nucleophilic substitution reactions

nucleophilic substitution reactions - SN1 SN 2 Mechanism...

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Unformatted text preview: SN1 SN 2 Mechanism Two steps: R-L R+ Kinetics 1st order (unimolecular) Rate = k[R-L] Ionization is the rate determining step 2nd order (bimolecular) Rate = k[R-L][Nu:] Stereochemistry Racemization Inversion Carbon (sp3) electrophile Favored by electrophiles which can best stabilize a carbocation. 3° > benzylic ~ allylic ~2° >1° Steric hindrance controlled CH3 > 1° > 2° (no tertiary) Nucleophile Nature of nucleophile has no effect on rate Favored by better nucleophiles RS->NC->I->RO->HO->Cl- Leaving group Favored by better leaving groups: -N2 > -Otos > -I > -Br > -Cl> -OH2 Favored by better leaving groups: -N2 > -Otos > -I > -Br > -Cl Solvent effect Favored by polar, ionizing solvents. Solvolysis: polar protic solvents act as nucleophile H2O>H2O/ROH (mix)>ROH Favored by polar aprotic solvents. HMPA>CH3CN>DMF>DMSO>TH F Disfavored by polar protic solvents (H-bonding) Competing reactions E1, rearrangements E2 by strongly basic nucleophiles R-Nu One step: R-L + Nu: R-Nu ...
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