Kaplan MCAT General Chemistry Review

Medical professionals aren’t the only ones concerned about drug solubilities and routes of administration. There’s a science to illegal drugs, too. People in the general population who use illegal drugs (and those who produce them) are knowledgeable about their available forms, as well as the most effective and efficient modes of delivery. One of the clearest examples of this is the difference in the ways that people use the two forms of cocaine. C₁₇H₂₁NO₄ (cocaine) is a large alkaloid compound derived from the coca plant. It is a central nervous stimulant that has been used medicinally, ceremonially, and recreationally since at least the pre-Columbian era. About 125 years ago, Pope Leo XIII purportedly carried around a hipflask filled with cocaine-laced wine called Vin Mariani. The 1886 original recipe for what is now the world’s most famous cola included coca leaves (from which this famous cola derives its name). Cocaine was once used to treat heroin addiction. Sigmund Freud wrote rhapsodically about its ability to cause “exhilaration and lasting euphoria.” By 1903, however, the American Journal of Pharmacy was warning that most cocaine abusers were “bohemians, gamblers, high- and low-class prostitutes, night porters, bell boys, burglars, racketeers, pimps, and casual laborers.”

Today, cocaine is used primarily in two different forms. Most commonly, the alkaloid compound is reacted with hydrochloric acid (which protonates its tertiary amine functional group), extracted with water, and dried to an aqueous soluble powder (cocaine hydrochloride); this powder either is snorted (insufflated) into the nasal cavity, where it is absorbed into the capillary beds, or is injected directly into the venous circulation. The salt form, however, because it has a very high boiling point close to the temperature at which it burns, cannot be smoked. To produce a form of cocaine that can be vaporized and inhaled from a pipe, the cocaine hydrochloride must be reacted with a base, typically either ammonia (to produce pure “freebase cocaine”) or sodium bicarbonate (to produce the less-pure “crack” cocaine). The base reacts with the protonated tertiary amine, removing the hydrogen ion to re-form the neutral alkaloid compound. The freebase cocaine is water-insoluble and usually extracted with ether, or it is left in the aqueous solution, which is heated and evaporated. The freebase or crack form of cocaine has a much lower boiling point; consequently, it can be smoked without risk of burning (combusting).

What a difference a little hydrogen ion can make! The complexities of drug delivery can in part be related to the presence or absence of the hydrogen ion—a mere proton! In this chapter, our focus will be those two classes of compounds—acids and bases—that are involved in so many important reactions. Acid-base reactions are an important focus for the MCAT; in fact, the neutralization reaction is one of the most commonly tested reaction types on Test Day. We will begin with a review of the different definitions of acids and bases and their properties, including the characterization of acids and bases as either strong or weak. Focusing on weak acids and bases, we will discuss the significance of the equilibrium constants, K_a and K_b, for acids and bases, respectively. Finally, we will review acid-base titrations and buffer systems.

Definitions

Over the last century, chemists have used different definitions to identify compounds as acids or bases. Three definitions have been proposed, and each is progressively more inclusive: Every Arrhenius acid (or base) can also be classified as a Brønsted-Lowry acid (or base), and every Brønsted-Lowry acid (or base) can also be classified as a Lewis acid (or base).

ARRHENIUS

The first definitions of acids and bases were formulated by Svante Arrhenius toward the end of the 19th century. Arrhenius defined an acid as a species that dissociates in water to produce a hydrogen ion, H⁺, and a base as a species that dissociates in water to produce a hydroxide ion, OH^-. These definitions, though useful for many reactions, fail to describe acidic and basic behavior in nonaqueous media.

MCAT Expertise

This is the most specific definition of acids and bases and is the least useful on the MCAT.

BRØNSTED-LOWRY