Lecture+1_Blank.pdf - 09-218 Organic Chemistry II Spring...

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Unformatted text preview: 09-218 Organic Chemistry II Spring 2019 Lecture 1: Conjugated systems 1.1 Learning Objectives: 1. To be able to define a diene, understand the differences between a conjugated dienes and non-conjugated dienes. 2. To be able to understand the stability and spectroscopic properties of conjugated dienes 3. To be able to draw all the products formed upon electrophilic addition of dienes. 4. Be able to determine the kinetic and thermodynamic product for both symmetrical and unsymmetrical dienes 5. Be able to sketch a reaction coordinate representing the electrophilic addition to dienes including ranking of products and intermediates 1.2 Introduction: ​diene, conjugated and isolated dienes 1.3 Conjugated Dienes: Molecular Orbitals and UV-vis Spectroscopy Page 1 Distributed on January 13, 2019 Note that the gap between the highest occupied and lowest unoccupied molecular orbitals decreases in going from ethylene to 1,3-butadiene. 1.4 Stability of Conjugated Dienes The overlap between the two double bonds in a conjugated diene leads to lower energy relative to an isolated (i.e. nonconjugated) diene. 2 Distributed on January 13, 2019 1.5 Electrophilic Addition to Conjugated Dienes Recall the electrophilic addition of HBr to an unsymmetrical alkene such as propene: When the same reaction is performed with a ​conjugated d ​ iene such as 1,3-butadiene, a mixture of products can result, with the relative amounts of the two products being strongly dependent on the temperature: 3 Distributed on January 13, 2019 How can we explain these observations? 4 Distributed on January 13, 2019 Reaction coordinate: Now consider the reaction of Br​2​ with 1,4-dimethyl-1,3-cyclohexadiene: 5 Distributed on January 13, 2019 1.6 Reaction with Unsymmetrical Conjugated Dienes The situation is more complicated when the conjugated diene is unsymmetrical. Now, four different products are possible because there will be two different-resonance stabilized carbocations. However, the same logic used to analyze the previous cases applies to these reactions as well. Consider the reaction of 4-methyl-1,3-pentadiene with HBr: Predict what product will be favored at low temperature and at high temperature. Also, draw an energy diagram for this reaction, identifying the starting materials, intermediates and products. Product D will be favored at low temperature because it comes from the most stable resonance form of the most stable intermediate. This reaction path will have the lowest energy, making this the most favorable product at low temperature. Product C will be favored at high temperature because it is the most stable alkene (trisubstituted). 6 Distributed on January 13, 2019 ...
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  • Fall '19
  • Electrophilic addition, Reaction coordinate, Diene, Unsymmetrical Conjugated Dienes

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