboxylic acid Numbering of the chain begins with the carboxyl group CHCH -CH(CH2)CO2H 6-Phenyloctanoic acid 19.15(a) Carboxylic acids are the most acidic class of organic compounds containing only the elements C, H, and O. The order of decreasing acidity is Acetic acid CHaCO, H 18×10-5 Ethanol CH3,OH Ethane H3 CH3 ≈1 (b) Here again, the carboxylic acid is the strongest acid and the hydrocarbon the weakest Benzoic acid CH,CO, H 6.7X 10-5 Benzyl alcohol CBH CH,OH 16-18 C6H6 ≈43 (c) Propanedioic acid is a stronger acid than propanoic acid because the electron-withdrawing effect of one carboxyl group enhances the ionization of the other. Propanedial is a 1,3-dicar- bonyl compound that yields a stabilized enolate; it is more acidic than 1, 3-propanediol. Propanedioic acid HO, CCH, CO, H 14×10-3 Propanoic acid CHa CH,CO, H 1.3×10-5 ≈10 1, 3-Propanediol HOCH,CH, CH,OH 10 ≈16 (d) Trifluoromethanesulfonic acid is by far the strongest acid in the group. It is structurally related to sulfuric acid, but its three fluorine substituents make it much stronger. Fluorine substituents Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(g) A carboxylic acid function is attached as a substituent on a seven-membered ring. The compound is cycloheptanecarboxylic acid. (h) The aromatic ring is named as a substituent attached to the eight-carbon carboxylic acid. Numbering of the chain begins with the carboxyl group. 19.15 (a) Carboxylic acids are the most acidic class of organic compounds containing only the elements C, H, and O. The order of decreasing acidity is Ka pKa Acetic acid CH3CO2H 1.8 105 4.7 Ethanol CH3CH2OH 1016 16 Ethane CH3CH3 1046 46 (b) Here again, the carboxylic acid is the strongest acid and the hydrocarbon the weakest: Ka pKa Benzoic acid C6H5CO2H 6.7 105 4.2 Benzyl alcohol C6H5CH2OH 1016–1018 16–18 Benzene C6H6 1043 43 (c) Propanedioic acid is a stronger acid than propanoic acid because the electron-withdrawing effect of one carboxyl group enhances the ionization of the other. Propanedial is a 1,3-dicarbonyl compound that yields a stabilized enolate; it is more acidic than 1,3-propanediol. Ka pKa Propanedioic acid HO2CCH2CO2H 1.4 103 2.9 Propanoic acid CH3CH2CO2H 1.3 105 4.9 Propanedial O?CHCH2CH?O 109 9 1,3-Propanediol HOCH2CH2CH2OH 1016 16 (d) Trifluoromethanesulfonic acid is by far the strongest acid in the group. It is structurally related to sulfuric acid, but its three fluorine substituents make it much stronger. Fluorine substituents 6-Phenyloctanoic acid CH(CH2)4CO2H CH2CH3 CO2H 510 CARBOXYLIC ACIDS __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOXYLIC ACIDS 511 ncrease the acidity of carboxylic acids and alcohols relative to their nonfluorinated analogs, but not enough to make fluorinated alcohols as acidic as carboxylic acids Ka Trifluoromethanesulfonic acid CF. OH Trifluoroacetic acid CF3CO2H 59×10-1 CH COH 1.8×10-5 2. 2-Trifluoroethanol CF3CH,OH Ethanol CHCHOH ≈16 (e) The order of decreasing acidity is carboxylic acid> B-diketone ketone hydrocarbon K pk COH Cyclopentanecarboxylic acid 1×10 5.0 2. 4-Pentanedione CH, CCH, CCH3 Cyclopentanone Cyclopentene -4545 19.16 (a) A trifluoromethyl group is strongly electron-withdrawing and acid-strengthening. Its ability to attract electrons from the carboxylate ion decreases as its distance down the chain increases 3, 3,3-Trifluoropropanoic acid is a stronger acid than 4, 4, 4-trifluorobutanoic acid CF CH, CO,H CF3CH,CH,CO, H 3. 3.3-Trifluoropropanoic acid 4, 4.4-TriHluorobutanoic acid k9.6×1 K69×10-3 (pK 3.0) (pK24.2) (b) The carbon that bears the carboxyl group in 2-butynoic acid is sp-hybridized and is, therefore, more electron-withdrawing than the sp-hybridized a carbon of butanoic acid. The anion of 2- butynoic acid is therefore stabilized better than the anion of butanoic acid, and 2-butynoic acid is a stronger acid CH3CECCO, H CH, CH, CH,CO,H Butanoic acid K25 (pk24.8) (c) Cyclohexanecarboxylic acid is a typical aliphatic carboxylic acid and is expected to be sim- lar to acetic acid in acidity. The greater electronegativity of the sp--hybridized carbon Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
increase the acidity of carboxylic acids and alcohols relative to their nonfluorinated analogs, but not enough to make fluorinated alcohols as acidic as carboxylic acids. Ka pKa Trifluoromethanesulfonic acid CF3SO2OH 106 6 Trifluoroacetic acid CF3CO2H 5.9 101 0.2 Acetic acid CH3CO2H 1.8 105 4.7 2,2,2-Trifluoroethanol CF3CH2OH 4.2 1013 12.4 Ethanol CH3CH2OH 1016 16 (e) The order of decreasing acidity is carboxylic acid -diketone ketone hydrocarbon. Ka pKa Cyclopentanecarboxylic acid 1 105 5.0 2,4-Pentanedione 109 9 Cyclopentanone 1020 20 Cyclopentene 1045 45 19.16 (a) A trifluoromethyl group is strongly electron-withdrawing and acid-strengthening. Its ability to attract electrons from the carboxylate ion decreases as its distance down the chain increases. 3,3,3-Trifluoropropanoic acid is a stronger acid than 4,4,4-trifluorobutanoic acid. (b) The carbon that bears the carboxyl group in 2-butynoic acid is sp-hybridized and is, therefore, more electron-withdrawing than the sp3 -hybridized carbon of butanoic acid. The anion of 2- butynoic acid is therefore stabilized better than the anion of butanoic acid, and 2-butynoic acid is a stronger acid. (c) Cyclohexanecarboxylic acid is a typical aliphatic carboxylic acid and is expected to be similar to acetic acid in acidity. The greater electronegativity of the sp2 -hybridized carbon CH3C CCO2H 2-Butynoic acid Ka 2.5 103 (pKa 2.6) CH3CH2CH2CO2H Butanoic acid Ka 1.5 105 (pKa 4.8) CF3CH2CO2H 3,3,3-Trifluoropropanoic acid (pKa 3.0) Ka 9.6 104 CF3CH2CH2CO2H 4,4,4-Trifluorobutanoic acid (pKa 4.2) Ka 6.9 105 O CH3CCH2CCH3 O O CO2H CARBOXYLIC ACIDS 511 __ __ __ Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website