Orgo I Exam 2: Stereo, Alkene rxns, Alkyne rxns, Sn1/Sn2/E1/E2

What makes a good leaving group?

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Terms in this set (9)

What makes a good leaving group?

Weak base (The lower the pka value of the conjugate acid, the weaker the base)
In order of best --> worst LGs: I-, Br-, Cl-, F-
Other LGs: sulfonate, tosylate, triflate

Sulfonate group

(sulfonic acid pictured)
Sulfonic acids are very strong acids, pKa < 0. Conj. base of acid pictured is mesylate (OMs). Conj. acid pKa = -9.

Image: Sulfonate group

Another good leaving group.
Abbreviation: OTs
pKa of conjugate acid: -6.5.

Image: Tosylate

Good LG.
Abbreviation: OTf
pKa of conjugate acid: -12

Image: Triflate

Alcohols are very easily converted into ____?

Alcohols are very easily converted into sulfonates (poor LG --> good LG)

Bad LGs for Sn2

OH-, OR-, NH2-, NR2-, CR3, H- (these are strong bases!!)

Moderate LG for Sn2

Carboxylate (RCO2-)

Image: Moderate LG for Sn2

What type of nucleophile is required for Sn2?

a stong nucleophile
-negatively charged nucleophiles are best
-exampes of good nucs:
OH-, OR-, SCN-, CN-, R-C≡C-, Cl-, Br-, I-, SH-, SR-. NR2-, NR3
-general rule: best nucleophiles are the strongest bases

The best bases are the strongest nucleophiles when...

1. When comparing the same nucleophilic atom