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.

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

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

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

phenols, syntheses:

1. From diazonium salts

2. Alkali fusion of sulfonates

N2

H2O,H+

OH

SO3 Na NaOH,H2O

300o

ONaH+ OH

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

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

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

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

insoluble soluble insoluble

insoluble soluble soluble

water 5% NaOH 5% NaHCO3

phenols

carboxylic acids

CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF

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?

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

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.

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

SO3H COOH OH CH2OH

1 2 3 4

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

O

COOH

CH3CO

aspirin

analgesicanti-inflamatoryantipyrreticanticoagulant

Reye's syndromenot to be used by childrenwith high fevers!

OH

NHCH3C

O

acetaminophen

aspirin substituteTylenol

Kidney damage!

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

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

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

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).

d) Friedel-Crafts alkylation.

OH

+ H3C C CH3

CH3

Cl

AlCl3

OH

C CH3

CH3

H3C

e) Friedel-Crafts acylation

OH

CH3CH2CH2CO

Cl+

AlCl3

OH

O

Do not confuse FC acylation with esterification:

OH

CH3CH2CH2CO

Cl+ O

O

OH

O

OH

CH3CH2CH2CO

Cl+ O

O

AlCl3

Fries rearrangement of phenolic esters.

f) nitrosation

OHHONO

OH

NO

EAS with very weak electrophile NO+

OH

CH3 NaNO2, HCl

OH

CH3

NO

p-nitrosophenol

g) coupling with diazonium salts

(EAS with the weak electrophile diazonium)

OH

CH3+

N2 Cl

benzenediazoniumchloride

CH3

OH

N

N

an azo dye

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

i) Reimer-Tiemann reaction

OH

CHCl3, aq. NaOH

70oC

H+

OH

CHO

salicylaldehyde

The salicylaldehyde can be easily oxidized to salicylic acid

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)

o-cresol

C--O

O--H

o-cresol

OH

CH3 a

b

c

c b a

ethyl salicylate (intramolecular hydrogen bonding)

OH

CO

O

CH2CH3

abc

d

d c b a