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CHAPTER 21 PHENOLS AND ARYL HALIDES NUCLEOPHILIC AROMATIC SUBSTITUTION

CHAPTER 21 PHENOLS AND ARYL HALIDES NUCLEOPHILIC AROMATIC SUBSTITUTION. 21.1 STRUCTURE AND NOMENCLATURE OF PHENOLS Phenol: Compounds that have a hydroxyl group directly attached to a benzene ring For example:. Naphthols or phenanthrols : Compounds that have a hydroxyl group attached

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CHAPTER 21 PHENOLS AND ARYL HALIDES NUCLEOPHILIC AROMATIC SUBSTITUTION

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  1. CHAPTER 21PHENOLS AND ARYL HALIDES NUCLEOPHILIC AROMATIC SUBSTITUTION 21.1 STRUCTURE AND NOMENCLATURE OF PHENOLS Phenol: Compounds that have a hydroxyl group directly attached to a benzene ring For example:

  2. Naphthols or phenanthrols: Compounds that have a hydroxyl group attached to a polycyclic benzenoid ring. For example: 21.1A NOMENCLATURE OF PHENOLS (1) In many compounds phenol is the base name. For example:

  3. (2) The methylphenols are commonly called cresols. For example: (3) The benzenediols also have common names.

  4. 21.2 NATURALLY OCCURRING PHENOLS Phenols and related compounds occur widely in nature. For example:

  5. 21.3 PHYSICAL PROPERTIES OF PHENOLS • Having higher boiling points: phenols are able to form strong intermolecular hydrogen bonds . For example: phenol • (bp,182℃) has a boiling point more than 70℃ higher than toluene(bp,110.6℃),even though the two molecular have almost the same molecular weight. (2) Modest solubility in water: the ability to form strong hydrogen bonds to molecules of water 21.4 SYNTHESIS OF PHENOLS 21.4A LABORATORY SYNTHESIS General Reaction:

  6. Specific Examples: 21.4B INDUSTRIAL SYNTHESIS 1. Hydrolysis of Chlorobenzene

  7. 2. Alkali Fusion of Sodium Benzenesulfonate 3. From Cumene Hydroperoxide

  8. Corresponding Mechanism:

  9. 25.1 REACTIONS OF PHENOLS AS ACIDS 21.5A STRENGTH OF PHENOLS AS ACIDS

  10. Phenols are much stronger acids than alcohols. For example: The reason : (1) The carbon atom that bears the hydroxyl group in phenol is sp2-hybridized, whereas, in cyclohexane , it is sp3 –hybridized. (2) Resonance structures for phenol:

  11. 21.5B DISTINGUISHING AND SEPARATING PHENOLS FROM ALCOHOLS AND CARBOXYLIC ACIDS Phenols dissolve in aqueous sodium hydroxide : Phenols are more acidic than water. Whereas most alcohols with six carbon atoms or more do not dissolve in aqueous sodium hydroxide . we can distinguish And separate phenols from most alcohols by this way.

  12. 21.6 OTHER REACTION PF THE OH- GROUP OF PHENOL Phenols react with carboxylic acid anhydrides and acid chlorides to form esters. For example: 21.6A PHENOLS IN THE WILLIAMSON SYNTHESIS Phenols can be convert to ethers through the williamson synthesis. General reaction:

  13. Specific Examples: 21.7 CLEAVAGE OF ALKYL ARYL ETHERS When alkyl aryl ethers react with strong acids such as HI and HBr, the reaction produces an alkyl halide and a phenol. For example:

  14. Specific Example: 21.8 REACTION OF THE BENZENE RING OF PHENOLS Bromination:

  15. Nitration: Sulfonation:

  16. Kolbe Reaction: 21.9 THE CLAISEN REARRANGEMENT Claisen rearrangement: heating allyl phenyl ether to 200℃ effects an intramolecular reaction.

  17. Mechanism: A Claisen rearrangement also takes place when allyl vinyl ethers are heated.

  18. 21.10 QUINONES Hydroquinones produces ρ-Benzoquinone by mide oxidizing agents ρ-Benzoquinone is easily reduced by mild reducing agents to hydroquinones 21.11 ARYL HALIDES AND NUCLEOPHILIC AROMATIC SUBSTITUTION

  19. Aryl halides and vinylic halides are relatively unreactive toward nucleophilic substitution under conditions that give facile nucleophilic substitution with alkyl halides. Reason: (1) Phenyl cations are very unstable. (2) Halogen bonds of aryl (and vinylic) halides are shorter and stronger than those of alkyl, allylic, and benzylic halides because of the hybridized state and the resonance. But aryl halides can be remarkably reactive toward nucleophiles if they bear certain substituents or when we allow them to react under the proper conditions.

  20. 21.11A NUCLEOPHILIC AROMATIC SUBSTITUTION BY ADDITION – ELIMINATION: THE SNAr MECHANISM Nucleophilic substitution can occur when strong electron-withdrawing groups are ortho or para to the halogen atom.

  21. But the meta-nitro group does not produce a similar activating effect. Mechanism: The delocalized carbanion is stabilized by electron-withdrawing groups in the positions ortho and para to the halogen atom.

  22. 21.11B NUCLEOPHILIC AROMATIC SUBSTITUTION THROUGH AN ELIMINATION-ADDITION MECHANISM: BENZYNE Chlorobenzene can be converted to phenol by heating it with aqueous sodium hydroxide in a pressurized reactor . Bromobenzene reacts with the very powerful base, in liquid ammonia

  23. C-14 bromobenzene is treated with amide ion in liquid ammonia, the aniline that is produced between the 1 and 2 position. When the ortho derivative 1 is treated with sodium amide, the only organic product obtained is m-(trifluoromethyl)aniline.

  24. Mechanism: Carbanion 3 is more stable than 4 because the carbon atom bearing the negative charge is closer to the highly electronegative trifluoro- methyl group.

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