CHEM301 F08 CA12B key

CHEM301 F08 CA12B key - CHEM301, Fall 2008 Dr. Ruder 1...

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CHEM301, Fall 2008 Dr. Ruder 1 Class Activity 12B Electrophilic Addition Part B: Additions Involving Cyclic Intermediates or Products Model 1: Halogenation of Alkenes The addition of bromine or chlorine to an alkene is called halogenation. The first step of this reaction forms a cyclic intermediate which goes on to form a vicinal dihalide product. + H Br H Br H H Br Step 1 Step 2 Br Br cyclic bromonium ion vicinal dibromide δ + δ - Questions: 1. (a). Is an alkene electron rich or electron deficient? Based on this answer, would the alkene act as a nucleophile or an electrophile in an addition reaction? (b). Consider the Br-Br bond. If an electron rich species were to approach one end of this bond (for here assume the electron rich species approaches the left side), what affect would this have on the dipole of the Br-Br bond? If an electron rich species approached the left side of the Br-Br molecule, then the electrons will be repelled from the electron rich source and move to the right side of the molecule, thus inducing a dipole in the bond . Draw δ + and δ - signs on the bromine in Model 1 to illustrate this induced dipole. Label the atom that would act as the electrophile. The Br labeled δ + would act as the electrophile. 2. (a). In step 1 identify what bonds are being formed and broken. Two C-Br bonds are being formed, one C=C pi bond is being broken, and one Br-Br bond is being broken. (b). Draw curved arrows for step 1 to illustrate electron movement. Where must the electrons come from to form the second C-Br bond? From the lone pairs on Br. (c). The cyclic brominium ion has a positive charge on Br. How many lone pair electrons are present on the Br? Draw the lone pair electrons in on the above model. Two lone pairs on Br. (d). In forming the second C-Br bond, the cyclic bromonium ion results. If this bond did not form, the following cation would have resulted. Compare the stability of this cation to the cyclic bromonium ion. Which would you expect to be more stable? How does this explain why the cyclic bromonium ion is formed rather than the cation shown? The cyclic bromonium ion would be the most stable of the two cations. Although Br has a positive charge in the cyclic bromonium, all atoms in this structure have a complete octet of electrons. In the cation shown below the carbon does not have a complete octet, thus making this structure less stable. Br H H (e). From what face of the original cyclopentene did the bromine approach? (look at the stereochemistry of the cyclic bromonium ion). The top face. Would you expect the bromine to approach the other face of the alkene (not shown) as well? Explain. Yes, the bromine should
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CHEM301, Fall 2008 Dr. Ruder 2 approach both faces equally. Since the alkene is planar, attack of the bromine should be just as easy from either face.
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This note was uploaded on 02/17/2011 for the course CHEM 301 taught by Professor Sahli during the Fall '07 term at VCU.

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CHEM301 F08 CA12B key - CHEM301, Fall 2008 Dr. Ruder 1...

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