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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Since the K a is small, we can make a few valid assumptions that simplify the calculation of x. First, since the reactions extent is very small as reflected by the K a value, we can assume that the change in concentration of HA is negligible. In addition, we can also assume that the amount of x produced by the reaction is much larger than the pre-existing concentration of H 3 O + . This allows us to express the K a as: In this case, the x value represents the concentrations of both H 3 O + and the conjugate base of phenobarbital [A¯]. What we see from this analysis is that the larger the initial concentration of HA, the larger the x value is and hence the lower the pH is. A similar procedure can be used to determine the equilibrium concentrations of species when bases react in water. The only difference is that in the case bases, the x value is equal to the OH¯ concentration. It is common to only provide the pK a values for acids and bases. This only means that the relationship between K a , K b and K w must be used to calculate the K b such that x is determined properly. Equilibrium Perturbations
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This note was uploaded on 03/29/2010 for the course CHEM 152 taught by Professor Grall during the Spring '08 term at Arizona.

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

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