s20 2011 week 2 day 4

s20 2011 week 2 day 4 - Chemistry S-20ab Week 2 Chemistry...

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E2 vs. S N 2: The “GFP” Reaction Hierarchy To help decide which reaction will predominate, apply the “GFP” (good-fair-poor) reaction hierarchy. If all else is equal , choose the reaction that is at the top of the list: 1. S N 2 reactions: Require good steric interactions Good if the halide is methyl or 1° Fair if the halide is 2° Poor if the halide is 3° or if the nucleophile is very bulky (e.g. t BuO ) 2. E2 reactions: Require a strong base Good if the base is at least as strong as OH Fair if the base is between H 2 O and OH in strength Poor if the base is weaker than H 2 O Let’s see some examples: Determine the primary product (and identify possible secondary products) for each of the following reactions: O Br Br Br O Br Br CH 3 I + OH CH 3 I + + OH + CN + CN + + OH Reading : Section 9.5 Chemistry S-20ab Week 2 116
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Reactions Involving Carbocations: S N 1 and E1 Consider the following reaction. Why is it unlikely that this reaction proceeded by either an S N 2 or E2 mechanism? Br EtOH O + 81% 19% 25°C Can you propose a mechanism for this reaction? Hint: the rate law for this reaction is simply: rate = k [tBuBr] What do you suppose is a requirement for this reaction? Reading : Section 9.6 Chemistry S-20ab Week 2 117
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The Stereochemistry of the S N 1 Reaction Given that the S N 1 mechanism proceeds through a carbocation intermediate, what would you expect to find when you subject the following alkyl halide to S N 1 conditions: H Cl H 2 O H OH HO H + S R In fact, the products are: 33% S and 67% R . How can we explain this observation?
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s20 2011 week 2 day 4 - Chemistry S-20ab Week 2 Chemistry...

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