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Aryl Halides - Bonding, Physical Properties and Reactions

Aryl Halides - Bonding, Physical Properties and Reactions Nucleophilic Aromatic Substitution of Chlorobenzene Nucleophilic Aromatic Substitution: Addition-Elimination. What’s the Difference Between Ar- and Ph-?. Bonding in Aryl Halides. Aryl Halides.

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Aryl Halides - Bonding, Physical Properties and Reactions

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  1. Aryl Halides - Bonding, Physical Properties and Reactions Nucleophilic Aromatic Substitution of Chlorobenzene Nucleophilic Aromatic Substitution: Addition-Elimination

  2. What’s the Difference Between Ar- and Ph-?

  3. Bonding in Aryl Halides

  4. Aryl Halides Aryl halides are halides in which the halogen is attached directly to an aromatic ring. Carbon-halogen bonds in aryl halides are shorter and stronger than carbon-halogen bonds in alkyl halides.

  5. H2C CHX sp2 452 (108) 368 (88) sp2 469 (112) 406 (97) X Dissociation Energies of Selected Compounds Bond Energy:kJ/mol (kcal/mol) X = H X = Cl sp3 410 (98) 339 (81)

  6. Resonance Picture C-X bonds in aryl halides have more double bond character than C-X bonds in alkyl halides

  7. Sources of Aryl Halides

  8. Preparation of Aryl Halides Halogenation of arenes (Section 12.5) electrophilic aromatic substitution

  9. Physical Properties of Aryl Halides

  10. resemble alkyl halides are essentially insoluble in water less polar than alkyl halides Cl Cl  2.2 D  1.7 D Physical Properties of Aryl Halides

  11. Reactions of Aryl Halides:A Review and a Preview

  12. Reactions Involving Aryl Halides Electrophilic aromatic substitution (Section 12.14) halide substituents are ortho-para directing & deactivating

  13. Substitution Reactions Involving Aryl Halides Nucleophilic substitution on chlorobenzene occurs so slowly that forcing conditions are required.

  14. Nucleophilic Substitution of Chlorobenzene This reaction does not proceed via SN2……..

  15. the SN2 is not reasonable because the aromatic ring blocks back-side approach of the nucleophile. Inversion is not possible. Why is Chlorobenzene Unreactive?

  16. SN1 Also Unlikely: Aryl Cations are Highly Unstable SN1 not reasonable because: C—Cl bond is strong; therefore, ionization to a carbocation is a high-energy process aryl cations are highly unstable

  17. SN1 Reaction is Possible with Very Powerful Leaving Groups such as Dinitrogen This is a unique case: halides are not good enough leaving groups for this process to occur.

  18. What is the Mechanism of This Reaction?

  19. Nucleophilic Substitution inNitro-Substituted Aryl Halides

  20. Nucleophilic Aromatic Substitution (SNAr)?

  21. 1.0 7 x 1010 2.4 x 1015 too fast to measure The More Electron-Deficient the Haloarene, the Faster the Substitution

  22. Direct Displacement Doesn’t Occur!

  23. Kinetics of Nucleophilic Aromatic Substitution follows second-order rate law: rate = k [aryl halide] [nucleophile] inference: both the aryl halide and the nucleophile are involved in rate-determining step

  24. Reaction Rate Depends on X: I > Br > Cl > F Effect of Leaving Group Upon Rate of SN2 During SN2 reactions, the C-X bond breaks during the rate-determining step C-F (485 kJ/mol), C-Cl (327 kJ/mol) C-Br (285 kJ/ml, C-I (213 kJ/mol)

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