Phenols (Ar-OH)

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.

 

Dr. Talat R. Al-Ramadhany

Phenol                                  Benzylalcohol

                  −OH is directly linked           −OH is not directly linked

                to the aromatic ring carbon     to the aromatic ring carbon 

Dr. Talat R. Al-Ramadhany

Dr. 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.

 

Dr. Talat R. Al-Ramadhany

      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. 

Dr. Talat R. Al-Ramadhany

      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. 

Dr. Talat R. Al-Ramadhany

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.

 

Dr. Talat R. Al-Ramadhany

      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. 

Dr. Talat R. Al-Ramadhany

Industrial sours of Phenols:

i. 

Conc.

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

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

Preparation of Phenols in the laboratory

Hydrolysis of diazonium salts

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Oxidation of arylthallium compounds

This method has two advantages over the diazonium route:

The speed and high yield.

Orientation control in the thallation step.

Dr. Talat R. Al-Ramadhany

Alkaline hydrolysis of aryl halides

Dr. Talat R. Al-Ramadhany

Dr. Talat R. Al-Ramadhany

Reaction of Phenols

There are two type of reaction:

Reaction of (O–H) bond.

Reaction of aromatic ring

          (Electrophilic Aromatic Substitution).

Dr. Talat R. Al-Ramadhany

Reaction of (O–H) bond.

Acidity, salt formation.

Ether formation (Williamson Synthesis)

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

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

Dr. Talat R. Al-Ramadhany

Ester formation

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

 Phenol               Benzoyl chloride                                    Phenyl benzoate

(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.

B. Reaction of aromatic ring

(Electrophilic Aromatic Substitution).

Halogenation (Bromination)

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

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

Some group can replace by bromine

Sulfonation:

Nitration:

Friedel-Crafts alkylation

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

      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.

Nitrosation:

Nitrous acid converts Phenols into nitrosophenols

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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.

Dr. Talat R. Al-Ramadhany

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.

Dr. Talat R. Al-Ramadhany