17786481
Description
Flashcards by John Rivada, updated more than 1 year ago
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Created by John Rivada
about 5 years ago
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| Question | Answer |
| What is the mechanism for an Sn1 reaction? Include transition states |
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| What is the mechanism for an Sn2 reaction? Include reaction states |
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| If a stable carbocation can be formed,is Sn1 preffered or Sn2? If no carbocation can be formed, what happens? | Stable carbocation = Sn1 None = Sn2 |
| Slide 8 | ** How does HBr prefer S1 and PBr3 prefer Sn2? |
| What kind of cations are stable enough to do Sn1? | -Benzylic -Allylic -3° Alkyl carbocations -2° Alkyl carbocations work but not as good -Heteroatom stabilized |
| Trityl carbocation? | **important? I dunno |
| Why are 3° cabocations stable enough to do Sn1? |
Hyperconjugation with C-H or C-C sigma bonds
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| What reagent can you use to promote Sn1? | Silver salts: AgBF4 AgPF6 AgSbF6 |
| Slide 13 | I think I was asleep for this slide |
| Whats so good about MOM and BOM? | Resonance from oxgen and methoxymeythl cation make it great for Sn1 (?) |
| Order of SN1 reaction rate |
Left is slow, right is fast
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| Why do methyl and 1° alkyl groups always go through an Sn2 pathway? | Low steric hindrance Cation not stable |
| Draw the reaction coordination diagram for an Sn1 and Sn2 Reaction |
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| What happens to the stereochemistry of a chiral atom in an Sn2 and Sn1 reaction? | Sn1: racemic Sn2: inversion |
| Order of Sn2 reaction rate |
Left is slow
Right is fast
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| What does the rate of Sn2 depend on? | 1. Nucleophile -what type of heteroatom is attacking -anionic or neutral -structure of Nu (Pkb and sterically hindered) 2. Electrophile -structure of El (1°, 2°, 3°) LG (lower pkb = better LG) |
| pKb of nucleophile goes up, does rate of Sn2 go up or down? | Rate goes up |
| pkb of LG goes up, does rate of Sn2 go up or down? | Rate goes down |
| Does an EDG promote Sn1 or Sn2? | Promotes Sn1 |
| Does an EWG promote Sn1 or Sn2 | Promotes Sn2 by inhibiting Sn1 |
| Good solvents for Sn1 | -Polar solvents best (transition state in RDS involves formation of ions) -eg. H2O, ROH, RCO2H |
| Good solvents for Sn2 | Less polar solvents are better Polar aprotic solvents are a good compromise |
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Trick question! If a Nu is good enough to displace OH then its more than good enough to deprotonate
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| What can you do if you want to perform an Sn rx on an alcohol? |
Protonate it!!!
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| What is the Mitsunobu reaction? |
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Mechanism?
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| What prerequisite does the Nu need for the mitsunobu reaction? | Nu-H must have a pka of less than 11 |
| When performing the mitsunobu reaction, how is the steriochemistry | Stereochemistry gets inverted |
| Whats a good way to turn an OH into a LG? | Sulfur chemistry! Add sulfonates |
| What are common sulfonates that you can use to turn OH into a LG? |
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| What reagent do you use to add a sulfonate group onto an alcohol? | Pyridine I think |
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Which more likely to happen and why?
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Blue is because less sterically hindered Red still happens because of the positive charge |
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| What is an old school way of turning 1° and 2° alcohol into a halide? | P-X electrophiles e.g. PBr3 and PCl5 |
| Using 1° and 2° alcohols, how can you use a halide acid (HCl, HBr, HI) to make a 1° or 2° halide? | You can't-ish. It's not that good! |
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Mechanism?
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| What is the Finelstein halide exchange reaction? | RCH2-X + NaI in acetone = RCH2I + NaBr X = Cl, Br, OTs, OMs Na-X is insoluble in acetone |
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Carbonyl attack usually predomanites over attack at ester R-group
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| What do you get when you use a nitrogen nucleophile for Sn2 reactions? | A flippin' mess! |
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| Are thiolates a good Nu? | Yes! Especially for Sn2 |
| How to make a thiol | ** |
| What is a good carbon nucleophile for Sn2? | -CN! NaCN or KCn |
| What can RCN be transformed into? | Amines Aldehydes Carboxylic Acids |
| What is a useful way to extend a carbon chane by 1 atom? | Use NaCN on an alkyl halide! |
| What is the trend compared to basicity if the atom forming the new bond is the same over a range of nucleohiles? |
Basicity parallels nucleophilicity
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| Nucleohpile rate scale |
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