Oh—what is that smell? You and your mother are taking an afternoon walk through the public rose garden, a pleasant activity for a pleasant summer day, and everything was so pleasant until you smelled . . . it, whatever it is. Your mother doesn’t have to wonder long until her own olfactory system is screeching the alarm, too. Oh, it’s just horrible. It smells like rancid almonds. Although your better instincts tell you to walk quickly away, you pause and, holding your breath, crouch down to get a better look. You are a premed, after all, and you did just synthesize cinnamaldehyde in your orgo lab, so your curiosity is understandable.
Scanning the leaves of some plants at the base of the rosebush, you notice a few green bugs whose backs give the impression of a shield. A small stick lies temptingly close to your foot. You can’t resist and so you grab the stick and position it for optimal poking, but just as the pointy end gets within a few inches of the insect, which now senses that its own pleasant afternoon is about to be ruined, it raises its hind quarters in the direction of the stick—that is to say, in your direction—exposing its sophisticated defense weaponry. Stink bugs! you exclaim, as you drop the stick and take what you believe are a sufficient number of steps to remove yourself from the bug’s bombing range.
Our world is filled with aromas and odors, some pleasant and some not so much. What is it that we are smelling when we “smell something”? Well, every odor that we perceive is the result of an interaction between the chemical receptors of our olfactory systems and some chemical compound. A stink bug “stinks” because it produces a highly concentrated solution of volatile compounds that we (and anything else that would dare to disturb it) perceive as malodorous, noxious, and irritating. It’s a pretty ingenious defense system: Make yourself so disgusting, so distasteful, that everyone will leave you alone, or at least think twice before bothering or eating you. Interestingly enough, the primary compounds in the stink bug’s stink bomb are hydrogen cyanide (a highly toxic compound that inhibits cytochrome c oxidase, thereby blocking aerobic respiration) and benzaldehyde.
Wait, benzaldehyde? Yes, benzaldehyde—that relatively simple compound consisting of a benzene ring substituted with an aldehyde functional group. You need to recognize it for the Biological Sciences section of the MCAT. Remember that we classify benzene ring compounds as part of the larger class of molecules called aromatic compounds. Therefore, you can smell benzaldehyde because it is aromatic and, like some other aromatic compounds, it vaporizes at room temperature and reaches your olfactory system as gas particles. Now, you might be surprised to learn that benzaldehyde is the key ingredient in artificial almond extract. At low concentrations, it gives a pleasant aroma of toasted almonds. However, at high concentrations, its odor is that of rotten, stinking almonds, and it is a noxious irritant to skin, eyes, and the respiratory tract. We do not recommend stink bug juice as a suitable cooking substitute for almond extract.
Benzaldehyde is a compound composed of seven carbon atoms, six hydrogen atoms, and one oxygen atom. One mole of benzaldehyde has a mass of 106.12 grams. It can react with other atoms or compounds to form new compounds (pure substances composed of two or more elements in a fixed proportion). They can be broken down by chemical means to produce their constituent elements or other compounds. They are characterized according to the same systems of traits: physical properties and chemical reactivities.
This chapter focuses on compounds and their reactions. It reviews the various ways in which compounds are represented: Empirical and molecular formulas and percent composition will be defined and explained. There is a brief overview of the major classes of chemical reactions, which we will examine more closely in subsequent chapters, and finally, there is a recap of the steps involved in balancing chemical equations with a particular focus on identifying limiting reagents and calculating reaction yields.
Molecules and Moles
A molecule is a combination of two or more atoms held together by covalent bonds. It is the smallest unit of a compound that displays the identifying properties of that compound. Molecules can be composed of two atoms of the same element (e.g., N2 and O2) or may be composed of two or more atoms of different elements, as in CO2, SOCl2, and C6H5CHO (benzaldehyde). Because reactions usually involve a very large number of molecules, far too many to count individually, we usually measure amounts of compounds in terms of moles or grams, using molecular weight to interconvert between these units.
Ionic compounds do not form true molecules because of the way in which the oppositely charged ions arrange themselves in the solid state. As solids, they can be considered as nearly infinite, three-dimensional arrays of the charged particles that comprise the compound. Remember, in Chapter 3 we mentioned that NaCl in the solid state is a 6:6 coordinated lattice in which each of the Na+ ions is surrounded by six Cl- ions and each of the Cl- ions is surrounded by six Na+ ions. As you might imagine, this makes it rather difficult to clearly define a sodium chloride molecular unit. Because no molecule actually exists, molecular weight becomes meaningless, and the term formula weight is used instead. (However, this is a technical distinction over which you ought not to sacrifice too much sleep.)
Bridge
Ionic compounds form from combinations of elements with large electronegativity differences (that sit far apart on the periodic table), such as sodium and chlorine. Molecular compounds form from the combination of elements of similar electronegativity (that sit close to each other on the periodic table), such as carbon with oxygen.
MOLECULAR WEIGHT
We’ve mentioned already that the term atomic weight is a misnomer, because it is a measurement of mass, not weight (another distinction not worth any sacrifice of sleep), and the same applies here to our discussion of molecular weight: It’s really a measurement of mass. Molecular weight, then, is simply the sum of the atomic weights of all the atoms in a molecule, and its units are atomic mass units (amu). Similarly, the formula weight of an ionic compound is found by adding up the atomic weights of the constituent ions according to its empirical formula (see the following example), and its units are grams.