sign in sign up
<<
Home , Aqueous solution, Polar aprotic solvents, Protic solvent, Solvation, Solvent

08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 339

8.4 IN ORGANIC 339

8.14 Label each of the following as a -bond acceptor, donor, or both. Indicate

1 the hydrogen that is donated1 or the atom that serves as the hydrogen-bond acceptor. (a) H Br (b) H F (c) O L 1 3 L 1 3 S3 3

H3C C CH3 L L 1

| (d) O 1 (e) (f) H3C CH2 NH3 S3 3 OH L L H3C C NH CH3 cL 1 L L L

8.4 SOLVENTS IN

A is a used to dissolve a compound. Solvents have tremendous practical impor- tance. They affect the acidities and basicities of solutes. In some cases, the choice of a solvent can have dramatic effects on reaction rates and even on the outcome of a reaction. Understand- ing effects like these requires a classification of solvent types, to which Section 8.4A is devoted. The rational choice of a solvent requires an understanding of —that is, how well a given compound dissolves in a particular solvent. Section 8.4B discusses the principles that will allow you to make general predictions about the of both covalent organic com- pounds and in different solvents. The effects of solvents on chemical reactions are closely tied to the principles of solubility. Solubility is also important in biology. For example, the solubilities of determine the forms in which they are marketed and used, and such important characteristics as whether they are absorbed from the gut and whether they pass from the bloodstream into the brain. Some of these ideas are explored in Section 8.5. Because certain , alkyl halides, and are among the most important organic solvents, this is a good point our survey of organic chemistry to study solvent properties.

A. Classification of Solvents There are three broad solvent categories, and they are not mutually exclusive; that is, a solvent can be in more than one category. 1. A solvent can be protic or aprotic. 2. A solvent can be polar or apolar. 3. A solvent can be a donor or a nondonor. A consists of molecules that can act as hydrogen-bond donors. , alcohols, and carboxylic are examples of protic solvents. Solvents that cannot act as hy- drogen-bond donors are called aprotic solvents. , methylene chloride, and are examples of aprotic solvents. A polar solvent has a high dielectric constant; an apolar solvent has a low dielectric con- stant. The dielectric constant is defined by the electrostatic law, which gives the interaction en-

ergy E between two ions with respective charges q1 and q2 separated by a distance r: q q ᎏ1 2 (8.3) E k er = In this equation, k is a proportionality constant and e is the dielectric constant of the solvent in which the two ions are imbedded. This equation shows that when the dielectric constant e is large, the magnitude of E, the energy of interaction between the ions, is small. This means that 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 340

340 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, , AND SULFIDES

both attractions between ions of opposite charge and repulsions between ions of like charge are weak in a polar solvent. Thus, a polar solvent effectively separates, or shields, ions from one another. This means, in turn, that the tendency of oppositely charged ions to associate is less in a polar solvent than it is in an apolar solvent. If a solvent has a dielectric constant of about 15 or greater, it is considered to be polar. Water (e 78), (e 33), and formic (e 59) are polar solvents. Hexane (e 2), ether (e= 4), and = (e 6) are ap- olar solvents.= = = = Unfortunately, the word polar has a double usage in organic chemistry. When we say that a is polar, we mean that it has a significant moment, m (Sec. 1.2D). When we say that a solvent is polar, we mean that it has a high dielectric constant. In other words, sol- vent polarity, or dielectric constant, is a property of many molecules acting together, but mol- ecular polarity, or dipole moment, is a property of individual molecules. Although it is true that all polar solvents consist of polar molecules, the converse is not true. The contrast be- tween acetic acid and is particularly striking: O O S S H3C C OH H C OH L L L L acetic acid formic acid m 1.5–1.7 D m 1.6–1.8 D =e 6.1 =e 59 = = These two compounds contain identical functional groups and have very similar structures and dipole moments. Both are polar molecules. Yet they differ substantially in their dielectric constants and in their solvent properties! Formic acid is a polar solvent; acetic acid is not. Donor solvents consist of molecules that can donate unshared electron pairs—that is, mol- ecules that can act as Lewis bases. Ether, THF, and methanol are donor solvents. Nondonor solvents cannot act as Lewis bases; and are nondonor solvents. Table 8.2 on p. 341 lists some common solvents used in organic chemistry along with their abbreviations and their classifications. This table shows that a solvent can have a combination of properties, as noted at the beginning of this section. For example, some polar solvents are protic (such as water and methanol), but others are aprotic (such as ).

PROBLEM 8.15 Classify each of the following substances according to their solvent properties (as in Table 8.2). (a) 2-methoxyethanol (e 17) (b) 2,2,2-trifluoroethanol (e 26) (c) O = (d) 2,2,4-trimethylpentane (e= 2) S = H3C C CH2CH3 (e 19) L L =

B. Solubility One role of a solvent is simply to dissolve compounds of interest. Although finding a suitable solvent can involve some trial and error, certain principles can help us choose a solvent ratio- nally. The discussion of solubility is divided into two parts: the solubility of covalent com- pounds, and the solubility of ionic compounds.

Solubility of Covalent Compounds In determining a solvent for a covalent compound, a useful rule of thumb is like dissolves like. That is, a good solvent usually has some of the mol- ecular characteristics of the compound to be dissolved. For example, an apolar aprotic solvent is likely to be a good solvent for another apolar aprotic substance. In contrast, a protic solvent 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 341

8.4 SOLVENTS IN ORGANIC CHEMISTRY 341

TABLE 8.2 Properties of Some Common Organic Solvents

(Listed in order of increasing dielectric constant) Class Common Boiling Dielectric Solvent Structure abbrevation point, °C constant e*PolarProticDonor

hexane CH3(CH2)4CH3 —68.71.9 1,4-dioxane† —101.32.2 x O O

benzene† —80.12.3

(C2H5)2 OEt2O34.64.3 x

CHCl3 —61.24.8 ethyl acetateO EtOAc 77.1 6.0 x S CH3COC2H5 acetic acidO HOAc 117.9 6.1 x x S CH3COH THF 66 7.6 x O

methylene chloride CH2Cl2 —39.88.9

acetoneO Me2CO 56.3 21 x x S CH3CCH3

C2H5OH EtOH 78.3 25 x x x N-methylpyrrolidone NMP 202 32 x x N O

CH3

methanol CH3OH MeOH 64.7 33 x x x

CH3NO2 MeNO2 101.2 36 x x N, N- O DMF 153.0 37 x x S HCN(CH3)2

CH3C'NMeCN81.638x x — 287 (dec) 43 x x S OO@ @ dimethylsulfoxideO DMSO 189 47 x x S CH3SCH3 formic acidO — 100.6 59 x x x S HCOH

water H2O— 100.078xxx formamideO — 211 (dec) 111 x x x S HCNH2

*Most values are at or near 25 C †Known ° 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 342

342 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES

in which significant hydrogen-bonding interactions occur between molecules is likely to dis- solve another protic substance in which hydrogen bonding between molecules is also an impor- tant cohesive interaction. To illustrate, let’s consider the water solubility of organic compounds. This is an important issue in biology, because water is the solvent in living systems. Consider the water solubility of the following compounds of comparable size and molecular mass:

CH3CH2CH2CH3 CH3CH2Cl CH3CH2 O CH3 CH3CH2CH2 OH L L L water solubility: virtually insoluble soluble miscible

Of these compounds, the , 1-propanol, is most soluble; in fact, it is miscible with water. This means that a is obtained when the alcohol is mixed with water in any pro- portion. Of the compounds shown, the alcohol is also most like water because it is protic. The ability both to donate a to water and to accept a hydrogen bond from water is an important factor in water solubility. .. an alcohol can accept O hydrogen bonds from water H .. H

...... H O CH3CH2CH2 O.. H H an alcohol can donate a hydrogen bond to water

The ether contains an atom () that can accept hydrogen bonds from water, although it cannot donate a hydrogen bond; hence, it has some water-like characteristics, but is less like water than the alcohol. .. O.. H H .. an ether can accept hydrogen bonds from water O.. CH3CH2 CH3 Finally, the (butane) and the alkyl halide (ethyl chloride) can neither donate nor accept STUDY GUIDE LINK 8.2 Boiling Points and hydrogen bonds and are therefore least like water; they are also the least water-soluble com- Solubilities pounds on the list. The same effect occurs in the following series:

CH3OH CH3CH2OH CH3CH2CH2OH CH3CH2CH2CH2OH CH3CH2CH2CH2CH2CH2OH water solubility: miscible 7.7 mass % 0.58 mass %

Alcohols with long chains—that is, large alkyl groups—are more like than are alcohols containing small alkyl groups. Because alkanes cannot form hydrogen bonds, they are insoluble in water, but they are soluble in other apolar aprotic solvents, Further Exploration 8.1 including other alkanes. Hence, alcohols (as well as any other organic compounds) with long Solubility of Covalent Compounds: hydrocarbon chains are relatively insoluble in water and are more soluble in apolar aprotic sol- A Deeper Look vents than alcohols with small alkyl chains. Solvents consisting of polar molecules lie between the extremes of water on the one hand and on the other. For example, consider the widely used solvent tetrahydrofuran 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 343

8.4 SOLVENTS IN ORGANIC CHEMISTRY 343

(THF; see Table 8.2). Because THF can accept hydrogen bonds, it dissolves water and many alcohols. Because its dipole moment can interact favorably with other , it also dissolves polar compounds (for example, alkyl halides). On the other hand, because most of its structure is hydrocarbon, it also dissolves hydrocarbons. As a solvent for the reaction of a water-insol- uble compound with water, THF is typically an excellent choice because it dissolves both compounds. For example, THF is the solvent of choice in the oxymercuration of alkenes (Sec. 5.4A); it dissolves both water and alkenes. What you should begin to see from this discussion are the trends to be expected in the sol- ubility behavior of various compounds. You cannot be expected to remember absolute solubil- ities, but you should be able to make an intelligent guess about the relative solubilities of a given compound in different solvents or the relative solubilities of a series of compounds in a given solvent. This ability, for example, is required to solve the following problems.

PROBLEMS 8.16 In which of the following solvents should hexane be least soluble: diethyl ether, methylene

chloride (CH2Cl2), ethanol, or 1-octanol? Explain. 1 8.17 (a) Into a is poured 200 mL of methylene chloride ( 1.33 g mL_ ) and 55 mL of water. This forms two layers. One milliliter of methanol= is added to the mixture, which is then stoppered and shaken. Two layers are again formed. In which layer is the methanol likely to be dissolved? Explain. (b) The experiment is repeated, except that 1 mL of 1-nonanol is added instead of methanol. In which layer is the alcohol dissolved? Explain. 8.18 A widely used undergraduate experiment is the recrystallization of acetanilide from water. Acetanilide (see following structure) is moderately soluble in hot water, but much less solu- ble in cold water. Identify one structural feature of the acetanilide molecule that would be ex- pected to contribute positively to its solubility in water and one that would be expected to

contribute negatively. 1 O S3 3 NH C CH3 cL L L

acetanilide

Solubility of Ionic Compounds Because of the importance of both ionic and ionic reactive intermediates in organic chemistry, the solubility of ionic compounds is worth special attention. Ionic compounds in solution can exist in several forms, two of which, pairs and dissociated ions, are shown in Fig. 8.2 on p. 344. In an ion pair, each ion is closely associated with an ion of opposite charge. In contrast, dissociated ions move more or less in- dependently in solution and are surrounded by several solvent molecules, called collectively the shell or solvent cage of the ion. Solvation refers to the favorable interaction of a dissolved molecule with solvent. When solvent molecules interact favorably with an ion, they are said to solvate the ion. Ion separation and ion solvation are mechanisms by which ions are stabilized in solution. If you think of the ion dissolution sequence in Fig. 8.2 as an ordinary , you can see that anything that favors the right side of this equilibrium tends to make ions sol- uble. The separation and solvation of ions reduce the tendency of the ions to associate into ag- gregates and, ultimately, to precipitate as from solution. Hence, ionic compounds are relatively soluble in solvents in which ions are well separated and solvated. What solvent properties contribute to the separation and solvation of ions? 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 344

344 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES

of the positive ion

S

+ – S S + – + –+ – + S +– +– + – + – + – more S – – ++–– ++ – + – + –+ solvent solvent + – ++–– ++–– ++–– ++ ––+ S – + ––+++–– ++ – ++ – + – S – + – ++–– ++–––+++––– + – + S S + –– +–– ++–– ++–– ++ –

– + – +++–––++–– ++ – + –+ S S + – + –– + –– ++–––++– + + – S –++ ––+++–– ++–– ++––– + – + – + –– +–––++–– + + S solvation shell of – + –– + –– ++–– +–– ++ – the negative ion – + – +++–––++–– +++–– +– + S + + –– + –– + –––++– + – ––++ ––++ –– ++ –– +–– + – + – + –++ –– ++–– ++ – + – + – + – ++ – + – + – dissolved ion pairs S + – – ++ –

+ – or higher aggregates S S (typically not S – very soluble) S crystalline material S

dissociated ions, each separately solvated

Figure 8.2 Ions in solution can exist as ion pairs and dissociated ions.The blue spheres are positive ions and the red spheres are negative ions.The solubility of an depends on the ability of the solvent to break the electrostatic attractions between ions and form separate solvation shells around the dissociated ions.Solvent molecules are represents by the gray ellipses.Solvation is very dynamic; that is, solvent molecules in the solvation shells are not fixed, but are rapidly exchanging with molecules in the bulk solvent.

The ability of a solvent to separate ions is measured by its dielectric constant e in Eq. 8.3 on p. 339. Look carefully at this equation again. The energy of attraction of two ions of oppo- site charge is reduced in a solvent with a high dielectric constant. Hence, ions of opposite charge have a reduced tendency to associate in solvents with high dielectric constants, and thus a greater solubility in those solvents. Solvent molecules solvate dissolved ions in various ways, which are illustrated in Fig. 8.3 for the interaction of the solvent water with dissolved chloride. A donor solvent can act as a Lewis to donate its unshared electron pairs to a cation, which acts as a Lewis acid. This covalent interaction is called a donor interaction. In addition, the dipole moments of solvent molecules can interact electrostatically with the charge of the ion. This means that the water molecule is oriented so that the negative end of its dipole moment vector is pointing to- wards the positive ion, thus creating a favorable electrostatic interaction by Eq. 8.3. This is called a charge–dipole interaction. Because the orientation of the water molecule is almost the same in both charge–dipole and donor interactions, the two interactions are sometimes considered to be different aspects of the same interaction. For solvation of the anion, a favor- able charge–dipole interaction can occur in which the solvent molecules are turned so that the positive ends of their dipole moment vectors are pointing towards the negative ion. Finally, if the solvent is protic and the anion can accept hydrogen bonds, the solvent can solvate a nega- tive ion by a hydrogen-bonding interaction. Solvation is dynamic. That is, although the solvent shells in Figs. 8.2 and 8.3 are shown as static structures, this figure represents a “snapshot” of a rapidly changing situation. The water molecules are constantly exchanging places with water molecules from bulk solvent, and the solvation mechanism within a solvent shell can change rapidly as well. 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 345

8.4 SOLVENTS IN ORGANIC CHEMISTRY 345

donor interaction (covalent interaction of an oxygen lone pair + hydrogen bonding with Na ) (hydrogen bonding of a water hydrogen with a chloride lone pair)

Na+ Cl–

charge–dipole interaction (electrostatic interaction of the dipole moment of water with the charge of the ion)

Figure 8.3 A “snapshot” of the interaction of solvent water molecules in the solvent shells of dissolved sodium and chloride ions.Although two donor interactions are shown for the cation and two hydrogen-bonding interac- tions for the anion, a greater number of such interactions can occur. Solvation shells can also contain more than six water molecules. Solvation is a dynamic process in which water molecules from the bulk solvent rapidly ex- change with water molecules in the solvent shell.

To summarize: The high dielectric constant of a polar solvent reduces the attraction be- tween ions of opposite charge; as a result, these ions easier to separate and bring into solution. Dissolved ions are stabilized (that is, kept in solution) by three general types of interaction: 1. Charge–dipole interactions, by which the dipole moment vectors of the molecules in a polar solvent are oriented so as to create an attractive (stabilizing) interaction with the charge of the ion. 2. Hydrogen-bonding interactions, by which dissolved ions can be stabilized by hydrogen bonding with solvent molecules. 3. Donor interactions, by which solvent molecules with unshared electron pairs can act as Lewis bases toward dissolved cations. These points show why water is the ideal solvent for ionic compounds, something you probably know from experience. First, because it is polar—it has a very large dielectric con- stant—it is effective in separating ions of opposite charge. Second, because it is protic—a good hydrogen-bond donor—it readily solvates anions. Third, because it is a Lewis base—an electron-pair donor—it can solvate cations by a donor interaction. Finally, its significant di- pole moment enables water to provide stabilizing ion–dipole attractions to both cations and anions. In contrast, hydrocarbons such as hexane do not dissolve ordinary ionic compounds because such solvents are apolar, aprotic, and nondonor solvents. Some ionic compounds, however, have appreciable solubilities in such as acetone or DMSO (see Table 8.2). Although these solvents lack the protic character that solvates anions, their donor 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 346

346 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES

capacity solvates cations, their substantial dipole moments provide favorable charge–dipole interactions, and their polarity separates ions of opposite charge. However, it is not surprising that, because polar aprotic solvents lack the protic character that stabilizes anions, most are less soluble in these solvents than in water, and salts dissolved in polar aprotic solvents exist to a greater extent as ion pairs (see Fig. 8.2).

PROBLEMS 8.19 Dimethylsulfoxide (DMSO, Table 8.2) has a very large dipole moment (3.96 D). Using struc- tures, show the stabilizing interactions to be expected between DMSO solvent molecules and (a) a dissolved sodium ion; (b) dissolved water; (c) dissolved chloride ion. 8.20 Acetone (Table 8.2) has a significant dipole moment (2.7 D). Using structures, show the sta- bilizing interactions to be expected between acetone solvent molecules and (a) a dissolved potassium ion; (b) dissolved water; (c) dissolved iodide ion.

APPLICATIONS OF SOLUBILITY 8.5 AND SOLVATION PRINCIPLES

A. Cell Membranes and Solubility Solubility is a crucial issue in drug action. If a drug is to be administered in an , it must have adequate aqueous solubility. However, water solubility is not the whole story. For drugs to act, they must get to their sites of action. For many drugs, this means that they must enter cells. The only way for a drug to get into a cell is for it to pass through the cell membrane, the “envelope” that surrounds the cell. Drugs and other substances pass through cell membranes by a variety of mechanisms; in some cases, transport requires carrier molecules imbedded in the membrane; and, in some cases, transport requires the expenditure of metabolic energy. However, in many cases, drugs simply pass unassisted through the cell membrane. It turns out that the abil- ity of a molecule to penetrate a cell membrane is very much a solubility issue. To understand this, let’s examine the structure of a cell membrane. Cell membranes contain a high of molecules called phospholipids. To under- stand what phospholipids are, we first need to understand what a lipid is. A lipid is a com- pound that shows significant solubility in apolar solvents. Because lipids are defined by a be- havior rather than a precise structure, a number of different biomolecule types fit into this category. For example, steroids (Sec. 7.6D) are lipids. Even though lipids might contain polar functional groups, their solubility behavior is dominated by their significant hydrocarbon character. Phospholipids are lipids that contain phosphate groups. However, membrane phospho- lipids have specialized structures. To understand these structures, let’s build a membrane phospholipid, phosphatidylethanolamine, from its component parts. To start with, all mem- brane phospholipids are built on a “scaffold.”

CH2 CH2 HO C OH (8.4) HO H glycerol (1,2,3-propanetriol) Two of the hydroxy groups of glycerol form linkages to fatty acids, which are themselves lipids consisting of carboxylic acids with long, unbranched hydrocarbon chains. These chains can contain 15–17 atoms, and they can also contain one or more cis double bonds. For