phenols (ar-oh). phenols are compounds of the general formula ar-oh, where ar- is phenyl,...

Phenols (Ar-OH)

Phenols are compounds of the general formula Ar-OH, where Ar- is phenyl, substituted phenyl, or one of the other aryl groups.

I. Structure and nomenclature:

Phenols differ from alcohols in having the -OH group attached directly to an aromatic ring. Hydroxybenzene, the simplest member of the phenols, is generally referred to as phenol.

OH

G

OH

G

Dr. Talat R. Al-Ramadhany

OH CH2OH

Phenol Benzylalcohol

−OH is directly linked −OH is not directly linked

to the aromatic ring carbon to the aromatic ring carbon


OH

Cl

o-Chlorophenol

OH

CH3

o-cresol

OH

CH3

m-Cresol

COOH

OH

o-Hydroxybenzoic acid

COOH

OH

p-Hydroxybenzoic acidDr. Talat R. Al-Ramadhany

II. Physical properties:

The simplest Phenols are liquid or low-melting solids.

Phenols have high boiling points.

Phenol itself is somewhat soluble in water

most other Phenols are essentially insoluble in water.

Phenols are colorless, but they easily oxidized by atmospheric air and become colored compounds.


m- and p- isomers have higher boiling point because of the intermolecular hydrogen bonding and their solubility in water is due to the hydrogen bonding with water.


For o-nitrophenol, the –NO2 and –OH groups are closed to each other and they form intramolecular hydrogen bonding (within a single molecule). Therefore o-nitrophenol does not have the low volatility of an associated liquid, cannot form hydrogen bonding with water, therefore it have lower solubility in water.


III. Acidity of Phenols:

Phenols are fairly acidic compounds, and in this respect differ markedly from alcohols, which are even more weakly acidic than water.

Carboxylic acid > Phenol > Water > Alcohol

Aqueous hydroxides convert Phenols into their salts; aqueous mineral acids convert salts back into the free Phenols.

Ar-OH ArO-OH-

H+

A Phenol A Phenoxide ion(acid) (salt)

Insoluble Solublein water in water


The acidity of phenols is mainly due to an electrical charge distribution in phenols that causes the -OH oxygen to be more positive. As a result, the proton is held less strongly, and phenols can easily give this loosely held proton away to form a phenoxide ion as outlined below.


Industrial sours of Phenols:

i. Conc.


Cl

NaOH, 360o

4500 lb/in.2

O-Na+

HCl

OH

Sodium phenoxideChlorobenzene Phenol

ii. Dow process, in which Chlorobenzene is allowed to react the aqueous sodium hydroxide at a temperature of about 360ºC.

iii. Oxidation of Cumene: Cumene is converted by air oxidation into cumene hydroperoxide, which is converted by aqueous acid into Phenol and acetone.

C HH3C

CH3

C OOHH3C

CH3

OH

C O

CH3

H3C+

Cumene Cumenehydroperoxide

PhenolAcetone

O2 H2O, H+


Preparation of Phenols in the laboratory

i. Hydrolysis of diazonium salts

ArN2+ + H2O ArOH + H+ + N2

N2+HSO4

-

Cl

OH

Cl

H2O, H+

+ N2heat


ii. Oxidation of arylthallium compounds

This method has two advantages over the diazonium route:

A.The speed and high yield.

B.Orientation control in the thallation step.

Tl(OOCCF3)3

Tl(O

Pb(OAc)4

Ph3P

OC

O

CF3)2 C CF3

O

Aryltrifluoroacetate

H2O, OH-

heat

H+

O- OH

Arylthalliumtrifluoroacetate


iii. Alkaline hydrolysis of aryl halides

Cl OH

NaOH (10%)

350o

10%

Cl OH

NaOH (15%)

160o

15%

NO2 NO2


Cl O-Na+

NO2 NO2

NO2

NaOHNO2

H+

OH

NO2

NO2

OH

NO2

NO2

HNO3

H2SO4

O2N

Picric acid

Cl

NO2

NO2O2N

H2O

OH

NO2

NO2O2N

90%


Reaction of Phenols

There are two type of reaction:

A. Reaction of (O–H) bond.

B. Reaction of aromatic ring

(Electrophilic Aromatic Substitution).

O H

A. Reaction of (O–H) bond.

1) Acidity, salt formation.

OH

+ NaOH

O Na

+ H2O

OH

+ NaHCO3

O Na

+ H2CO3

2) Ether formation (Williamson Synthesis)

aqueous NaOH

OH O-

R X

O R

+ Xheat

Phenols are converted into ethers by reaction in alkaline solution with alkyl halides.

OH

+ C2H5Iaqueous NaOH

heat

OC2H5


heat

OH

aq.NaOH

O-

phCH2Br

OCH2ph

Cl

+ phCH2O-


Aryl halide must be containing strong electron-withdrawing group to form corresponding ether.

Cl

+ CH3O- Na+

NO2

NO2

OCH3

NO2

NO2

3) Ester formation

Phenols are usually converted into their esters by the reaction with carboxylic acids, acid chlorides or anhydrides.

OH + COCl O C

O

NaOH

Phenol Benzoyl chloride Phenyl benzoate

OH + (CH3CO)2O O C

O

CH3COONaO2N CH3O2N

p-Nitrophenyl acetatep-Nitrophenol Acetic anhydride

(Fries rearrangement)

When esters of Phenols are heated with aluminum chloride, the acyl group migrates from the Phenolic oxygen to an ortho or para position of the ring and yield a ketone. This reaction is called the Fries rearrangement, is often used to prepare phenolic ketones.

OH O

C2H5COCl

C C2H5

O

AlCl3CS2

OH

C C2H5

O

+

OH

CO

C2H5Phenol Phenyl propionate o-Hydroxyphenyl

ethyl ketone

p-Hydroxyphenylethyl ketone

B. Reaction of aromatic ring

(Electrophilic Aromatic Substitution).

1. Halogenation (Bromination)

Treatment of Phenols with aqueous solution of bromine results in replacement of every hydrogen ortho or para to the –OH group

OH

+ 3Br2

H2O

OH

Br

Br

Br

2,4,6-tribromophenol

+ 3HBr

If halogenation is carried out in a solvent of low polarity:

OH

+ 2Br2

CHCl3

OH

Br

+ 2HBr

OH

Br

+nonpolarsolvent

OH

+ 2Br2

CS2+ HBr

OH

Br

0oC

OH

CH3

+ 2Br2 (aq)

OH

CH3Br

Br

2,4-Dibromo-6-methylphenol

+ 2HBr

o-Cresol

Some group can replace by bromine

+ 3Br2 (aq)

OH

BrBr

Br

+ 3HBr + H2SO4

OH

SO3H

p-Phenolsulfonic acid 2,4,6-Tribromophenol

2) Sulfonation:

OH

H2SO4

15-20oC

100oC

OH

SO3H

OH

SO3H

o-Phenolsulfonic acid

p-Phenolsulfonic acid

H2SO4, 100oC

Conc.

Conc.

3) Nitration:

OH OH

NO2dilute HNO3

20oC +

OH

NO2

p-Nitrophenolo-Nitrophenol40% yield 13% yield

OH OH

NO2

NO2

O2NConc.HNO3

Picric acid

20oC

4) Friedel-Crafts alkylation

OH + H3C C

CH3

CH3

Cl

HO C

CH3

CH3

CH3

p-tert-Butylphenoltert-Butylchloride

HF

Alkyl phenols can be prepared by Friedel-Crafts alkylation of Phenols, but the yields are often poor.

OH

CH3

m-Cresol

O

CH3

C

O

CH3

m-Cresyl acetate

AlCl3

25oC

160oC

(CH3CO)2O

OH

CH3

C CH3O2-Methyl-4-hydroxyacetophenone

OH

CH3

CH3C

O

4-Methyl-2-hydroxyacetophenone

Phenolic ketones can be made by direct Friedel-Crafts acylation of Phenols, they are more often prepared in two steps by means of the Fries rearrangement.


5) Nitrosation:

Nitrous acid converts Phenols into nitrosophenols

OH

NaNO2 , H2SO4

7 - 8oC

OH

NOp-Nitrosophenol

80% yield


6) Synthesis of Phenolic acids (Kolbe reaction):

Treatment of the salts of a Phenol with carbon dioxide brings about substitution of the carboxyl group, -COOH, for hydrogen of the ring. This reaction is known as the Kolbe reaction; its most important application is in the conversion of Phenol into o-Hydroxybenzoic acid, known as salicylic acid.

ONa

+

O

C

O

125oC

4-7 atm.

OH

COONa

Sodium salicylate(Chief product)

H+

OH

COOH

Salicylic acid


7) Synthesis of Phenolic aldehydes (Reimer-Tiemann reaction):

Treatment of Phenol with chloroform and aqueous hydroxide introduces an aldehyde group, –CHO, into the aromatic ring, generally ortho to the –OH. This reaction is known as the Reimer-Tiemann reaction.

OH O-

CHCl2CHCl3, aq.NaOH

70oC

O-

CHO

OH

CHOHCl

Salicyladehyde(Chief product)

phenols (ar-oh). phenols are compounds of the general formula ar-oh, where ar- is phenyl,...

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