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Unformatted text preview: 1 CHAPTER 3 ACIDS AND BASES The tens of millions of organic compounds known undergo hundreds of millions of reactions. In order for us to be able understand and predict the course of these reactions in any manageable way, we need to systematize our knowledge and identify principles and patterns of chemical behavior. One important approach is to identify and define groups of compounds or chemical species that display similar chemical behavior. Two of the most important groups are acids and bases. All compounds identified as acids undergo essentially the same chemical reactions when they act as acids. All compounds identified as bases undergo essentially the same chemical reactions when they act as bases. And most important, any acid will react with any base in the same way. The usefulness and scope of this approach will depend how many compounds we can define as acids and as bases. 3.1 Brnsted Acids and Bases The first reasonably general definition of acids and bases was proposed by J. N Brnsted in the first quarter of the 20 th century. 1. An acid (also called a Brnsted Acid) is a chemical species that can donate a proton (H + ) 2. A base (also called a Brnsted base) is a chemical species that can accept a proton. These definitions make it very clear why an acid and a base react with each other. They also make it very clear what that reaction will be. The reaction between a Brnsted acid and a Brnsted base is the transfer 2 of a proton from the acid to the base. Such reactions are almost always reversible and almost always occur in solution. These definitions seem very simple and very general. But they have some important ramifications that help us to understand many important aspects of the chemical behavior of these compounds. We can represent a Brnsted Acid by the symbol HA, which is not intended to convey any information about the charge of the acid. Acids can be neutral, positively charged, or negatively charged. But no matter what their charge, they must have an H which can be donated by the acid as H + . We can then symbolize this behavior with a skeleton equation that is not balanced with respect to electrical charge. HA A + H + The equation indicates that the acid HA is donating a proton. It does not indicate the recipient of this donation and therefore it does not describe an actual chemical process. An acid must donate a proton to something; free protons cannot exist to any significant extent under ordinary chemical conditions. The actual chemical process is a proton transfer from the acid to another chemical species that can accept the proton. A chemical species that can accept a proton is by definition a Brnsted Base, We can represent a Brnsted Base by the symbol B, which is not intended to convey any information about the charge of the base. Bases also can be neutral, positively charged, or negatively charged. We can symbolize this behavior with a skeleton equation that is not balanced with respect to electrical charge. electrical charge....
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- Fall '09