Unit 6- Module 2

Unit 6- Module 2 - Module 2 Unit 6 Changing the Environment...

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Module 2- Unit 6 Changing the Environment John Pollard University of Arizona One of the most amazing and powerful things that can emerge from chemical thinking is the ability to control chemical reactions. When thinking about designing a pharmaceutical drug to target a specific part of the body, this thinking is crucial. How can we predict the effect of different environmental conditions on the structure and properties of drugs in our bodies AND how can we take advantage of this knowledge to control their behavior? To answer these questions, we first need to understand from a thermodynamics and kinetics perspective what affect changing the concentrations of reactants and products, the temperature and pressure of the environment and the solvent environment has on chemical reactions in general. To illustrate these effects, we will focus on the very fast process of proton transfer, which is the primary characteristic of an acid-base reaction. There are three main items that we will explore to further our understanding of how to control the extent of these process: concentration, temperature and solvent effects. Concentration To better illustrate the effect of concentration on reaction extent, let’s consider a common acidic drug phenobarbital. We can represent in a generic sense the reaction between this substance and water as the addition of a common weak acid reaction. The K a for this reaction is: To start, we must first establish what the concentrations of [H 3 O + ], [A¯] and [HA] are at equilibrium. The pK a of phenobarbital is 7.40 which means the K a = 3.98 x 10 -8 . If the initial concentration of phenobarbital is represented by C o , then we can say the following: ] [ ] ][ [ 3 HA A O H K a + =
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