6. Nucleophilic Substitution

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Chemistry Flashcards on 6. Nucleophilic Substitution, created by John Rivada on 21/04/2019.
John Rivada
Flashcards by John Rivada, updated more than 1 year ago
John Rivada
Created by John Rivada over 5 years ago
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Question Answer
What is the mechanism for an Sn1 reaction? Include transition states
What is the mechanism for an Sn2 reaction? Include reaction states
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
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
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
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
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
Trick question! If a Nu is good enough to displace OH then its more than good enough to deprotonate
What can you do if you want to perform an Sn rx on an alcohol? Protonate it!!!
What is the Mitsunobu reaction?
Mechanism?
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?
What reagent do you use to add a sulfonate group onto an alcohol? Pyridine I think
Which more likely to happen and why? Blue is because less sterically hindered Red still happens because of the positive charge
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!
Mechanism?
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
Carbonyl attack usually predomanites over attack at ester R-group
What do you get when you use a nitrogen nucleophile for Sn2 reactions? A flippin' mess!
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
Nucleohpile rate scale
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