Solubility

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Solubility and Functional Groups _______________________________________________________ You will recall from general chemistry that a solution has two components: the solvent, which is the substance present in greater amount, and the solute, which is dissolved in the solvent. Solubility is defined as the mass (in grams) of solute dissolved in 100 g of solute at saturation. Molar solubility is defined as the amount (in moles) of solute per liter of saturated solution. The solubility of one compound in another is related to the strength and type of intermolecular forces that exist between the two components. These forces arise from factors of molecular shape and electronegativity difference (∆χ), and are influenced by the specific functional groups contained within the molecule. A functional group is a group of atoms bonded in a particular way, with a predictable chemical and physical behavior. Examples are shown in Figure 1. Figure 1. Examples of functional groups

The class of compound is given under each structure; the functional group name, if different, is in parentheses. H N amine (amino)

OH

Br alkyl bromide (bromo) alkene (alkenyl) O O H aldehyde ketone (carbonyl) alkyne (alkynyl) O

alcohol (hydroxyl)

O N C nitrile epoxide

ether

O OH carboxylic acid (carboxyl)

O Cl acyl halide (haloformyl)

O O

O

O O

O N H amide (carboxamide)

anhydride

ester

carboxylic acid derivatives

Solute molecules that experience strong intermolecular attractions to solvent molecules will be more likely to dissolve. On the other hand, if the solute molecules experience more attraction to each other than to the solvent molecules, then it is more energetically favorable for them to remain together and for the solvent particles to remain together, so

we would observe little solubility. Applying the broad generalization like dissolves like, we can make reasoned predictions about the solubility of a chosen substance in a given solvent. For example, polar compounds tend to dissolve other polar compounds, while non-polar compounds tend to dissolve other non-polar compounds. Recall that a polar molecule has a permanent electric dipole moment, but a nonpolar molecule has no net molecular dipole moment. Sucrose (table sugar) a polar compound with many hydroxyl functional groups, dissolves readily in water (another polar compound), but does not dissolve in hexane (a nonpolar six-carbon hydrocarbon). As you can see in Figure 1, the presence of most functional groups suggests significant electronegativity differences, and thus the potential for an overall molecular dipole. On the other hand, molecules containing only hydrogen and carbon–through a combination of small ∆χ values and symmetric molecular structure–are precluded from having a permanent dipole. A brief review of the intermolecular forces between neutral molecules follows: Dipole-dipole: When two different molecules are both polar, strong dipole-dipole attractions may result. The strength of these attractions will increase with the magnitudes of the dipole moments and as the molecules approach each other more closely. See Figure 2. Molecules with permanent electric dipole moments orient in such a way as to maximize attractions, which will lower the potential energy of the system. The molecules align so that the positive pole of one molecule approaches the negative pole of a neighboring molecule. Positive-positive or negative-negative approach would cause repulsions, raising the potential energy of the two-molecule system. We can consider hydrogen bonding as a special case of dipole-dipole attraction. See Figure 2(b). For hydrogen bonding to occur, one molecule must O have at least one lone pair O H H H H O of nonbonding electrons, O O and the other atom must H H have a hydrogen atom a b bound covalently to a small and highly Figure 2. (a) Dipole-dipole interaction in acetone. electronegative atom– (b) Hydrogen bonding between water molecules. either N,...
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