CHEM301 F08 CA9B key

CHEM301 F08 CA9B key - CHEM301, Fall 2008 Dr. Ruder 1 Class...

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CHEM301, Fall 2008 Dr. Ruder 1 Class Activity 9B Substitution Nucleophilic Unimolecular, SN1 Two-step nucleophilic substitution. Model 1 : Two-Step Nucleophilic Substitution (SN1) In a two step substitution reaction (SN1) mechanism, the first step is slow and is the rate determining step, while the second step is fast. Br H 3 C H 3 C H 3 C CH 3 O CH 3 CH 3 CH 3 CH 3 H 3 C CH 3 + Br HOCH 3 H CH 3 O CH 3 CH 3 CH 3 H + SLOW FAST intermediate carbocation SN1 (two-step) mechanism Step 1 Step 2 Step 3 deprotonation E Nu Questions 1. For the reaction shown in Model 1 above: (a). What bonds are formed/broken in the first step of the SN1 reaction? C-Br bond (b). What bonds are formed/broken in the second step of the SN1 reaction? C-O bond (c). Label the nucleophile (Nu-) and the electrophile (E+) and circle the leaving group. See above (d). Draw curved arrows to show electron movement. See above (e). Step 3 in Model 1 (deprotonation) involves loss of the proton that was part of the incoming group, to return the oxygen to a neutral charge. Draw curved arrows to illustrate this proton loss. 2. The mechanism for an SN1 reaction occurs in two steps as shown in Model 1: (a). Why do you expect the first step to be slow and the second step to be fast? The first step is slow because you are breaking a bond and going from a stable reactant to a higher energy charged intermediate. The second step is fast because the higher energy intermediate is reacting to form a neutral stable product. (b). Which step would be the rate determining step? Step 1. (c). How many reactants are involved in the rate determining step? one (d). Based on your answer to 2c above, explain why the SN1 reaction is unimolecular. Since only one reactant is involved in the rate determining step, the reaction is termed “uni” molecular. (e). The rate equation for an SN1 reaction is Rate = k[CH 3 Br]. Explain how the rate equation supports a unimolecular reaction. Since only one reagent is involved in the rate, this supports the unimolecular (one molecule) reaction mechanism. 3. Focus on the carbocation intermediate formed in the SN1 mechanism. (a). What is the hybridization of the carbon with the charge? Sp2
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CHEM301, Fall 2008 Dr. Ruder 2 (b). What is the shape of the carbocation structure (tetrahedral, trigonal planar etc)? trigonal planar (c). How many valence electrons does the carbon with the charge have? 6 (d). Is the carbocation electron rich or electron deficient? Electron deficient. (e). Which will the carbocation most likely react with, electron rich or electron deficient reagents? Based on this answer, would the carbocation be a Lewis Acid or a Lewis Base? (f). Rank the following carbocations from most stable (1) to least stable (4). H
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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 CA9B key - CHEM301, Fall 2008 Dr. Ruder 1 Class...

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