09 - Chemistry 5.12 Spring 2003 Handout#9 Substitution...

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1 Chemistry 5.12 Spring 2003, Handout #9 Substitution Reactions (S N 2 versus S N 1) Substrate: Sterics (methyl > 1° > 2°) No S N 2 with 3°! Orbital Overlap (methyl 2245 benzylic > allylic > 1°) Cation Stability (benzylic > allylic > 3° > 2°) No 1° or methyl R + without extra stabilization! Nucleophile: Not Important Strong/Moderate Required strong: RS , I , R 2 N , R 2 NH, RO , CN moderate: RSH, Br , RCO 2 Leaving Group: Moderately Important (same trend as S N 1) Very Important (–OTf >> –OTs 2245 –OMs >> –I > –Br > –Cl) Solvent: Polar Aprotic Polar Protic Mechanism: Concerted Two Steps (Look for carbocation rearrangements.) Rate Equation: Rate = k r [R–X][Nuc] Rate = k r [R–X] Stereochemistry: Stereospecific (inversion) Loss of Stereochemistry S N 2 S N 1 Elimination Reactions: E2 versus E1 Substrate: Alkene Stability (3° > 2° > 1°) Cation Stability (benzylic > allylic > 3° > 2°) Base: Not Important: Usually Weak (ROH, R 2 NH) Strong Base Required
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09 - Chemistry 5.12 Spring 2003 Handout#9 Substitution...

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