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Phenols Ar-OH

Phenols Ar-OH Phenols are compounds with an –OH group attached to an aromatic carbon. Although they share the same functional group with alcohols, where the –OH group is attached to an aliphatic carbon, the chemistry of phenols is very different from that of alcohols. Nomenclature.

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Phenols Ar-OH

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  1. Phenols Ar-OH Phenols are compounds with an –OH group attached to an aromatic carbon. Although they share the same functional group with alcohols, where the –OH group is attached to an aliphatic carbon, the chemistry of phenols is very different from that of alcohols.

  2. Nomenclature. Phenols are usually named as substituted phenols. The methylphenols are given the special name, cresols. Some other phenols are named as hydroxy compounds.

  3. physical properties phenols are polar and can hydrogen bond phenols are water insoluble phenols are stronger acids than water and will dissolve in 5% NaOH phenols are weaker acids than carbonic acid and do not dissolve in 5% NaHCO3

  4. Intramolecular hydrogen bonding is possible in some ortho-substituted phenols. This intramolecular hydrogen bonding reduces water solubility and increases volatility. Thus, o-nitrophenol is steam distillable while the isomeric p-nitrophenol is not.

  5. phenols, syntheses: • From diazonium salts • 2. Alkali fusion of sulfonates

  6. Reactions: • alcohols phenols • HX NR • PX3NR • dehydration NR • as acids phenols are more acidic • ester formation similar • oxidation NR

  7. Phenols, reactions: • as acids • ester formation • ether formation • EAS • a) nitration f) nitrosation • b) sulfonation g) coupling with diaz. salts • c) halogenation h) Kolbe • d) Friedel-Crafts alkylation i) Reimer-Tiemann • e) Friedel-Crafts acylation

  8. as acids: with active metals: with bases: CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF

  9. CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF water 5% NaOH 5% NaHCO3 phenols carboxylic acids

  10. effect of substituent groups on acid strength? Electronwithdrawing groups will decrease the negative charge in the phenoxide, lowering the PE, decreasing the ΔH, shifting the equil farther to the right, stronger acid. Electrondonating groups will increase the negative charge in the phenoxide, increasing the PE, increasing the ΔH, shifting the equilibrium to the left, weaker acid.

  11. Electrophilic Aromatic Substitution • The –OH group is a powerful activating group in EAS and an ortho/para director. • a) nitration

  12. b) halogenation

  13. c) sulfonation At low temperature the reaction is non-reversible and the lower Eact ortho-product is formed (rate control). At high temperature the reaction is reversible and the more stable para-product is formed (kinetic control).

  14. d) Friedel-Crafts alkylation.

  15. e) Friedel-Crafts acylation

  16. f) nitrosation

  17. g) coupling with diazonium salts (EAS with the weak electrophile diazonium)

  18. h) Kolbe reaction (carbonation)

  19. i) Reimer-Tiemann reaction

  20. Spectroscopy of phenols: Infrared: O—H stretching, strong, broad 3200-3600 cm-1 C—O stretch, strong, broad ~1230 cm-1 (alcohols ~ 1050 – 1200) nmr: O—H 4-7 ppm (6-12 ppm if intramolecular hydrogen bonding)

  21. o-cresol C--O O--H

  22. o-cresol c b a

  23. Ready for Question

  24. Question • Which type of chemical bonding characterized phenols ?

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