Lesson15 - Understanding Reactivity Chemistry is the study...

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Understanding Reactivity Chemistry is the study of change. Up to this point, the molecular changes we have studied involved internal changes such as conformational rotation about single bonds of alkanes or ring flipping of cyclohexane. None of these changes involved the breaking and making of covalent bonds. The chemical change we will study from this point forward involves changes in electron configuration. Much of the language that was developed in previous lessons will apply to covalent bond changes. Our description of these changes will greatly benefit from the curved arrow notation that we began to study when we discussed resonance. The discussion of intermediates and transition states from the last lesson will be utilized as well. Our goal in this course is to learn and understand the chemical transformations (i.e., reactions ) common to molecular compounds. To achieve this goal, we will analyze reactions by breaking them down into a sequence of elementary steps ”. This sequence of steps is called the reaction mechanism . In order to write mechanisms, you will need a working knowledge of the electronic structure of molecules and the skill in writing arrow notation. This lesson will begin to put these skills to use as we learn to write the mechanism for a simple reaction - the proton transfer reaction. Organizing chemistry around mechanisms allows the process of change to be understood , not just memorized. Once basic principles of reactivity are understood, you’ll learn to reason-by-analogy t o rationalize and predict outcomes of reactions that you’ve never previously seen. “Elementary step”? Elementary steps describe the process of change between adjoining minima on the MEP of an overall reaction. Many reactions have several minima (i.e., intermediates) located between starting and ending points. Thus, the overall reaction mechanism requires a sequence of elementary steps to describe the entire process. Each elementary step has: •A reaction arrow ( ) connecting the starting and ending structures of that step •Balanced charge •Balanced stoichiometry
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Nucleophiles and Electrophiles We will expand the concept of acids and bases to a related classification scheme known as electrophiles and nucleophiles . The word nucleophile derives from the Greek nucleo , for “nucleus,” and philos , for “loving.” The word electrophile is derived from the Greek electros , “electron,” and philos , “loving.” Most elementary steps involve a nucleophile reacting with an electrophile. Electrophiles are electron deficient. They are characterized by partial or fully developed positive charges and by incomplete octets. Electrophiles have low-lying LUMOs. Nucleophiles are electron rich. They are characterized by partial or fully developed negative charges, and by electron lone pairs or pi bonds. Nucleophiles have high-lying HOMOs. It should not surprise you to learn that all acids are electrophiles, and all bases are nucleophiles.
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Lesson15 - Understanding Reactivity Chemistry is the study...

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