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Chapter 6

Chapter 6. Ionic Reactions Nucleophilic Substitution and Elimination Reactions of Alkyl Halides. Organic Halides. Alkyl Halides: alkane molecule in which a halogen has replaced a hydrogen. Physical Properties Of Organic Halides. Low solubilities in water

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Chapter 6

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  1. Chapter 6 Ionic Reactions Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

  2. Organic Halides • Alkyl Halides: alkane molecule in which a halogen has replaced a hydrogen

  3. Physical Properties Of Organic Halides • Low solubilities in water • Miscible with each other and other relatively nonpolar solvent • E.g CH2Cl2 CHCl3 CCl4

  4. Reaction Intermediates • 3 major types • Carbocation (C+) • Carbanion (C-) • Free radical (C· )

  5. Reaction Sites • Nucleophiles: (nucleus-loving) any species containing electron pairs • Electrons are (-), so Nu: are attracted to (+) site • Charge Nu: are better than neutral one E.g

  6. Reaction Sites • Electrophiles (electron-loving): any species or site in molecule that’s deficient in electron density because it’s near a electronegative atom or lacking of e- altogether

  7. Multiple bonds • The electrons are available to be donated to another species

  8. Nucleophilic Substitution Reactions • General Reaction

  9. Reaction Schemes

  10. Leaving Groups • Relatively stable, weakly basic molecule or anion • Halogen atom of an alkyl halide is a good leaving group because once departed it is a weak base and table anion

  11. A mechanism for SN2Reaction • The nucleophile approaches the carbon bearing the leaving group from the back side • Directly opposite the leaving group • As the reaction progresses, the bond between nucleophile and the carbon strengthens, and the bond between the carbon atom and the leaving group weakens • Carbon atom has its configuration turned inside out  inversion

  12. Transition State • Arrangement of the atoms in which nucleophile and the leaving group are both partially bonded to the carbon atom undegoing substitution • Bond breaking and forming and occurred simultaneouly • Concerted reaction

  13. Kinetics of a Nucleophilic substitution: an SN2 reaction • 1 step reaction • Second order • Rate of reaction depends an alkyl halides and Nu: • Rate Rxn = k [alkyl halide] x [Nu:]

  14. Stereochemistry of SN2 Reactions • Nucleophiles approaches from the back side, that is directly opposite the leaving group. • Consider the cis-1-chloro-3-methylclyclopentane • When undergoes SN2, the product become trans

  15. examples • Give the structure of the product that would be formed when trans-1-bromo-3-methylcyclobutane undergoes an SN2 reaction with NaI

  16. A mechanism for SN1Reaction

  17. The Relative stabilities of Carbocations • The order of stability of carbocations can be explained on the basic of hyperconjugation. • Involves electrons delocalization from a filled bonding orbital to an adjacent unfilled orbital • Any time a charge can be disperred or delocalized, a system will be stabilized

  18. The Relative stabilities of Carbocations

  19. Kinetics of a Nucleophilic substitution: an SN1 reaction • 2 step reaction • 1st order rate determination • Rate of reaction depends the slowest step • Heterocleavage of halide • Rate Rxn = k [alkyl halide]

  20. Multistep Reactions and The Rate-Determining Step

  21. Multistep Reactions and The Rate-Determining Step • The step is intrinsically slower than all other is called the rate-limiting step or rate determining step

  22. Transition State • Stabilization of leaving group • I- > Br- > Cl- > F- • Weaker conjugated base  stronger leaving group

  23. Mechanism for SN1 Reaction • Show a complete mechanism with stereochemisty for the following reaction

  24. Mechanism for SN1 Reaction • Show a complete mechanism with stereochemisty for the following reaction

  25. Factors Affecting the Rates of SN1 and SN2 Reactions • The structure of the substrate, • The concentration and reactivity of the nucleophile • The effect of the solvent • The nature of the leaving group

  26. The Effect of the Structure of the substrate • SN2 reaction shows the following general order of reactivity • Methyl > primary > secondary >> (tertiary – unreactive) • steric hindrance • SN1 reaction • Tertiary > secondary > methyl • Hyperconjugation between p orbitals

  27. Hammond-Leffler Postulate • The structure of a transition state resembles the stable species that is nearest it in free energy

  28. The effect of the Concentration and Strength of the Nucleophile • A negatively-charged nucleophile is always a more reactive nucleophile than its conjugated acid • HO- is a better Nu: then H2O and RO- is better than ROH • In a group of nucleophiles in which the nucleophilic atom is the same, nucleophilicities parallel to basicities • RO- > HO- >> RCO2- >> ROH > H2O • equilibrium favors the side with weaker acid

  29. Solvent Effects on SN2 Reactions: Polar Protic and Aprotic solvent • The effect of hydrogen bonding with the nucleophile is to encumber the Nu: and hinder its reactivity in a substitution reaction

  30. Solvent Effects on SN2 Reactions: Polar Protic and Aprotic solvent • Hydrogen bonds to a small nucleophile atom are more stronger than those to larger nucleophilic atoms • Halide Nucleophilic in Protic Solvent I- > Br- > Cl- > F- Larger atoms have greater polarizability  larger nucleophile atom can donate a greater degree of electron density to substrate SH- > CN- > I- > -OH > N3- > Br- > CH3CO2- > Cl- > F- > H2O

  31. Solvent Effects on SN2 Reactions: Polar Protic and Aprotic solvent • Aprotic solvents are those solvents whose molecules do not have a hydrogen that is attached to an atom of an electronegative element • They do the same way as protic solvents solvate cations; by orienting their negative ends around cation and by donating unshared electron pairs to vacant orbitals of the cation

  32. Solvent Effects on SN2 Reactions: Polar Protic and Aprotic solvent • They cannot form H-bond because their positive centers are well shielded by the steric effects from any interaction with anions • Rate of SN2 reaction generally increased when they are carried out in a polar protic solvent

  33. Solvent Effects on SN1 Reactions: The Ionizing Ability of the solvent • The use of polar protic solvent will greatly increase the rate of ionization of an alkyl halide in SN1 reaction • Able to solvate cations and anions more affectively • Stabilize transition state leading to the intermediate carbocation and halide ion more than it does the reactant • Lower activation energy

  34. Summary of SN1 and SN2 Reactions Leaving Group: I > Br > Cl > F for both SN1 and SN2 ( the weaker the base after the group departs the better the leaving group)

  35. Elmination Reactions of Alkyl Halides • General Reaction • If carbon next to alkyl halide has a halogen, Elimination is possible • Require a strong base

  36. Dehydrohalogenation • General Reaction • Elements of hydrogen halide are eliminated from a haloalkane

  37. 1,2 Elimination • Alpha (α) carbon atom: carbon that bears the substituent • Beta (β) hydrogen atom: hydrogen that attached to the carbon adjacent to α carbon

  38. Bases used in Dehydrohalogenation

  39. Bases used in Dehydrohalogenation

  40. Mechanism for E2 Reaction

  41. Example • Show a complete mechanism for E2 reaction

  42. Mechanism for E1 Reaction • Unimolcular reaction

  43. Example • Show a complete mechanism for E1 reaction

  44. Substitution Versus Elmination • As a rule: • If your subsitution mechanism is SN1, E1 is the elimination mechanism • If your substitution mechanism is SN2, E2 is the elmination mechanism • 2o alkyl halides: if 2o ; weak Nu: E1 if 2o; strong Nu: E2

  45. Substitution Versus Elmination

  46. Examples • Prove the possible mechanisms (SN1, SN2, E1 and/or E2) and possible products for the reaction below

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