2.9 Protic and Aprotic Solvents

As we learned in Section 2.7, ionic compounds can have very high solubility in a polar solvent such as water, due to the strong solvation set up by ion–dipole interactions in the resulting solution. For example, about 35 g of sodium chloride (NaCl) dissolves in 100 mL of water (dipole moment = 1.9 D). Dimethyl sulfoxide (DMSO) is even more polar (dipole moment = 4.0 D), but dissolves only about 0.4 g NaCl.

Two equations showing the solubility of sodium chloride in water or H2O, and in dimethyl sulfoxide or DMSO. The first equation, numbered (2-2a), shows 35 grams of solid NaCl being dissolved in H2O to give sodium cations and chloride anions. The second equation, numbered (2-2b), shows 0.4 grams of solid NaCl being dissolved in DMSO to give sodium cations and chloride anions. An accompanying note reads, �NaCl is highly soluble in water but not in DMSO.�

Connections In addition to being an important solvent in chemistry, DMSO has a variety of medicinal uses. It is used topically to treat some chronic inflammatory conditions and, because it was discovered to penetrate organs without damaging them, it can be used to deliver some drugs into biological systems. Interestingly, when DMSO contacts the skin, some people taste garlic in the mouth.

Condensed structural formula of dimethyl sulfoxide or DMSO. The structure shows a sulfur atom bonded to two methyl groups by single bonds and to an oxygen atom by a double bond.

There must be more to the issue than just the magnitude of the solvent’s dipole moment. What also comes into play is how close the ions can approach the partial charges of the polar solvent molecule. As shown in Figure 2-24, the partial negative and partial positive charges of water are well exposed to the Na+ and Cl– ions, respectively, so these ions are able to approach the partial charges rather closely. Thus, the solvation of each ion is relatively strong.

Two illustrations show the interactions between sodium cation and chloride anion when salt dissolves in water. The first illustration shows a sodium cation enclosed in a blue circle and surrounded by electrostatic potential maps of six water molecule, each represented by a ball-and-stick model. The triangular electrostatic map of each water molecule is shaded in red at the upper end representing the oxygen atom, and in blue at the two lower ends, each representing a hydrogen atom. The central portion is shaded in yellow, green, and turquoise from top to bottom. The red end of each electrostatic map shows a partial negative charge, and points toward the central ion. A note above this illustration reads, �The negative end of water�s dipole solvates the sodium cation strongly.� The second illustration shows a chloride anion enclosed in a red circle and surrounded by electrostatic potential maps of six water molecule, each represented by a ball-and-stick model. The triangular electrostatic map of each water molecule is shaded in the same manner as in the first illustration. Here, the blue ends of each electrostatic map show a partial positive charge and point toward the central ion. A note above this illustration reads, �The positive end of water�s dipolesolvates the chloride anion strongly.� The caption reads, Solvation of NaClin water: a. The sodium ion is stronglysolvated because the partial negativecharge of water is well exposed.b. The chloride ion is strongly solvatedbecause the partial positive charge ofwater is well exposed.
FIGURE 2-24 Solvation of NaCl in water (a) The Na+ ion is strongly solvated because the partial negative charge of water is well exposed. (b) The Cl– ion is strongly solvated because the partial positive charge of water is well exposed.

Figure 2-25a shows that the partial negative charge of DMSO is also well exposed, so Na+ is strongly solvated by DMSO, just as it is strongly solvated by water. However, notice that the partial positive charge of DMSO is flanked by two bulky CH3 groups. This makes it difficult for the Cl– ion to approach DMSO’s partial positive charge (Fig. 2-25b). We say that the methyl groups introduce steric hindrance. Thus, Cl– is not solvated very strongly.

Two illustrations show the interactions between sodium cation and chloride anion when salt dissolves in dimethyl sulfoxide or DMSO. The first illustration shows a sodium cation enclosed in a blue circle and surrounded by six DMSO molecules, each represented a space-filling model. The models show a central sulfur atom bonded to two methyl groups by single bonds and to an oxygen atom by a double bond. The oxygen atom in each DMSO molecule carries a partial negative charge, and points toward the central ion. A note pointing to these partial negative charges, �Partial negative charge very accessible.� The second illustration shows a chloride anion enclosed in a blue circle and surrounded by fourDMSO molecules. The central sulfur atom in each DMSO molecule carries a partial positive charge, and points toward the central ion. A note pointing to these partial negative charges, �Partial positive charge less accessible.� The caption reads, Solvation in DMSO: a.Sodium ion is solvated strongly in DMSO because thepartial negative charge of DMSO is well exposed. b.Chloride ion is not solvated very strongly byDMSO, because DMSO�s partial positive charge is buried inside the molecule.
FIGURE 2-25 Solvation in DMSO (a) Na+ is solvated strongly in DMSO because the partial negative charge of DMSO is well exposed. (b) Cl– is not solvated very strongly by DMSO, because DMSO’s partial positive charge is buried inside the molecule.

These characteristics of water and DMSO, as they pertain to the solubility of ionic compounds, are not unique to just these two solvents. Water is an example of a protic solvent, because it possesses a H-bond donor—in this case, an O—H covalent bond. DMSO has no H-bond donors, so it is an aprotic solvent. Other examples of polar protic solvents and polar aprotic solvents are shown in Table 2-7.

Table 2-6 is titled, common polar protic solvents and polar aprotic solvents. The table has four columns and five rows. The rows represent different solvents. The columns represent the structures and names of these solvents. Data are included in the accompanying table. | Polar protic solvents Polar protic solvents Polar aprotic solvents Polar aprotic solvents | Structure Name Structure Name | Central oxygen atom bonded to two hydrogen atoms, one of which carries partial positive charge. Water Central sulfur atom with a partial positive charge bonded by single bonds to two methyl groups and by a double bond to an oxygen atom. Dimethyl sulfoxide (DMSO) | Two-carbon chain with one carbon bonded to a hydroxyl group, in which the hydrogen atom carries partial positive charge. Ethanol Central carbon atom with a partial positive charge bonded by single bonds to two methyl groups and by a double bond to an oxygen atom. Propanone (Acetone) | Carbon atom bonded to three hydrogen atoms and a carboxyl group, in which the hydrogen atom carries partial positive charge. Ethanoic acid (Acetic acid) Nitrogen atom bonded to two methyl groups and an aldehyde group, in which the carbon atom carries a partial positive charge. N,N-Dimethylformamide(DMF)

YOUR TURN 2.17
SHOW ANSWERS

In Table 2-7, circle and label each potential hydrogen-bond donor.

Hydrogen-bond donors can be H—O, H—N, or H—F bonds. The only solvents in Table 2-7 that have a hydrogen-bond donor are water, ethanol, and ethanoic acid.

A table with two columns and four rows show the structural formulae and name of the compounds. Data are included in the accompanying table.

Like water, all polar protic solvents have easily accessible partial negative and positive charges. Polar aprotic solvents, on the other hand, have well-exposed partial negative charges, but their partial positive charges tend not to be very accessible. Therefore:

●  Polar protic solvents tend to solvate both cations and anions very strongly.

●  Polar aprotic solvents tend to solvate cations very strongly, but not anions.

The importance of these attributes extends far beyond understanding the solubilities of ionic compounds. As we will see in Chapter 9, the solvation characteristics of protic and aprotic solvents play a major role in governing the outcomes of reactions.

problem 2.27 Will KSCH3 be more soluble in ethanol or acetone? Explain.