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CHEM 2OA3 Lecture Notes - Leaving Group, Nucleophile, Weak Base

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Jeff Landry

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Nucleophilic Substitution
Nu- + R-X ----(solvent)→ Nu-R + X- (leaving group)
o If X- is a weak base, the reaction is favorable
o Strong base X- is poor leaving group
Stronger Acid + Stronger Base → Weaker Acid + Weaker Base
Good nucleophile has to be strongly basic and have a good concentrated negative charge
o Concentrated electron pair
Resonance spreads the negative charge, bad for nucleophilicity
o O- is good nucleophile
Chemical reactions occur when there is overlap of orbitals between atoms
o Anything that blocks them from overlapping hurts the reaction
E.g. Big attached atoms (hindered)
The O- in CH3CH2CH2O- is a poor nucleophile (hindered) while the O- in
CH3O- is good nucleophile (not hindered)
Reactive Rates of R-X Reactivity
o CH2 R-X > CHR2X >> CR3X (C in center, with the 4 other things attached to it)
Solvents in Nucleophilic Substitution
Solvation Interaction between solute and solvent molecules
Most important thing for solvent is to have something that will solvate the cation and
NOT the anion
o If you solvate the anion, the reaction slows down
SN2 Reactions work better in dipolar aprotic solvents!!!
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