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 form 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
Acidity of Phenols. Phenols are more acidic than aliphatic alcohols
ROH H2O H3O RO-
ArOH H2O H3O ArO-

4. Phenol Acidity
Phenols (pKa ~10) are much more acidic than alcohols (pKa ~ 16) due to resonance stabilization of the phenoxide ion
Phenols react with NaOH solutions (but alcohols do not), forming soluble salts that are soluble in dilute aqueous
A phenolic component can be separated from an organic solution by extraction into basic aqueous solution and is isolated after acid is added to the solution

5. Factors that influence acidity: Inductive effect:
CH3CH2OH FCH2CH2OH F2CHCH2OH F3CCH2OH (F3C)3COH
pKa ~
Electron-withdrawing groups make an
alcohol a stronger acid by stabilizing
the conjugate base (alkoxide)
A benzene ring is generally considered electron withdrawing
and stabilizes the negative charge through inductive effects
Resonance effect: the benzene ring stabilizes the phenoxide
ion by resonance delocalization of the negative charge

6. Substituent Effects on the Acidity of Phenols
Electron-donating substituents make a phenol less acidic by destabilizing the phenoxide ion (resonance effect)
X= -H -CH3 -OCH3 -NH2
pKa ~

7. The influence of a substituent on phenol acidity is also
Electron-withdrawing substituents: make a phenol more acidic by stabilizing the phenoxide ion through delocalization of the negative charge and through inductive effects.
X= -H -Cl -Br -NO2
pKa ~
The influence of a substituent on phenol acidity is also
dependent on its position relative to the -OH
pKa X= para meta
-NO
-OCH
-CH

8. The effect of multiple substituents on phenol acidity is additive.

9. 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.

10. Phenols, syntheses:
From diazonium salts
2. Alkali Fusion of Sulfonates

11. From Hydrolysis of Aryl Halides (Dow Process)
From the Oxidation of isopropenyl benzene (Cumene)
When exposed to oxygen of the air , hydroperoxide forms

12. From aryl ketones Reactions: alcohols phenols HX NR PX3 NR
dehydration NR
as acids phenols are more acidic
ester formation similar
oxidation NR

13. 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

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

15. NaHCO3 is weak base while NaOH is a strong base
CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF
NaHCO3 is weak base while NaOH is a strong base

16. water 5% NaOH 5% NaHCO3
insoluble
soluble
phenols
carboxylic acids

17. We use the ionization of acids in water to measure acid strength (Ka):
HBase H2O H3O Base-
Ka = [H3O+ ][ Base ] / [ HBase]
ROH Ka ~
ArOH Ka ~ 10-10
Why are phenols more acidic than alcohols?

18. ROH H2O H3O RO-
ArOH H2O H3O ArO-
Resonance stabilization of the phenoxide ion, lowers the PE of the products of the ionization, decreases the ΔH, shifts the equil farther to the right, makes phenol more acidic than an alcohol

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

20. Number the following acids in decreasing order of acid strength (let # 1 = most acidic, etc.)

21. Ester Formation (similar to alcohols)

23. Ether Formation (Williamson Synthesis)
Ar-O-Na R-X  Ar-O-R NaX
note: R-X must be 1o or CH3
Because phenols are more acidic than water, it is possible to generate the phenoxide in situ using NaOH.

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

25. b) Halogenation

26. 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).

27. d) Friedel-Crafts alkylation.

28. e) Friedel-Crafts acylation

29. Fries rearrangement of phenolic esters.
Acylation of Phenols. In the absence if AlCl3, phenols
react with acid chlorides to afford phenyl esters.

30. f) Nitrosation

31. g) Coupling with Diazonium Salts
(EAS with the weak electrophile diazonium)

32. h) Kolbe reaction (carbonation)

34. i) Reimer-Tiemann reaction

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

36. o-cresol
C--O
O--H

37. o-cresol
c b a

38. ethyl salicylate (intramolecular hydrogen bonding)
d c b a