Ch08Su08

Ch08Su08 - Reactions of Alkenes Since bonds are stronger...

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Reactions of Alkenes Since σ bonds are stronger than π bonds, double bonds tend to react to convert the double bond into σ bonds This is an addition reaction. (Other types of reaction have been substitution and elimination). Addition reactions are typically exothermic. Electrophilic Addition The π bond is localized above and below the C-C σ bond. The π electrons are relatively far away from the nuclei and are therefore loosely bound. Figure 8-1 400cd6a2817b3a0b9fcfaa36f5a025f253ce5730.doc Page1 + X-Y Y X
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An electrophile will attract those electrons, and can pull them away to form a new bond. This leaves one carbon with only 3 bonds and a +ve charge (carbocation). The double bond acts as a nucleophile (attacks the electrophile). In most cases, the cation produced will react with another nucleophile to produce the final overall electrophilic addition product. Electrophilic addition is probably the most common reaction of alkenes. Consider the electrophilic addition of H-Br to 2-butene: The alkene abstracts a proton from the HBr, and a carbocation and bromide ion are generated. 400cd6a2817b3a0b9fcfaa36f5a025f253ce5730.doc Page2 + E + E + Nuc: - Nuc E H 3 C CH 3 H H H 3 C H H CH 3 H + H 3 C H Br H H CH 3 H Br Br: -
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The bromide ion quickly attacks the cationic center and yields the final product. In the final product, H-Br has been added across the double bond. Orientation of Addition Consider the addition of H-Br to 2-methyl-2-butene: There are two possible products arising from the two different ways of adding H-Br across the double bond. But only one is observed. The observed product is the one resulting from the more stable carbocation intermediate. Tertiary carbocations are more stable than secondary. The protonation occurs at the least highly substituted end so that the cation produced is at the most highly substituted end (and therefore more stable). 400cd6a2817b3a0b9fcfaa36f5a025f253ce5730.doc Page3 H 3 C CH 3 H 3 C H H 3 C CH 3 H CH 3 H + H 3 C CH 3 Br H H CH 3 H Br Br: - CH 3 H CH 3 H 3 C H + CH 3 H Br CH 3 H H 3 C Br: - Observed Product Not Observed H 3 C CH 3 H CH 3 H + CH 3 H CH 3 H 3 C H + >
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Markovnikov’s Rule : The addition of a proton acid to the double bond of an alkene results in a product with the acid hydrogen bound to the carbon atom that already has the greater number of hydrogens attached. (More generally: In an electrophilic addition to an alkene, the electrophile adds in such a way that it generates the most stable intermediate). Reactions that give such products are said to have Markovnikov orientation, and are Markovnikov products. The electrophilic addition of HBr is said to be regiospecific , since it only gives one orientation of addition. HCl and HI undergo analogous electrophilic addition reactions.
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This note was uploaded on 02/23/2010 for the course CHEM 202 taught by Professor Smith during the Spring '10 term at Duke.

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Ch08Su08 - Reactions of Alkenes Since bonds are stronger...

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