《有机化学》课程教学资源（教材文献，英文版）CHAPTER 19 CARBOXYLIC ACIDS.pdf_大学文库

lowing this one. Together, this chapter and the next tell the story of some of the most fundamental structural types and functional group transformations in organic and biological chemistry. 19.1 CARBOXYLIC ACID NOMENCLATURE Nowhere in organic chemistry are common names used more often than with the carboxylic acids. Many carboxylic acids are better known by common names than by their systematic names, and the framers of the IUPAC nomenclature rules have taken a liberal view toward accepting these common names as permissible alternatives to the systematic ones. Table 19.1 lists both the common and the systematic names of a number of important carboxylic acids. Systematic names for carboxylic acids are derived by counting the number of carbons in the longest continuous chain that includes the carboxyl group and replacing the -e ending of the corresponding alkane by -oic acid. The first three acids in the table, methanoic (1 carbon), ethanoic (2 carbons), and octadecanoic acid (18 carbons), illustrate this point. When substituents are present, their locations are identified by number; numbering of the carbon chain always begins at the carboxyl group. This is illustrated in entries 4 and 5 in the table. 19.1 Carboxylic Acid Nomenclature 737 TABLE 19.1 Systematic and Common Names of Some Carboxylic Acids 1. 2. 3. 4. 5. 6. 7. 9. 10. 11. 12. 8. Methanoic acid Ethanoic acid Octadecanoic acid 2-Hydroxypropanoic acid 2-Hydroxy-2-phenylethanoic acid Propenoic acid (Z)-9-Octadecenoic acid o-Hydroxybenzenecarboxylic acid Propanedioic acid Butanedioic acid 1,2-Benzenedicarboxylic acid Systematic name Benzenecarboxylic acid Formic acid Acetic acid Stearic acid Lactic acid Mandelic acid Acrylic acid Oleic acid Benzoic acid Salicylic acid Malonic acid Succinic acid Phthalic acid Structural formula Common name HCO2H CH3CO2H CH3(CH2)16CO2H CH3CHCO2H W OH CH2œCHCO2H CH3(CH2)7 (CH2)7CO2H H H CœC ± ± ± ± HO2CCH2CO2H HO2CCH2CH2CO2H CHCO2H W OH CO2H CO2H OH CO2H CO2H Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER NINETEEN Carboxylic Acids Notice that compounds 4 and 5 are named as hydroxy derivatives of carboxylic acids, rather than as carboxyl derivatives of alcohols. We have seen earlier that hydroxyl groups take precedence over double bonds, and double bonds take precedence over halo- gens and alkyl groups, in naming compounds. Carboxylic acids outrank all the common groups we have encountered to this point Double bonds in the main chain are signaled by the ending -enoic acid, and their position is designated by a numerical prefix. Entries 6 and 7 are representative carboxylic acids that contain double bonds. Double-bond stereochemistry is specified by using either the cis-trans or the e-Z notation When a carboxyl group is attached to a ring, the parent ring is named(retaining the final -e) and the suffix - carboxylic acid is added, as shown in entries 8 and 9 Compounds with two carboxyl groups, as illustrated by entries 10 through 12,are distinguished by the suffix -dioic acid or-dicarboxylic acid as appropriate. The final -e in the base name of the alkane is retained PROBLEM 19.1 The list of carboxylic acids in Table 19. 1 is by no means exhaus- tive insofar as common names are concerned. many others are known by their common names, a few of which follow. Give a systematic IUPAC name for each (a) CH2-CCO,H (c) HO2 CCO2H (b)H3C CO,H CO2H (p-Toluic acid) SAMPLE SOLUTION (a)Methacrylic acid is an industrial chemical used in the preparation of transparent plastics such as lucite and Plexiglas. The carbon chain that includes both the carboxylic acid and the double bond is three carbon atoms in length. The compound is named as a derivative of propenoic acid. It is not nec essary to locate the position of the double bond by number, as in 2-propenoic acid, "because no other positions are structurally possible for it. The methyl group is at C-2, and so the correct systematic name for methacrylic acid is 2-methyl 19.2 STRUCTURE AND BONDING The structural features of the carboxyl group are most apparent in formic acid. Formic acid is planar, with one of its carbon-oxygen bonds shorter than the other, and with bond angles at carbon close to 1200 Bond distances Bond Angles H-C=0 134 H oxidization at carbon, and a o T carbon-oxygen double bond analogous to that of aldehydes and ketones Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

Notice that compounds 4 and 5 are named as hydroxy derivatives of carboxylic acids, rather than as carboxyl derivatives of alcohols. We have seen earlier that hydroxyl groups take precedence over double bonds, and double bonds take precedence over halogens and alkyl groups, in naming compounds. Carboxylic acids outrank all the common groups we have encountered to this point. Double bonds in the main chain are signaled by the ending -enoic acid, and their position is designated by a numerical prefix. Entries 6 and 7 are representative carboxylic acids that contain double bonds. Double-bond stereochemistry is specified by using either the cis–trans or the E–Z notation. When a carboxyl group is attached to a ring, the parent ring is named (retaining the final -e) and the suffix -carboxylic acid is added, as shown in entries 8 and 9. Compounds with two carboxyl groups, as illustrated by entries 10 through 12, are distinguished by the suffix -dioic acid or -dicarboxylic acid as appropriate. The final -e in the base name of the alkane is retained. PROBLEM 19.1 The list of carboxylic acids in Table 19.1 is by no means exhaustive insofar as common names are concerned. Many others are known by their common names, a few of which follow. Give a systematic IUPAC name for each. (a) (c) (b) (d) SAMPLE SOLUTION (a) Methacrylic acid is an industrial chemical used in the preparation of transparent plastics such as Lucite and Plexiglas. The carbon chain that includes both the carboxylic acid and the double bond is three carbon atoms in length. The compound is named as a derivative of propenoic acid. It is not necessary to locate the position of the double bond by number, as in “2-propenoic acid,” because no other positions are structurally possible for it. The methyl group is at C-2, and so the correct systematic name for methacrylic acid is 2-methylpropenoic acid. 19.2 STRUCTURE AND BONDING The structural features of the carboxyl group are most apparent in formic acid. Formic acid is planar, with one of its carbon–oxygen bonds shorter than the other, and with bond angles at carbon close to 120°. This suggests sp2 hybridization at carbon, and a carbon–oxygen double bond analogous to that of aldehydes and ketones. Bond Distances CœO C±O 120 pm 134 pm Bond Angles H±CœO H±C±O O±CœO 124° 111° 125° C H O H O CH3 CO2H (p-Toluic acid) C H H CO2H H3C C (Crotonic acid) HO2CCO2H (Oxalic acid) CH2 CH3 CCO2H (Methacrylic acid) 738 CHAPTER NINETEEN Carboxylic Acids Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

19.3 Physical Properties Additionally, sp- hybridization of the hydroxyl oxygen allows one of its electron pairs to be delocalized by orbital overlap with the T system of the roup(Figure 19.1). In resonance terms, this electron delocalization is represented as H →>H—( OH OH Lone-pair donation from the hydroxyl oxygen makes the carbonyl group less elec trophilic than that of an aldehyde or ketone. The graphic that opened this chapter is an GURE 19.1 Carbon electrostatic potential map of formic acid that shows the most electron-rich site to be the and both oxygens g acid oxygen of the carbonyl group and the most electron-poor one to be, as expected, the OH The T component of the c=o group and the p or- Carboxylic acids are fairly acid, and benzoic acid have dipol system that includes carbo and the two oxygens 19.3 PHYSICAL PROPERTIES The melting points and boiling points of carboxylic acids are higher than those of hydro- carbons and oxygen-containing organic compounds of comparable size and shape and ndicate strong intermolecular attractive forces ng By Modeling and notice how much more intens carbon 2-Methyl-l-butene 2-Butanone 2-Butanol bp(I atm) 99°C A unique hydrogen-bonding arrangement, shown in Figure 19. 2, contributes to erties these attractive forces. The hydroxyl group of one carboxylic acid molecule acts as a proton donor toward the carbonyl oxygen of a second. In a reciprocal fashion, the n Appendix hydroxyl proton of the second carboxyl function interacts with the carbonyl oxygen of the first. The result is that the two carboxylic acid molecules are held together by two hydrogen bonds. So efficient is this hydrogen bonding that some carboxylic acids exist as hydrogen-bonded dimers even in the gas phase. In the pure liquid a mixture of hydrogen-bonded dimers and higher aggregates is present. In aqueous solution intermolecular association between carboxylic acid molecules is replaced by hydrogen bonding to water. The solubility properties of carboxylic acids are similar to those of alcohols. Carboxylic acids of four carbon atoms or fewer are mis cible with water in all proportions. FIGURE 19.2 Attrac positive(blue)and negative (red)electrostatic potentia ecular hydrogen bonding between two molecules of acetic acid Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

Additionally, sp2 hybridization of the hydroxyl oxygen allows one of its unshared electron pairs to be delocalized by orbital overlap with the system of the carbonyl group (Figure 19.1). In resonance terms, this electron delocalization is represented as: Lone-pair donation from the hydroxyl oxygen makes the carbonyl group less electrophilic than that of an aldehyde or ketone. The graphic that opened this chapter is an electrostatic potential map of formic acid that shows the most electron-rich site to be the oxygen of the carbonyl group and the most electron-poor one to be, as expected, the OH proton. Carboxylic acids are fairly polar, and simple ones such as acetic acid, propanoic acid, and benzoic acid have dipole moments in the range 1.7–1.9 D. 19.3 PHYSICAL PROPERTIES The melting points and boiling points of carboxylic acids are higher than those of hydrocarbons and oxygen-containing organic compounds of comparable size and shape and indicate strong intermolecular attractive forces. A unique hydrogen-bonding arrangement, shown in Figure 19.2, contributes to these attractive forces. The hydroxyl group of one carboxylic acid molecule acts as a proton donor toward the carbonyl oxygen of a second. In a reciprocal fashion, the hydroxyl proton of the second carboxyl function interacts with the carbonyl oxygen of the first. The result is that the two carboxylic acid molecules are held together by two hydrogen bonds. So efficient is this hydrogen bonding that some carboxylic acids exist as hydrogen-bonded dimers even in the gas phase. In the pure liquid a mixture of hydrogen-bonded dimers and higher aggregates is present. In aqueous solution intermolecular association between carboxylic acid molecules is replaced by hydrogen bonding to water. The solubility properties of carboxylic acids are similar to those of alcohols. Carboxylic acids of four carbon atoms or fewer are miscible with water in all proportions. bp (1 atm): 2-Methyl-1-butene 31°C O 2-Butanone 80°C OH 2-Butanol 99°C O OH Propanoic acid 141°C H OH C O H C O OH H O C OH 19.3 Physical Properties 739 FIGURE 19.1 Carbon and both oxygens are sp2 - hybridized in formic acid. The component of the CœO group and the p orbital of the OH oxygen overlap to form an extended system that includes carbon and the two oxygens. A summary of physical properties of some representative carboxylic acids is presented in Appendix 1. Examine the electrostatic potential map of butanoic acid on Learning By Modeling and notice how much more intense the blue color (positive charge) is on the OH hydrogen than on the hydrogens bonded to carbon. FIGURE 19.2 Attractions between regions of positive (blue) and negative (red) electrostatic potential are responsible for intermolecular hydrogen bonding between two molecules of acetic acid. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

740 CHAPTER NINETEEN Carboxylic Acids 19. 4 ACIDITY OF CARBOXYLIC ACIDS Carboxylic acids are the most acidic class of compounds that contain only carbon, hydro- gen, and oxygen. With ionization constants Ka on the order of 10(pKa s 5), they ar much stronger acids than water and alcohols. The case should not be overstated, how- ever. Carboxylic acids are weak acids; a 0. 1 M solution of acetic acid in water, for exam To understand the greater acidity of carboxylic acids compared with water and alcohols, compare the structural changes that accompany the ionization of a representa- tive alcohol (ethanol) and a representative carboxylic acid(acetic acid). The equilibria that define K ar CH3CH2OH H+ CHCHO IHCH3CH,0 ICH3CH,OH Ethoxide ion lonization of acetic acid CH3COH H+ CHz CO k、田CH3CO2 [CH3CO2H) 18×10-5 From these Ka values, the calculated free energies of ionization (AGo) are 91 kJ/mol are calculated from equilib-(21.7 kcal/mol) for ethanol versus 27 kJ/mol (6.5 kcal/mol) for acetic acid. An energy rium constants according to diagram portraying these relationships is presented in Figure 19.3. Since it is equilibria, the relationship not rates, of ionization that are being compared, the diagram shows only the initial and △G°=-RTn final states. It is not necessary to be concerned about the energy of activation, since that affects only the rate of ionization, not the extent of ionization. The large difference in the free energies of ionization of ethanol and acetic acid reflects a greater stabilization of acetate ion relative to ethoxide ion lonization of ethanol yields an alkoxide ion in which the negative charge is localized on oxygen Solvation forces are the chief means by which ethoxide ion is stabilized. Acetate ion is also sta- bilized by solvation, but has two additional mechanisms for dispersing its negative charge that are not available to ethoxide ion: 1. The inductive effect of the carbonyl group. The carbonyl group of acetate ion is electron-withdrawing, and by attracting electrons away from the negatively charged oxygen, acetate anion is stabilized. This is an inductive effect, arising in the polar- ization of the electron distribution in the o bond between the carbonyl carbon and the negatively charged oxygen. Positively polarized CH2 group has carbon attracts elec- ons from negatively H on electron density CH3-CH, at negatively 2. The resonance effect of the carbonyl group. Electron delocalization, expressed by resonance between the following Lewis structures, causes the negative charge in acetate to be shared equally by both oxygens. Electron delocalization of this type is not available to ethoxide ion Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

19.4 ACIDITY OF CARBOXYLIC ACIDS Carboxylic acids are the most acidic class of compounds that contain only carbon, hydrogen, and oxygen. With ionization constants Ka on the order of 105 (pKa 5), they are much stronger acids than water and alcohols. The case should not be overstated, however. Carboxylic acids are weak acids; a 0.1 M solution of acetic acid in water, for example, is only 1.3% ionized. To understand the greater acidity of carboxylic acids compared with water and alcohols, compare the structural changes that accompany the ionization of a representative alcohol (ethanol) and a representative carboxylic acid (acetic acid). The equilibria that define Ka are Ionization of ethanol Ionization of acetic acid From these Ka values, the calculated free energies of ionization (G°) are 91 kJ/mol (21.7 kcal/mol) for ethanol versus 27 kJ/mol (6.5 kcal/mol) for acetic acid. An energy diagram portraying these relationships is presented in Figure 19.3. Since it is equilibria, not rates, of ionization that are being compared, the diagram shows only the initial and final states. It is not necessary to be concerned about the energy of activation, since that affects only the rate of ionization, not the extent of ionization. The large difference in the free energies of ionization of ethanol and acetic acid reflects a greater stabilization of acetate ion relative to ethoxide ion. Ionization of ethanol yields an alkoxide ion in which the negative charge is localized on oxygen. Solvation forces are the chief means by which ethoxide ion is stabilized. Acetate ion is also stabilized by solvation, but has two additional mechanisms for dispersing its negative charge that are not available to ethoxide ion: 1. The inductive effect of the carbonyl group. The carbonyl group of acetate ion is electron-withdrawing, and by attracting electrons away from the negatively charged oxygen, acetate anion is stabilized. This is an inductive effect, arising in the polarization of the electron distribution in the bond between the carbonyl carbon and the negatively charged oxygen. 2. The resonance effect of the carbonyl group. Electron delocalization, expressed by resonance between the following Lewis structures, causes the negative charge in acetate to be shared equally by both oxygens. Electron delocalization of this type is not available to ethoxide ion. CH3 C O Positively polarized O carbon attracts electrons from negatively charged oxygen. CH2 group has negligible effect on electron density at negatively charged oxygen. CH3 CH2 O Acetic acid CH3COH O Acetate ion CH3CO O H Ka [H][CH3CO2 ] [CH3CO2H] 1.8 105 Ethanol CH3CH2OH H Ethoxide ion CH3CH2O Ka [H][CH3CH2O] [CH3CH2OH] 1016 740 CHAPTER NINETEEN Carboxylic Acids Free energies of ionization are calculated from equilibrium constants according to the relationship G° RT In Ka Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

19.5 Salts of Carboxylic Acids 743 QUANTITATIVE RELATIONSHIPS INVOLVING CARBOXYLIC ACIDS uppose you take two flasks, one containing pure This relationship is one form of the Henderson- water and the other a buffer solution main- Hasselbalch equation It is a useful relationship in tained at a pH of 7.0 If you add 0. 1 mol of acetic chemistry and biochemistry. one rarely needs to cal- acid to each one and the final volume in each flask is culate the ph of a solution-pH is more often mea- 1 L, how much acetic acid is present at equilibrium? sured than calculated. It is much more common that low much acetate ion? In other words, what is the one needs to know the degree of ionization of an extent of ionization of acetic acid in an unbuffered acid at a particular pH, and the Henderson-Hassel- edium and in a buffered one? balch equation gives that ratio The first case simply involves the ionization of a For the case at hand the solution is buffered at weak acid and is governed by the expression that de- pH=7.0. Therefore fines k for acetic acid: [CH3CO 18×10 IH ICH3CO2] =18×10 [CH3 CO,H [CH3 CO2 H] A very different situation exists in an aqueous solu- Since ionization of acetic acid gives one h for each tion maintained at ph =7.0 from the situation in CH3CO2, the concentrations of the two ions are pure water. We saw earlier that almost all the acetic equal, and setting each one equal to x gives: acid in a 0.1 M solution in pure water was nonion- ized. At pH 7.0, however, hardly any nonionized 18×10-5 acetic acid remains; it is almost completely converted to its carboxylate ion Solving for x gives the acetate ion concentration as: This difference in behavior for acetic acid in pure water versus water buffered at ph =7.0 has X=1.3×10 some important practical consequences. Biochemists usually do not talk about acetic acid (or lactic acid, or Thus when acetic acid is added to pure water, the ra- salicylic acid, etc. ) They talk about acetate (and lac- tio of acetate ion to acetic acid is tate, and salicylate). Why? It's because biochemists are concerned with carboxylic acids as they exist in di- CHo21=13×10=0013 lute aqueous solution at what is called biological pH 0.1 lological fluids are naturally buffered. The ph of blood, for example, is maintained at 7. 2, and at this Only 1.3% of the acetic acid has ionized. Most of it h carboxylic acids are almost entirely converted to (98.7%)remains unchanged Now think about what happens when the same their carboxylate anions. amount of acetic acid is added to water that is Daics An alternative form of the Henderson-Hassel- buffered at pH =7.0. Before doing the calculation, let us recognize that it is the [CH3 CO2 1/(CH3CO2H] CH3 CO2-I ratio in which we are interested and do a little alge- pH=pka +log (CH3 CO2H From this equation it e seen that whe K,=田cHo2 ICH3 CO2= [CH3,H then the second term is lo 1=0, and ph=pKa. This means that when the ph of a solution is equal to the pka of a weak acid, the con- centration of the acid and its conjugate base are [CHa CO equal. This is a relationship worth remembering [CH, CO,H [H+ Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

19.5 Salts of Carboxylic Acids 743 QUANTITATIVE RELATIONSHIPS INVOLVING CARBOXYLIC ACIDS S uppose you take two flasks, one containing pure water and the other a buffer solution maintained at a pH of 7.0. If you add 0.1 mol of acetic acid to each one and the final volume in each flask is 1 L, how much acetic acid is present at equilibrium? How much acetate ion? In other words, what is the extent of ionization of acetic acid in an unbuffered medium and in a buffered one? The first case simply involves the ionization of a weak acid and is governed by the expression that de- fines Ka for acetic acid: Ka 1.8 105 Since ionization of acetic acid gives one H for each CH3CO2 , the concentrations of the two ions are equal, and setting each one equal to x gives: Ka 1.8 105 Solving for x gives the acetate ion concentration as: x 1.3 103 Thus when acetic acid is added to pure water, the ratio of acetate ion to acetic acid is 0.013 Only 1.3% of the acetic acid has ionized. Most of it (98.7%) remains unchanged. Now think about what happens when the same amount of acetic acid is added to water that is buffered at pH 7.0. Before doing the calculation, let us recognize that it is the [CH3CO2 ] ⁄[CH3CO2H] ratio in which we are interested and do a little algebraic manipulation. Since Ka then Ka [H] [CH3CO2 ] [CH3CO2H] [H][CH3CO2 ] [CH3CO2H] 1.3 103 0.1 [CH3CO2 ] [CH3CO2H] x2 0.1 x [H][CH3CO2 ] [CH3CO2H] This relationship is one form of the Henderson– Hasselbalch equation. It is a useful relationship in chemistry and biochemistry. One rarely needs to calculate the pH of a solution—pH is more often measured than calculated. It is much more common that one needs to know the degree of ionization of an acid at a particular pH, and the Henderson–Hasselbalch equation gives that ratio. For the case at hand, the solution is buffered at pH 7.0. Therefore, 180 A very different situation exists in an aqueous solution maintained at pH 7.0 from the situation in pure water. We saw earlier that almost all the acetic acid in a 0.1 M solution in pure water was nonionized. At pH 7.0, however, hardly any nonionized acetic acid remains; it is almost completely converted to its carboxylate ion. This difference in behavior for acetic acid in pure water versus water buffered at pH 7.0 has some important practical consequences. Biochemists usually do not talk about acetic acid (or lactic acid, or salicylic acid, etc.). They talk about acetate (and lactate, and salicylate). Why? It’s because biochemists are concerned with carboxylic acids as they exist in dilute aqueous solution at what is called biological pH. Biological fluids are naturally buffered. The pH of blood, for example, is maintained at 7.2, and at this pH carboxylic acids are almost entirely converted to their carboxylate anions. An alternative form of the Henderson–Hasselbalch equation for acetic acid is pH pKa log From this equation it can be seen that when [CH3CO2 ] [CH3CO2H], then the second term is log 1 0, and pH pKa. This means that when the pH of a solution is equal to the pKa of a weak acid, the concentration of the acid and its conjugate base are equal. This is a relationship worth remembering. [CH3CO2 ] [CH3CO2H] 1.8 105 107 [CH3CO2 ] [CH3CO2H] Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER NINETEEN Carboxylic Acids The metal carboxylate salts formed on neutralization of carboxylic acids are named by first specifying the metal ion and then adding the name of the acid modified by replac ing -ic acid by -ate. Monocarboxylate salts of diacids are designated by naming both the cation and hydrogen as substituents of carboxylate groups CHaCOLI CI CONa HOC(CH,)4COK Lithium Sodium p-chlorobenzoate Potassium hydrogen hexanedioate Metal carboxylates are ionic, and when the molecular weight isn' t too high, the sodium and potassium salts of carboxylic acids are soluble in water. Carboxylic acids therefore may be extracted from ether solutions into aqueous sodium or potassium hydroxide. The solubility behavior of salts of carboxylic acids having 12-18 carbons is unusual and can be illustrated by considering sodium stearate Sodium stearate ( sodium octadecanoate) Sodium stearate has a polar carboxylate group at one end of a long hydrocarbon chain The carboxylate group is hydrophilic ("water-loving") and tends to confer water solu bility on the molecule. The hydrocarbon chain is lipophilic("fat-loving")and tends to associate with other hydrocarbon chains. The compromise achieved by sodium stearate when it is placed in water is to form a colloidal dispersion of spherical aggregates called micelles. Each micelle is composed of 50-100 individual molecules. Micelles form spon taneously when the carboxylate concentration exceeds a certain minimum value called the critical micelle concentration. A representation of a micelle is shown in Figure 19 Polar carboxylate groups dot the surface of the micelle. There they bind to water molecules and to sodium ions. The nonpolar hydrocarbon chains are directed toward the interior of the micelle, where individually weak but cumulatively significant induced dipole/induced-dipole forces bind them together. Micelles are approximately spherical because a sphere encloses the maximum volume of material for a given surface area and FIGURE 19.5 A space filling model of a micelle formed by association of cal fatty acid. In general, the re inside and the carboxy- late ions on the surface bu contains voids tangled carbon with a metal ion such as Na Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

The metal carboxylate salts formed on neutralization of carboxylic acids are named by first specifying the metal ion and then adding the name of the acid modified by replacing -ic acid by -ate. Monocarboxylate salts of diacids are designated by naming both the cation and hydrogen as substituents of carboxylate groups. Metal carboxylates are ionic, and when the molecular weight isn’t too high, the sodium and potassium salts of carboxylic acids are soluble in water. Carboxylic acids therefore may be extracted from ether solutions into aqueous sodium or potassium hydroxide. The solubility behavior of salts of carboxylic acids having 12–18 carbons is unusual and can be illustrated by considering sodium stearate: Sodium stearate has a polar carboxylate group at one end of a long hydrocarbon chain. The carboxylate group is hydrophilic (“water-loving”) and tends to confer water solubility on the molecule. The hydrocarbon chain is lipophilic (“fat-loving”) and tends to associate with other hydrocarbon chains. The compromise achieved by sodium stearate when it is placed in water is to form a colloidal dispersion of spherical aggregates called micelles. Each micelle is composed of 50–100 individual molecules. Micelles form spontaneously when the carboxylate concentration exceeds a certain minimum value called the critical micelle concentration. A representation of a micelle is shown in Figure 19.5. Polar carboxylate groups dot the surface of the micelle. There they bind to water molecules and to sodium ions. The nonpolar hydrocarbon chains are directed toward the interior of the micelle, where individually weak but cumulatively significant induceddipole/induced-dipole forces bind them together. Micelles are approximately spherical because a sphere encloses the maximum volume of material for a given surface area and O Na O Sodium stearate (sodium octadecanoate) CH3COLi O Lithium acetate Cl CONa O Sodium p-chlorobenzoate HOC(CH2)4COK O O Potassium hydrogen hexanedioate 744 CHAPTER NINETEEN Carboxylic Acids FIGURE 19.5 A space- filling model of a micelle formed by association of carboxylate ions derived from a fatty acid. In general, the hydrophobic carbon chains are inside and the carboxylate ions on the surface, but the micelle is irregular, and contains voids, channels, and tangled carbon chains. Each carboxylate is associated with a metal ion such as Na (not shown). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

19.6 Substituents and acid Strength disrupts the water structure least. Because their surfaces are negatively charged, two micelles repel each other rather than clustering to form higher aggregates It is the formation of micelles and their properties that are responsible for the cleansing action of soaps. Water that contains sodium stearate removes grease by enclos- ing it in the hydrocarbon-like interior of the micelles. The grease is washed away with the water not because it dissolves in the water but because it dissolves in the micelles that are dispersed in the water. Sodium stearate is an example of a soap; sodium and potassium salts of other C12-Ci8 unbranched carboxylic acids possess similar properties Detergents are substances, including soaps, that cleanse by micellar action. A large number of synthetic detergents are known. One example is sodium lauryl sulfate Sodium lauryl sulfate has a long hydrocarbon chain terminating in a polar sulfate ion and forms tic potenti f sodium lau soap-like micelles in water. on Learning By Modeling. Sodium lauryl sulfate (sodium dodecyl sulfate) Detergents are designed to be effective in hard water, meaning water containing calcium salts that form insoluble calcium carboxylates with soaps. These precipitates rob the soap of its cleansing power and form an unpleasant scum. The calcium salts of synthetic deter- gents such as sodium lauryl sulfate, however, are soluble and retain their micelle-forming ability in water. 19.6 SUBSTITUENTS AND ACID STRENGTH Alkyl groups have little effect on the acidity of a carboxylic acid. The ionization con- stants of all acids that have the general formula CnH2n+I CO2H are very similar to one another and equal approximately 10(pKa 5). Table 19.2 gives a few examples An electronegative substituent, particularly if it is attached to the a carbon, increases the acidity of a carboxylic acid. As the data in Table 19.2 show, all the mono- haloacetic acids are about 100 times more acidic than acetic acid. Multiple halogen sub- stitution increases the acidity even more; trichloroacetic acid is 7000 times more acidic than acetic acid! The acid-strengthening effect of electronegative atoms or groups is easily seen as an inductive effect of the substituent transmitted through the g bonds of the molecule According to this model. the g electrons in the carbon-chlorine bond of chloroacetate ion are drawn toward chlorine, leaving the a-carbon atom with a slight positive charge The a carbon, because of this positive character, attracts electrons from the negativel harged carboxylate, thus dispersing the charge and stabilizing the anion. The more sta ble the anion, the greater the equilibrium constant for its formation H contains molecular models of CH3 COz(acetate)and Cl3Cco Chloroacetate anion is hese two ions with respect to tabilized by electron- the amount of negative charge withdrawing effect of on their oxygens. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

disrupts the water structure least. Because their surfaces are negatively charged, two micelles repel each other rather than clustering to form higher aggregates. It is the formation of micelles and their properties that are responsible for the cleansing action of soaps. Water that contains sodium stearate removes grease by enclosing it in the hydrocarbon-like interior of the micelles. The grease is washed away with the water, not because it dissolves in the water but because it dissolves in the micelles that are dispersed in the water. Sodium stearate is an example of a soap; sodium and potassium salts of other C12–C18 unbranched carboxylic acids possess similar properties. Detergents are substances, including soaps, that cleanse by micellar action. A large number of synthetic detergents are known. One example is sodium lauryl sulfate. Sodium lauryl sulfate has a long hydrocarbon chain terminating in a polar sulfate ion and forms soap-like micelles in water. Detergents are designed to be effective in hard water, meaning water containing calcium salts that form insoluble calcium carboxylates with soaps. These precipitates rob the soap of its cleansing power and form an unpleasant scum. The calcium salts of synthetic detergents such as sodium lauryl sulfate, however, are soluble and retain their micelle-forming ability in water. 19.6 SUBSTITUENTS AND ACID STRENGTH Alkyl groups have little effect on the acidity of a carboxylic acid. The ionization constants of all acids that have the general formula CnH2n1CO2H are very similar to one another and equal approximately 105 (pKa 5). Table 19.2 gives a few examples. An electronegative substituent, particularly if it is attached to the carbon, increases the acidity of a carboxylic acid. As the data in Table 19.2 show, all the monohaloacetic acids are about 100 times more acidic than acetic acid. Multiple halogen substitution increases the acidity even more; trichloroacetic acid is 7000 times more acidic than acetic acid! The acid-strengthening effect of electronegative atoms or groups is easily seen as an inductive effect of the substituent transmitted through the bonds of the molecule. According to this model, the electrons in the carbon–chlorine bond of chloroacetate ion are drawn toward chlorine, leaving the -carbon atom with a slight positive charge. The carbon, because of this positive character, attracts electrons from the negatively charged carboxylate, thus dispersing the charge and stabilizing the anion. The more stable the anion, the greater the equilibrium constant for its formation. Cl C C H O O H Chloroacetate anion is stabilized by electronwithdrawing effect of chlorine. O O S O Na O 2 Sodium lauryl sulfate (sodium dodecyl sulfate) 19.6 Substituents and Acid Strength 745 Compare the electrostatic potential maps of sodium lauryl sulfate and sodium stearate on Learning By Modeling. Learning By Modeling contains molecular models of CH3CO2 (acetate) and Cl3CCO2 (trichloroacetate). Compare these two ions with respect to the amount of negative charge on their oxygens. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website