Ranking Nucleophiles and Leaving Groups, Hammond's Postulate

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Comparing Nucleophile Quality withPotential Energy DiagramsLower electronegativitydestabilizes HS - .Some of this isresolved intransition state(but not all).E a (HS - )++EE a (Cl - )So, rate of attack by SH - isgreater than that of Cl - .reaction coordinate++The Hammond PostulateFor similar reactions, differences in starting material orproduct stabilities will be reflected in transition state(to a lesser degree).+Difference instability here……is reflectedhere.++E+Wade 4-14reaction coordinate
What Makes a Good Leaving Group?1. Wants to take electrons (electronegative).Again, lower electronegativitydestabilizes both HS - and S in transition state.E a (RSH)+E a (RCl)+EE a (RSH) > E a (RCl).So, Cl - leaves fasterthan SH - .because of TS stabilization,not product stabilizationreaction coordinate++What Makes a Good Leaving Group?2. Polarizable. (Stabilizes transition state.)good overlap,stabilizationpoor overlapIFSo, I is a better leaving group than F .Interesting consequence: I is both a good nucleophileand a good leaving group.
Page 1 and 2: What Makes a Good Nucleophile?+ +1.
Page 3: Some Common NucleophilesComparing N
Page 7: Inhibition of S N 2 by Neopentyl Gr

Ranking Nucleophiles and Leaving Groups, Hammond's Postulate

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