CHEM301 F08 CA12C key

CHEM301 F08 CA12C key - CHEM301 Fall 2008 Dr Ruder 1 Class...

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CHEM301, Fall 2008 Dr. Ruder 1 Class Activity 12C key Electrophilic Addition to Alkynes Model 1: Electrophilic Addition of HX to an Alkyne H 3 C H + H Br H 3 C Br H H Br H Br H H CH 3 HBr Step 1 Step 2 vinyl bromide geminal dibromide Questions: 1. (a). How many π -bonds does an alkyne have? 2 (b). Would an alkyne act as a nucleophile or an electrophile? Why? As a nucleophile because it has electrons to share. (c). Would you expect an alkyne to react in a similar fashion to an alkene? Explain. Alkenes should react in a similar manner to that of alkenes because they both have pi electrons that can be shared. (d). In the example in Model 1, how many π bonds reacted after step 1? How many after step 2? One after each step. 2. Consider the addition of HBr to an alkyne as shown in Model 1: (a). Draw the two possible vinyl cations (using curved arrows to show formation) after the addition of H + in step 1. H 3 C H + H Br H 3 C H H H 3 C H H + (b). Of the two vinyl cations drawn above, which would be the most favorable? The secondary vinylic cation. (c). Does this follow Markovniov’s rule? Explain. Yes because the H adds to the carbon of the alkyne that has the most H, to give the more stable cation. (d). Starting with the vinyl bromide, draw (using curved arrows) the two possible cations formed after the addition of H + in step 2. H 3 C Br H H H Br + H 3 C Br H H H 3 C Br H H + H H (e). Given the structure of the geminal dibromide in Model 1, which of the two cations from answer (d) must be the more favorable? Explain why (use resonance forms to help explain your answer). The cation connected to the Br group would be the most stable as the lone pairs on the Br can stabilize the charge via resonance.
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CHEM301, Fall 2008 Dr. Ruder 2 H 3 C Br H H H H 3 C Br H H H 3. In the addition of HBr to 2-pentyne: (a). Draw the two possible cations that could form after the addition of H + in the first step. H
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CHEM301 F08 CA12C key - CHEM301 Fall 2008 Dr Ruder 1 Class...

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