chapter11 - Chapter 11 Problems 11.1 40 11.1 The General...

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Chapter 11 Problems: 11.1 - 40 11.1 The General Mechanism The simplest mechanism for a addition reaction is a two step reaction. First, the electrophile adds to the double bond, generating a carbocationic intermediate. Second, the nucleophile adds to the carbocation. These reactions are termed electrophilic addidtion reactions. Example 11.2 Addition of Hydrogen Halides The addition of HX (X = F, Cl, Br, or I)occurs first via the addition of H + to the alkene then the nucleophilic X - adds to the carbocation. Additions to unsymmetrical alkenes occur regioselectively according to Markovnikov’s rule: The H bonds to the less substituted carbon and the halogen bonds to the more substituted carbon. This is due to the formation of the more stable carbocationinc intermediate. Example The stereochemistry of addition reactions to alkenes can occur as syn, anti, or mixed addition. Since the addition of HX to alkenes occurs via a carbocationic intermediate X - can add from either the same side or the opposite side as H + . Rearangement reactions can also occur when a more stable carbocation can be generated. Hydrogen halides can also be added to alkynes. The additions can occur to yield either vinyl halides or dihaloalkanes.
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The addition of water to an alkene can occur under acidic condition (H 2 SO 4 and H 2 O). The mechanism for this reaction is the opposite of the E1 mechanism. However, the yields are usually low and carbocationic rearrangements are frequently encountered. Ex. 11.4 Addition of Halogens The additions of Cl 2 or Br 2 in inert solvents such as CH 2 Cl 2 or CHCl 3 to alkenes produce dihalides. However, reactions of F 2 and I 2 are typically not carried out. The mechanism of the reaction begins with electrophilic attack of the halogen on the pi bond to form a bromonium ion. Then the bromide anion attacks the bromonium ion from the opposite face. Figure 11.1 This reaction occurs in a stereospecific fashon. When Br 2 is reacted with trans-2-butene
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chapter11 - Chapter 11 Problems 11.1 40 11.1 The General...

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