MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020
Bonding and Chemical Interactions
Introduction
Chapter Profile
The content in this chapter should be relevant to about 13% of all questions about general chemistry on the MCAT.
This chapter covers material from the following AAMC content categories:
4C: Electrochemistry and electrical circuits and their elements
5B: Nature of molecules and intermolecular interactions
Introduction
The Maillard reaction is one of the most important chemical processes that occurs while cooking. The reaction mechanism itself is one with which you are closely familiar from your studies of organic chemistry: a nucleophilic reaction between the amino terminus of the peptide chain of a protein and the carbonyl functionality of a sugar to form an N-substituted glycosylamine. This compound undergoes a complex series of rearrangements and other reactions to produce a set of compounds that gives cooked food its pleasing color and delectable flavor. This reaction is especially important for browning meat.
When the surface of the meat comes into contact with the hot surface of a pan or grill, the proteins and sugars on the meat’s exterior begin interacting via the Maillard reaction. The pan must be sufficiently hot to bring the exterior of the meat to a temperature of 155°C (310°F), the optimal temperature for the reaction to occur. So how does a grill master achieve the impossible: generating very high heat for the exterior but not overcooking the interior? The answer lies, in part, in drying the meat. When meat that has a lot of water on its exterior surface hits the hot pan, the first process that takes place is the boiling of the water. Boiling is a phase change from liquid to gas and occurs at a constant temperature; water’s boiling point is 100°C (212°F). Because this temperature is considerably lower than that necessary for the Maillard reaction, no browning will occur and the flavor compounds will not form. The lesson here is: if you want a tasty steak, always dry the surface of your meat!
Of course, the real lesson is the topic of discussion for this chapter: bonding and chemical interactions. We will not address complex chemical bonding, such as that which takes place in the Maillard reaction, in this chapter. Rather, this chapter will address the basics of chemical bonding and interactions. Here, we will investigate the nature and behavior of covalent and ionic bonds. We will also review a system—Lewis structures—by which bonding electrons are accounted for, and we will address the main principles of valence shell electron pair repulsion (VSEPR) theory. Finally, we will recount the various modes of interaction between molecules: intermolecular forces.