1 / 46

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

brady-baird
Download Presentation

Chapter 6

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related