Solvent

Polarity

I'm sure you remember that a water molecule consists of two hydrogen atoms covalently bonded to a single oxygen atom. Since the oxygen atom is much more electronegative than the hydrogen atoms, the bonds are polar and the electrons are pulled toward the oxygen. Now the molecule also has a bent shape, and angular shape, so that the molecule itself is a polar molecule.

Model of water molecule. [66mod01.JPG]

As a result, water is an excellent solvent for other polar molecules, particularly those which have hydrogen bonded to either oxygen or nitrogen atoms. This is because it is possible to have hydrogen bonding between the molecules.

Model of water molecules hydrogen bonding to alcohol and amine. [66mod02.JPG]

Water is also an excellent solvent for both positive and negatively charged ions. With a positively charged ion, water molecules can surround the ion with the negative oxygen end nearest the positive charge and with the negative ion, a water molecule can have its positively charged hydrogen end closest to the ion. The natural attraction between positive and negative keeps the ions surrounded by water molecules and therefore dissolved in solution.

Models of water ion-dipole bonding to ions. [66mod03.JPG]

Many biochemicals are either polar molecules or charged ions (or both) so water makes an excellent solvent for them. Being dissolved in water allows these biochemicals to react readily because they can mix rapidly, and it also allows them to be transported rapidly from where they are produced or ingested to where they are needed, whether that is just across a cell or moving from one end of the body to the other.

Models of water with hydrogen bonding and ion-dipole bonding to polar molecules and ions. [66mod04.JPG]

Insoluble Tissues

Water's high polarity makes it a very poor solvent for nonpolar molecules and, oddly enough, this is another important and useful quality. Certain body tissues should not dissolve readily. Things such as bone, hair, muscles and cell membranes would be useless if they dissolved in water. These materials contist of nonpolar molecules and that ensures that they will be able to maintain their integrity in the presence of water.

Hydrophilic and Hydrophobic

Quite often the terms hydrophilic and hydrophobic are used to classify compounds. Hydrophilic means water-loving and hydrophobic means water-fearing.

	Hydrophilic compounds are those which are polar and able to form hydrogen bonds or dipole-dipole bonds with water.
	Hydrophobic molecules are nonpolar and therefore they do not dissolve readily in water. They tend to form structures with each other that are stabilized by the weaker van der Waals bonds and not get incorporated into aqueous solutions.

In the past we have made this distinction but we referred to these compounds simply as being either soluble in water (hydrophilic) or insoluble in water (hydrophobic).

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E-mail instructor: Sue Eggling

Clackamas Community College
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