applications a nJ analogies "Crack - A High-Interest C O ~ D O U ~ ~ Illustrating Important concepts in General Chemistry William Hoyt Saint Joseph's College Windham, ME 04062 Although cocaine, like all narcotics, can be used as a n anesthetic to relieve ~ a i in n severe iniuries. the e ~ i d e m i c of "crack" cocaine aAd associated crime ekpha$ize the necative a s ~ e c t sof the alkaloid. Most people a r e a w a r e t h a t "crack" gives a larger initial "kick" because i t can be vaporized readily and affects the brain receptors more quickly. This knowledge is already i n most students' database. T h e difference between "crack" (1) and "regular" cocaine (2) graphically illustrates the effects caused by differences in intermolecular bonding a n d t h e acid-base chemistry of amines. Before presenting the following, I tell the class that "crack" is very dangerous a s well a s illegal and highly addictive. Use of cocaine can lead to a ~ a r a n o i d~svchosis or death by heart failure (i).This happened to Len Bias, top draft choice of t h e Boston Celtics. He took cocaine once, collapsed, and died rapidly from heart failure. I t also can cause spontaneous abortions in pregnant women ( I ) . The lethal dose is between 0.5 g and 1.2 g, although toxic effects have heen reported for as little as 20 m -s (1). , about 1/10 the size of a lead-~encil eraser. Cocaine can occur in two forms - an ionic hydrochloride or ammonium salt (2) and an uncharged molecule (1) called the "free base". Properties of the two forms are summarized in the table (2). CH3
I
CH3 0
V
0
Crack, 1
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Journal of Chemical Education
Middle Georgia College Cochran. GAS1014
Properties of the Two Forms of Cocaine Free Base (1) Melting Point Water solubility, gImL Chloroform solubility, glmL Ether solubility, glml
98 "C (very volatile above90"C) 1.67x 1o - ~ 1.43 0.29
Hydrochloride (2) > 195 "C
2.5 0.08 insoluble
(reaction 2). The Briinsted-Lowry concept of proton donor and acceptor is clearly illustrated. These acid-base reactions produce compounds with different types of intermolecular attractions. The increased volatility, lower melting point, and enhanced solubility of the free hase in low-polarity solvents are to be expected in a compound where there are no ionic attractions or Hbonds present, while t h e high melting point and high water solubility are characteristic of i o n ~ ccompounds. The solubility data in the table can be used to illustrate the concept of solvent extraction. An aqueous solution of t h e hydrochloride is treated with hydroxide transforming it into the free base. Then when ether i s added and the two solvents shaken then allowed to separate, the free base ends up in the ether layer, where i t can be recovered by evaporation of the ether. A Low-Interest Example Another examole of amine acid-base chemistry that students may be familiar with is addinz a touch of vinegar or citric acid to fish to remove the "fishy" (aminel odor. The crack illustration is better because i t is of such high interest.
0
Cocaine.2
Although purification is a multi-step procedure, the reaction is a simple acid-base reaction. The hydrochloride and free base are interconvertible. The basic nitrogen atom in the free base can accept a proton (reaction while the acidic proton of the hydrochloride can be donated to a base
322
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RONDELORENZO
~ 1.
i t cited ~ ~ ~ t ~ ~ ~ o~i,,~ . ~ " iR.. . ~ d .DFUSS . pncts compnriso.. sainthuir: F ~ C ~and ;- F~~URS,
1992 pp 2260-2251. 2. Budavan. Susan, Ed., The Memk Index: Rahway. NJ:Merck & Co.. 1989,p 383, item 2450.
