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Chapter 6: Substitution Reactions

Chapter 6: Substitution Reactions. Overview. Nu : Nucleophile (lone pair; negative charge) X : Halide (Cl, Br, I); Also Leaving Group R- X : Substrate; Where Chemistry “Happens”. Reactions are Just as Name Implies; One Group Replaces Another.

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Chapter 6: Substitution Reactions

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  1. Chapter 6: Substitution Reactions Overview Nu: Nucleophile (lone pair; negative charge) X: Halide (Cl, Br, I); Also Leaving Group R-X: Substrate; Where Chemistry “Happens” Reactions are Just as Name Implies; One Group Replaces Another Relevant Text: Solomons and Fryhle, Chapter 6: 230-260

  2. The Nucleophile Nucleophiles: (1) Have Lone Pairs/Negative Charge (2) Weak or Strong Lewis Bases (3) Can Be Neutral or Anionic Electronegativity Difference Results in Bond Polarization; Nucleophiles are Drawn to the Partially Positive C Atom Question: Why is it Necessary For Nucleophiles to be Lewis Bases?

  3. Leaving Groups Must Be Relatively Stable; Should Be Weak Bases For Halides, I- > Br- > Cl- >> F- “Fitness” as Leaving Group Parallels Weakness as Base KEY Consideration: When Does Leaving Group Really Leave?

  4. SN2 Reactions Substitution Nucleophilic bimolecular Rate = k [Nu] [Substrate] Second Order Reaction, Both Nucleophile and Substrate Concentration Affect the Reaction Rate Bond Making and Breaking Occur Simultaneously

  5. Typical SN2 Reaction Coordinate Diagram Transition State Free Energy of Activation DG‡ Reactants Free Energy Free Energy Change DG° Products Reaction Coordinate RECALL: Negative DG  Exergonic; Positive DG  Endergonic

  6. SN2: The Transition State Transition States Are Bracketed; Noted with a Double Dagger Transition State Structure Shows Simultaneous Bond Breaking and Bond Formation. Transition State Geometry? - (S) (R) SN2 Stereochemical Implications: INVERSION!

  7. SN1 Reactions Substitution Nucleophilic Unimolecular Rate = k [Substrate] First Order Reaction, Only the Substrate Concentration Affects the Reaction Rate Carbocation Intermediate Heterolytic C-Br Bond Cleavage - Bond Making and Breaking Occur in Separate Events

  8. Typical SN1 Reaction Coordinate Diagram DG‡ (2) Carbocation Intermediate Free Energy DG‡ (1) Reactants Products Reaction Coordinate First Transition State is Most Critical (Highest Activation Barrier) NOTE: Intermediates Reside in Energy WELLS

  9. SN1: The Critical Transition State Only C-Br Bond Breaking is Featured in TS‡ Partial Charges are Developing and Bond is Nearly Broken Polar Protic Solvents Stabilize (H2O in this Example) Resulting Intermediate Will be the Key Carbocation

  10. SN1: Carbocation Intermediates Relative Stabilities: Geometry and Carbocation Capture by a Nucleophile: Planar Carbocation SN1 Stereochemical Implications: RACEMIZATION! KEY Consideration: ALWAYS Examine C+ Stability for SN1

  11. SN1 and SN2: Factors Influencing Rate • Substrate Structure • SN2: methyl > 1° > 2 ° >> 3° (unreactive) • SN1: carbocation stability limiting factor (3°) • Nucleophile Concentration and Reactivity • SN2: anionic nucleophile better than conjugate acid • SN2: for like atoms, nucleophilicity || basicity • RO- > HO- > RCO2- > ROH > H2O

  12. SN1 and SN2: Factors Influencing Rate • Solvent • SN2: Strongly Solvated Nucleophiles are Weaker! • (this explains relative halide nucleophilicity) • SN1: Ionizing Ability of Solvent is Critical • Polar, Protic Solvents Best (Solvate Cations and Anions) • Leaving Group Stability • Typically Don’t See Strong Bases as Leaving Groups • More Leaving Group Stability = Greater Reaction Rate

  13. SN1 and SN2: Differentiating Factors Reactions Involving Alkyl Halide Substrates SN1 3° (Stable C+) Weak Lewis Base Neutral Polar Protic SN2 Methyl > 1° > 2° Strong Lewis Base High [Nu] Polar Aprotic Factor Substrate Nucleophile Solvent Leaving Group I > Br > Cl > F (SN1 and SN2) KEY Consideration: For a Given Reaction, Look at These Factors Now We’re Ready to Tackle Some Specific Examples

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