9 - Chapter 9 Addition to alkenes: an introduction to...

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Chapter 9 – Addition to alkenes: an introduction to reaction mechanisms Chemistry is the study of change. Up to this point, the molecular level changes we have studied have 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. In contrast, the chemical changes we will study from this point forward involve changes in electron configuration. Much of the language that was developed in previous chapters will apply to these covalent bond changes. Our description of these changes will greatly benefit from the curved arrow notation that we began to study in Chapter 2 when we discussed resonance. The discussion of intermediates and transition states that we learned in the last chapter will be utilized as well. Our initial discussion of chemical reactions will be centered on the carbon-carbon double bond (i.e., alkenes). Our goal in this course is to learn and understand the chemical transformations (i.e., reactions ) common to organic compounds. To achieve this goal, we will analyze reactions by breaking them down into a sequence of simple 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 curved arrow notation that was discussed in chapter 2. This chapter will begin to put these skills into use as we learn to write our first reaction mechanisms. Organizing organic chemistry around reaction mechanisms allows the process of change to be understood, not just memorized. We will discover unifying principles of reactivity that will allow you to reason by analogy in rationalizing reactions that you have never previously seen. Analyzing the reaction: addition of H-Br to an alkene The reaction mechanism for any given transformation must account for the overall change in electron configuration. When looking at a reaction, we know from our first encounter of a chemical reaction in Chapter 1 that this overall change can be understood by looking at the bonds made and broken. Our first reaction will involve the addition of H-Br to a carbon-carbon double bond. Literally, this reaction involves the addition of the elements of hydrogen and bromine across the alkene. A typical example is shown below, along with an analysis of bonds made and bonds broken. CH 3 + H–Br CH 3 H H Br H Bonds Made Bonds Broken C 1 -Br C 2 -H C 1 =C 2 2 1 2 1 H-Br Let’s follow the summary given in box 1 of Chapter 1. First notice that satisfactory Lewis structures have been drawn for all reactants and products. Also notice that some hydrogen atoms have been explicitly drawn; we are especially interested in showing those hydrogen atoms located around the region of change. The stoichiometry is balanced. Atoms have been numbered in the region of chemical change.
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Comparing the products and reactants we see that the carbon-carbon double bond is broken leaving just a carbon-carbon single bond.
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9 - Chapter 9 Addition to alkenes: an introduction to...

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