phenols ar-oh phenols are compounds with an –oh group attached to an aromatic carbon. although...
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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.
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Nomenclature.
Phenols are usually named as substituted phenols. The methylphenols are given the special name, cresols. Some other phenols are named as hydroxy compounds.
OH
phenol
OH
Br
m-bromophenol
CH3
OH
o-cresol
OH
COOH
salicylic acid
OH
OH
OH
OH
OH
OH
catechol resorcinol hydroquinone
COOH
OH
p-hydroxybenzoic acid
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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
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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.
N
OH
O
O
o-nitrophenolbp 100oC at 100 mm0.2 g / 100 mL watervolatile with steam
OH
NO2
p-nitrophenolbp decomposes1.69 g / 100 mL waternon-volatile with steam
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phenols, syntheses:
1. From diazonium salts
2. Alkali fusion of sulfonates
N2
H2O,H+
OH
SO3 Na NaOH,H2O
300o
ONaH+ OH
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Reactions:
alcohols phenols
1. HX NR
2. PX3 NR
3. dehydration NR
4. as acids phenols are more acidic
5. ester formation similar
6. oxidation NR
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Phenols, reactions:
1. as acids
2. ester formation
3. ether formation
4. 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
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as acids:
with active metals:
with bases:
CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF
OH
Na
ONa
sodium phenoxide
+ H2(g)
OH
+ NaOH
ONa
+ H2O
SA SB WB WA
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CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF
OH
+ NaOH
ONa
+ H2O
SA SB WB WA
water insoluble water soluble
OH
+ NaHCO3 NR phenol < H2CO3
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insoluble soluble insoluble
insoluble soluble soluble
water 5% NaOH 5% NaHCO3
phenols
carboxylic acids
CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF
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We use the ionization of acids in water to measure acid strength (Ka):
HBase + H2O H3O+ + Base-
Ka = [H3O+ ][ Base ] / [ HBase]
ROH Ka ~ 10-16 - 10-18
ArOH Ka ~ 10-10
Why are phenols more acidic than alcohols?
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ROH + H2O H3O+ + RO-
ArOH + H2O H3O+ + ArO-
OH OH O O O O O
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
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effect of substituent groups on acid strength?
OH
G + H2O
O
G + H3O
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.
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Number the following acids in decreasing order of acid strength (let # 1 = most acidic, etc.)
OH OH OH OH OH
NO2 CH3 Br
3 5 1 4 2
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SO3H COOH OH CH2OH
1 2 3 4
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2. ester formation (similar to alcohols)
OH
CH3+ CH3CH2C
O
OH
H+
CH3CH2CO
O
H3C
+ H2O
OH
COOH
salicyclic acid
+ (CH3CO)2O
O
COOH
CH3CO
aspirin
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O
COOH
CH3CO
aspirin
analgesicanti-inflamatoryantipyrreticanticoagulant
Reye's syndromenot to be used by childrenwith high fevers!
OH
NHCH3C
O
acetaminophen
aspirin substituteTylenol
Kidney damage!
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3. 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.
OH
CH3
+ CH3CH2Br, NaOH
OCH2CH3
CH3
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4. Electrophilic Aromatic Substitution
The –OH group is a powerful activating group in EAS and an ortho/para director.
a) nitration
OH OH
NO2
NO2
O2Npolynitration!
OH
dilute HNO3
OH OH
NO2
NO2
+
HNO3
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OH
Br2 (aq.)
OH
Br
Br
Br no catalyst required
use polar solvent
polyhalogenation!
OH
Br2, CCl4
OH OH
Br
Br
+
non-polar solvent
b) halogenation
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c) sulfonation
OH
H2SO4, 15-20oC
OH
SO3H
H2SO4, 100oC
OH
SO3H
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).
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d) Friedel-Crafts alkylation.
OH
+ H3C C CH3
CH3
Cl
AlCl3
OH
C CH3
CH3
H3C
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e) Friedel-Crafts acylation
OH
CH3CH2CH2CO
Cl+
AlCl3
OH
O
Do not confuse FC acylation with esterification:
OH
CH3CH2CH2CO
Cl+ O
O
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OH
O
OH
CH3CH2CH2CO
Cl+ O
O
AlCl3
Fries rearrangement of phenolic esters.
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f) nitrosation
OHHONO
OH
NO
EAS with very weak electrophile NO+
OH
CH3 NaNO2, HCl
OH
CH3
NO
p-nitrosophenol
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g) coupling with diazonium salts
(EAS with the weak electrophile diazonium)
OH
CH3+
N2 Cl
benzenediazoniumchloride
CH3
OH
N
N
an azo dye
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h) Kolbe reaction (carbonation)
ONa
+ CO2
125oC, 4-7 atm.
OH
COONa
sodium salicylate
H+
OH
COOH
salicylic acid
EAS by the weaklyelectrophilic CO2
O C O
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i) Reimer-Tiemann reaction
OH
CHCl3, aq. NaOH
70oC
H+
OH
CHO
salicylaldehyde
The salicylaldehyde can be easily oxidized to salicylic acid
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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)
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o-cresol
C--O
O--H
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o-cresol
OH
CH3 a
b
c
c b a
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ethyl salicylate (intramolecular hydrogen bonding)
OH
CO
O
CH2CH3
abc
d
d c b a