Lesson16 - Structure-Reactivity Relationships A change in...

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Structure-Reactivity Relationships A change in structure corresponds to a chemical reaction. Reactivity is the term we use to describe the potential of a structure to undergo chemical change. Some structures are more prone to react than others. Structures that are likely to undergo chemical change are said to be reactive . Structures that are resistant to chemical change are said to be unreactive. We sometimes choose to speak about the complementary property known as stability. Structures that are highly reactive are generally unstable . Structures that are unreactive are generally stable . We want to view molecular structure from the perspective of potential to undergo chemical change. A goal is to establish trends that will allow you to examine a structure and make predictions about its chemical characteristics. Just like a physician diagnoses patients, chemists learn to associate certain features of structure with the tendency to react. From these trends emerge the structure- reactivity relationships . Potential energy (specifically, chemical potential ) is the link between structure and reactivity. The greater a structure’s chemical potential, the greater is its reactivity. Understanding the link between structure and reactivity will help us decide which, among several possible changes, are most reasonable (e.g., pathways having intermediates with exceedingly high energies are not likely to be reasonable and should therefore be avoided, especially if a pathway with lower energy intermediates is available). structure chemical potential reactivity
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Analysis of Chemical Potential : Chemical potential makes sense in the context of a chemical reaction because we can compare what’s on the two sides of a reaction arrow ( ) and determine the difference in chemical potential. By comparing what’s on one side with what’s on the other, it becomes possible to know if the chemical change is favored or not. Does the left-hand-side or the right-hand-side have greater chemical potential? The favored direction will be from high to low chemical potential. An analysis of chemical potential can be done for individual elementary steps, or an overall chemical reaction, as the problem demands. The things that contribute to chemical potential - the things that need to be compared - are: •Bond energy changes •Strength (reactivity) of electrophiles and /or acids •Strength (reactivity) of nucleophiles and/or bases •Weak bonds •Low energy LUMO = strong acid (reactive electrophile) •High energy HOMO = strong base (reactive nucleophile) •Permanent charges What Factors Contribute to High Chemical Potential? A
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This note was uploaded on 01/25/2012 for the course CHEM 232 taught by Professor Miller during the Spring '08 term at University of Illinois, Urbana Champaign.

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Lesson16 - Structure-Reactivity Relationships A change in...

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