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Chemistry 5.12 Spring 2003, Handout #9 Substitution Reactions (S N2 versus S N1 )

Chemistry 5.12 Spring 2003, Handout #9 Substitution Reactions (S N2 versus S N1 ). S N 2. S N 1. Two Steps (Look for carbocation rearrangements.). Mechanism:. Concerted. Rate Equation:. Rate = kr[R–X][Nuc]. Rate = kr[R–X]. Stereospecific (inversion). Stereochemistry:.

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Chemistry 5.12 Spring 2003, Handout #9 Substitution Reactions (S N2 versus S N1 )

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  1. Chemistry 5.12 Spring 2003, Handout #9 Substitution Reactions (SN2 versus SN1) SN2 SN1 Two Steps (Look for carbocationrearrangements.) Mechanism: Concerted Rate Equation: Rate = kr[R–X][Nuc] Rate = kr[R–X] Stereospecific (inversion) Stereochemistry: Loss of Stereochemistry Substrate: Sterics (methyl > 1° > 2°) No SN2 with 3°! Orbital Overlap (methyl @ benzylic > allylic > 1°) Cation Stability (benzylic > allylic > 3° > 2°) No 1° or methyl R+ without extra stabilization! Nucleophile: Strong/Moderate Required strong: RS–, I–, R2N–, R2NH, RO–, CN– moderate: RSH, Br–, RCO2– Not Important Very Important (–OTf >> –OTs @ –OMs >> –I > –Br > –Cl) Leaving Group: Moderately Important (same trend as SN1) Solvent: Polar Aprotic Polar Protic

  2. Elimination Reactions: E2 versus E1 E2 E1 Two Steps (Look for carbocationrearrangements.) Mechanism: Concerted Rate Equation: Rate = kr[R–X][Base] Rate = kr[R–X] Stereospecific (antiperiplanar TS) Stereochemistry: Not Stereospecific Cation Stability (benzylic > allylic > 3° > 2° Substrate: Alkene Stability (3° > 2° > 1°) Base: Strong Base Required (RO–, R2N–) Not Important: Usually Weak (ROH, R2NH) Very Important (same trend as SN1) Leaving Group: Moderately Important (same trend as SN1) Solvent: Wide Range of Solvents Polar Protic Saytzeff Rule: The most highly substituted alkene usually predominates. Hofmann Product: Use of a sterically hindered base will result in formation of the least substituted alkene (Hofmann product). Product Ratio:

  3. Generic Reaction-Energy Diagrams Substitution Elimination energy energy reaction coordinate (SN2) reaction coordinate (E2) Bimolecular Unimolecular energy energy reaction coordinate (E1) reaction coordinate (SN1)

  4. Predicting the Products: Substitution versus Elimination Start no yes Bimolecular Reaction What kind of substrate? Is Nuc/Base strong? 1° 2°, 3°, or stabilized 1° Unfavorable Reaction Is Nuc/Base bulky? yes Unimolecular Reaction E2 no 3° yes mostly E1* What kind of substrate? Is Nuc/Base bulky? 2° no SN2+E2 methyl or 1° SN2 mostly SN1* * Under conditions that favor a unimolecular reaction (weak nuc/base and polar protic solvent), mixtures of SN1 and E1 are usually obtained.

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