5. Reactions of Alkenes & Alkynes

5. Reactions of Alkenes & Alkynes - 2010 Department...

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© 2010 Department of Chemistry, The University of Western Ontario 5.1 5. Reactions of Alkenes & Alkynes (all sections) A. Introduction to Alkene Reactions The main reactions of alkenes are a ddition reactions , where some AB species reacts to break the bond and form two new bonds. Addition reactions are usually exothermic and occur spontaneously at room temperature. CC AB A B (exothermic) and also oxidation
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5.2 To fully understand the structure and stereochemistry of the products formed, we need to look at the sequence of steps that occur as the reaction proceeds, i.e. the mechanism . B. Mechanism of Electrophilic Addition Reactions Electrophilic addition is the mechanism by which the addition of many reagents across the double bond occurs. In Chapter 4, we saw that bonds have electron density on both sides of the bond. The bond is reactive and will attract electrophiles (electron-seeking reagents). The addition of AB to a bond proceeds well when AB is ionized (behaves as A + and B ), polarized (A –B ), or electron-deficient (does not have a full octet). If A B, the reaction can form two constitutional isomers: CC Y Z X W AB A W X B Y Z + - B W X A Y Z 12 The constitutional isomers that can be formed from reactions are called regioisomers . If one of the regioisomers is formed in greater amount than the other, the reaction is said to be regioselective . Electrophilic addition reactions are usually regioselective.
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5.3 Two-step mechanism Step 1 (electrophilic step) CC Y Z X W AB A W X Y Z + - W X A Y Z + B and/ or slow The first step involves the electrophilic attack of the electron pair by a positively charged species. This results in: o The breaking of the original A–B bond, and the bonding electrons are transferred to B; o The breaking of the C=C bond; and o The formation of the C–A bond. Two bonds are broken ( + ), but only one is formed ( ), so this step is endothermic. It is the slow, rate-determining step of the entire electrophilic addition reaction. The product of this step is a positively charged intermediate known as a carbocation .
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5.4 Step 2 (nucleophilic step) and/or B fast and/or C A W X Y Z C W X A Y Z A W X Y Z B W X A Y Z B In this step, a nucleophile (nucleus- or positive-seeking species) attacks the carbocation. A new C–B bond is formed, and this step is fast and exothermic. Energy diagram If AB adds to asymmetrical alkenes, one of the two possible regioisomers is usually formed in a greater amount. CC Y Z X W AB A W X B Y Z + - B W X A Y Z 12 To predict the major product, it is necessary to know how the course of the reaction depends on the energy of the carbocation intermediate.
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5.5 The reaction coordinate diagram below shows two possible routes. The shape of the plot is typical for the addition of most AB species to alkenes.
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This note was uploaded on 05/01/2011 for the course CHEMISTRY Chemistry taught by Professor Dr.felixlee during the Fall '11 term at UWO.

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5. Reactions of Alkenes & Alkynes - 2010 Department...

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