Sn1 Sn2 E1 E2 question

[joonkimdds]

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Hi, I am studying substitution and elimination part of OCHEM.
All the materials that I have say
Strong base favors E2, weak base favors Sn2 but it doesn't tell me the reason.

my books say Sn1 favors polar protic and Sn2 favors polar aprotic but it doesn't state the reason either.

and what are the rules to find the better nucleophile, and better leaving group?

Maybe I can just memorize them but I like to understand instead of memorizing them.


And also, I learned that weak base is a good leaving group because they can stabilize – charge of leaving group effectively, but can someone explain to me why weak base can stabilize – charge and strong base can't?

 

[poc91nc]

McMurry, Loudon, Morrison and Boyd, Solomon, Maitland Jones, should be able to help you out with those types of questions! Good luck!

 

[joonkimdds]

McMurry, Loudon, Morrison and Boyd, Solomon, Maitland Jones, should be able to help you out with those types of questions! Good luck!

are they the elite OCHEM genius from SDN?

 

[IsRaEl21]

A good leaving group is usually a weak base like I- or tosylate. A good nucleophile is a strong base. If u really want to understand it u should read Kaplan Orgo chem Chapter # 4 or the destroyer which also explains it very well.

 

[Joey413]

To know a a strong base/ nucleophile follow the principles I outlined for the Acid/Base thread.
As for SN1,SN2, E1, E2:

SN1:
-Is a 2-step mechanism R-L-->R+-->R-Nu
-follows first order kinetics: Rate=k[R-L]
-Stereochemistry: Racemization/Inversion
-Substrate: Resonance stabalization favored tertiary>>sec>pri
-Nucleophile: weak, concentration does not affect the rate
-Leaving group: must be good (follow the acid/base thread)
-Solvent: polar protic
-Temp: low temp
-Reaction condition: acidic
-Rearrangement possible to a more stable carobation 3>>2>1

SN2:
-One step mechanism R-L + Nu--> R-Nu + L
-Second order kinetics: Rate=k[R-L][Nu]
-stereochem: inversion
-Substrate: steric hinderance unfavored primary> sec>> ter
-Strong Nucleophile
-poor leaving group
-Polar aprotic
-low temp
-basic reaction condition
-no rearrangement

E1:
-two step mechanism -C-C-L --> -C-C+- --> -C=C-
-first order: rate=k[R-L]
-Z isomer
-Substrate: steric and resonance stabilization favored
tert>>sec> prim
-Nucleophile: bulky and weak (concentration does not affect rate)
-good leaving
-polar protic
-high temp
-acidic reaction condition
-rearrangement possible

E2:
-One step mech -C-C-L --> -C=C-
-Second order kinetics: rate=k[R-L][Nu]
-Steric hinderance favorable 3>2>1
-substrate: bulky and strong
-poor leaving group
-polar aprotic
-high temp
-basic reaction condition
-rearranement not possible