# 4.10: Applications and Solubility of Covalent Compounds

Skills to Develop

• Know the basic difference between the terms polar and nonpolar.
• Apply the like dissolves like solubility rule for covalent compounds
• Understand why molecular shapes are important to pharmaceutical chemists and biologists.
• Appreciate how medications/vitamins need to be soluble in the body.
• Pick a vitamin and memorize highlighted information
• Realize that molecules are three dimensional in nature.
• Know the applications of covalent compounds in this section

### The Solubility of Covalent Compounds

Unlike ionic solubility, covalent compound solubility cannot be determined by a table. Instead, structures and three-dimensional shapes must be drawn. Once a correct geometry has been determined, the compound would be classified as being polar on nonpolar. Polar species are soluble in water, while nonpolar species are soluble in oils and fats. Covalent solubility uses the like dissolves like rule. This means that substances with the same type of polarity will be soluble in one another. Moreover, compounds with differing polarities will be insoluble in one another.

Oil and water form a heterogeneous mixture due to their differing polarities. These substances are immiscible (not mixable). In contrast, alcohol dissolves in water to form a homogeneous mixture.

Figure $$\PageIndex{1}$$: Two phases (water and oil) in the same state of aggregation (liquid). Images used with permission (CC BY-SA 4.0; Victor Blacus and Elizabeth R. Gordon, respectively L to R)

In this class, we will not explore molecular geometries that are used to determine polarity. Instead, the polarity of a substance will be provided. It is important to remember that water is polar and oil/fat is nonpolar. If the polarity of a substance is given, you should be able to classify it as being water or oil/fat soluble.

Phet Simulation: Molecular Shapes

In determining polarity, chemists look to the power of atom's' nucleus. The protons from an atom's nucleus are capable of attracting another atom's electrons. Within a covalent bond, valence electrons are pulled toward's an atom that has a more powerful nucleus. This pull is called electronegativity. If different atoms are connected in a bond, then one tends to be more electronegative than the other. Molecules that have an overall pull in one direction are labeled as being polar species. Look at the structure of water that is shown below. This structure of this molecule shows the bonding electrons being pulled towards oxygen. Therefore, oxygen has a more powerful nucleus than hydrogen. Water's bent molecular shape does not cancel out the individual dipole pulls. As a result, water has an overall pull and is classified as being polar.

Figure $$\PageIndex{2}$$: The molecular geometry of a molecule affects its polarity. Each CO bond has a dipole moment, but they point in opposite directions so that the net CO2 molecule is nonpolar. In contrast, water is polar because the OH bond moments do not cancel out.

Carbon dioxide also has dipoles (arrows) that pull in opposing directions. This molecule does not have an overall pull in one direction and is classified as being nonpolar. Some other molecules are shown in the figure below.  The top three assymetrical molecules are all polar. They all have dipoles (pulls) that do not cancel.  All of these molecules would be soluble in water. The bottom three molecules are nonpolar. These are symmetrical molecules that have dipoles that cancel. Both of these molecules would be oil or fat soluble.

Figure $$\PageIndex{3}$$: Some examples of polar and nonpolar molecules based on molecular geometry.

Covalent solubility is important in the pharmaceutical industry. If a medication is not water soluble, then it will not dissolve in the bloodstream and react in the active site of the body in a timely and potent fashion. Watch the video below to obtain a basic understanding of how ibuprofen travels through the body to reduce pain and/or inflammation.

The polarity of vitamins can affect how long they remain in the body. Vitamins B and C are both water soluble and remain in the body for a short period of time. Our bodies must intake these vitamins more frequently. On the other hand, vitamins A, D, E, and K are all fat or oil soluble. Nutrients that are fat soluble remain in the body longer. These vitamins accumulate easily and can be toxic if recommended doses are surpassed. Pick one vitamin from the table below and memorize the common/chemical names, one source, and one function. In addition, note the solubility (water or fat) of your selected vitamin.

Vitamins with their sources and functions. Image taken from: https://upload.wikimedia.org/wikiped...mins_Table.png

Drug Design