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AROMATIC SUBSTITUTION REACTIONS

AROMATIC SUBSTITUTION REACTIONS . NOMENCLATURE. Learn Nomeclature Either on Your Own or Use the Organic Nomeclature Software. 1-bromo-3-nitrobenzene. methylbenzene. (toluene). 1-chloro-3-methylbenzene. 1,4-dimethylbenzene. SOME SPECIAL NAMES. toluene. aniline. anisole.

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AROMATIC SUBSTITUTION REACTIONS

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  1. AROMATIC SUBSTITUTION REACTIONS

  2. NOMENCLATURE

  3. Learn Nomeclature Either on Your Own or Use the Organic Nomeclature Software 1-bromo-3-nitrobenzene methylbenzene (toluene) 1-chloro-3-methylbenzene 1,4-dimethylbenzene

  4. SOME SPECIAL NAMES toluene aniline anisole phenol o-xylene m-xylene benzoic acid p-xylene

  5. ortho, meta and para Positions m-nitrotoluene 3-nitrotoluene ipso 1 o- ortho 6 2 1-methyl-3-nitrobenzene meta m- 5 3 4 para p-dichlorobenzene p- 1,4-dichlorobenzene

  6. REVIEW OF BENZENE PROPERTIES

  7. BENZENE RESONANCE Review Sections 6.8 - 6.14 KEKULE STRUCTURES Resonance Energy = 36 Kcal / mole All bonds are equivalent The ring is symmetric. Bond lengths are between a single and a double bond. Very Stable Less reactive than other groupings of atoms.

  8. . . H H . . . . H H H H All 2p orbitals overlap equally.

  9. BENZENE Isodensity surfaces - electron potential mapped in color. (van der Waal’s) Highest electron density is red. Color adjusted to enhance the pi system. Note the symmetry.

  10. BENZENE - DETERMINATION OF RESONANCE ENERGY cyclohexatriene (hypothetical) benzene RESONANCE ENERGY 36 kcal/mol cyclohexene -49.8 kcal/mol -85.8 kcal/mol (calculated) -28.6 kcal/mol cyclohexane

  11. REACTIVITY

  12. The “Double Bonds” in a Benzene Ring Do Not React Like Others Alkene Benzene no reaction + + no reaction + + no reaction + + no reaction + +

  13. Benzene is a Weak Base and Poor Nucleophile Stronger base + Readily donates electrons to an electrophile. alkene + Donation of electrons would interrupt ring resonance (36 kcal / mole). Weaker base A strong electrophile is required - and often a catalyst. benzene

  14. Benzene Reactivity Benzene requires a strong electrophile and a catalyst …..and then it undergoes substitution reactions, not addition. + + catalyst substitution compare: + no catalyst addition

  15. Some Substitution Reactions of Benzene Halogenation + Friedel-Crafts Alkylation + Friedel-Crafts Acylation + - + + Nitration + - Sulfonation +

  16. MECHANISM All of the reactions follow the same pattern of mechanism. The reagents combine to form a strong electrophile E+ ,and its partner (:X ), which react as follows: ELECTROPHILIC AROMATIC SUBSTITUTION (+) + + HX (+) + slow :X intermediate restores ring resonance benzenium ion* resonance structures are shown by the (+) symbols * also called a benzenonium ion

  17. ENERGY PROFILE FOR AROMATIC SUBSTITUTION (+) benzenium intermediate Transition state 1 + (+) Transition state 2 intermediate Ea activation energy + + H slow fast STEP 1 STEP 2

  18. HALOGENATION

  19. Cl Al Cl Cl Formation of the Chloronium Ion Complex .. .. : : : : d+ .. .. .. .. .. .. d- : : : : : .. .. .. .. .. .. : : : : .. .. sp2 .. : : .. .. .. + - : : : .. .. .. : : .. chloronium ion complex

  20. Chlorination of Benzene - Cl2 + AlCl3 - + + [ ] benzenium ion chloronium ion complex + HAlCl4 HCl + AlCl3

  21. FRIEDEL-CRAFTS REACTIONS

  22. FRIEDEL-CRAFTS ALKYLATION

  23. Formation of a Carbocation Complex .. .. : : : : d+ .. .. .. .. d- : : : .. .. .. .. : : : : .. .. Other aliphatic R-Cl may be used .. : : .. .. - + : : .. .. : : .. carbocation

  24. Friedel-Crafts Alkylation - CH3Cl + AlCl3 - + + [ ] + HAlCl4 HCl + AlCl3

  25. REARRANGEMENTS ARE COMMON IN FRIEDEL-CRAFTS ALKYLATION AlCl3 - + - + carbocation rearrangement AlCl3 +

  26. FRIEDEL-CRAFTS ACYLATION

  27. Formation of an Acylonium Complex .. .. : : : : .. .. .. .. d+ d- : : : .. .. .. .. : : : : .. .. .. Other acid chlorides (RCOCl) may be used : : .. .. - : : .. .. + : : .. acylium ion (acylonium ion) Rearrangements DO NOT occur

  28. Friedel-Crafts Acylation - + AlCl3 - + + [ ] + HAlCl4 HCl + AlCl3

  29. LINEAR CHAINS ARE MADE VIA ACYLATION (no rearrangement) AND REMOVAL OF C=O Clemmensen X doesn’t work - rearranges

  30. NITRATION

  31. .. .. .. : : : .. .. .. + + + H2SO4 : + .. : : : : : .. .. - - .. Formation of Nitronium Ion nitronium ion .. : Powerful Electrophile + Reacts with benzene. : ..

  32. Nitration of Benzene : .. .. .. HNO3 + : : + : : + H2SO4 .. - : .. .. : .. : + .. -

  33. SULFONATION

  34. Fuming Sulfuric Acid . H2SO4 SO3 .. .. : : .. .. : .. .. : : .. .. sulfur trioxide

  35. Sulfonation of Benzene - . H2SO4 SO3 + H3O+ D can be reversed in boiling water or steam (acidic) + H2SO4

  36. REMOVAL OF THE SULFONATE GROUP - + heat or steam excess H2O - - +

  37. Benzoic Acid Syntheses

  38. Synthesis of Benzoic Acids and Benzoate Esters CH3Cl AlCl3 KMnO4 CH3OH 1 Li 2 CO2 3 H3O+ Br2 AlBr3

  39. AROMATIC ? WHERE DID THE TERM ORIGINALLY COME FROM ?

  40. A LOT OF NICE-SMELLING COMPOUNDS (SPICES IN PARTICULAR) HAVE BENZENE RINGS thymol anisaldehyde (thyme) cinnamaldehyde (anise) (cinnamon) eugenol cuminaldehyde (cloves) (cumin) Hence, compounds having benzene rings eventually came to be know as “AROMATIC COMPOUNDS. Today chemists have a different definition of “AROMATIC” which we will discuss later in the chapter.

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