Preparation of phenols:
Phenols can be prepared by the following methods.
1) From sodium benzenesulphonate: Phenol can be prepared by the fusion of sodium benzenesulphonate with solid NaOH at 300 ºC followed by acidification.
2) From chlorobenzene (Down process): Phenol is manufactured by heating chlorobenzene with 10% solution of NaOH in presence of copper catalyst at about 623 K and under 200 atmosphere pressure.
3) From benzene diazonium salts: Phenol is prepared in the laboratory by the hydrolysis of diazonium salts with water or dilute acids.
4) From Grignard reagent: When oxygen is bubbled through the solution of phenyl magnesium bromide in ether, it forms addition products which on acidification with dilute acid give phenol.
5) From sodium salt of salicylic acid: Phenol is prepared by fusing sodium salicylate with soda lime (NaOH and CaO).
Acidity of phenols:
Phenols are weakly acidic in nature. They turn blue litmus red and react with alkali metals and alkalies to form their salts. e.g.
However, phenol is less acidic than carboxylic acid. The acidic character of phenol is because of the presence of polar –OH group. Because of larger electronegativity of oxygen atom, the electron pair of – OH bond is withdrawn more towards O and H+ ion can be easily released as:
Phenol is more acidic than alcohols:
The more acidic character of phenols as compared to alcohols can be explained on the basis of resonance. The resonance hybrid of the following structures:
It is clear that three structure of phenol (III, IV and V) have positive charge on oxygen of – OH group. This oxygen attracts the electron pair of –OH bond strongly towards itself and, therefore, facilitates the release of H+. Similarly. Phenoxide ion is resonance stabilized as follows:
Thus, both phenol and phenoxide ion are stabilized by resonance but phenoxide ion is more resonance stabilized than phenol. Thus, the resonance hybrid of phenol is less stable than phenoxide ion and the reaction is very much favour of the phenoxide ion. Therefore, phenol is more acidic.
Note:
On the other hand, in case of alcohols, there is no such stability of alkoxide ion. Thus, phenols are more acidic than alcohols. However, phenols are less acidic than carboxylic acid.
Effect of substituents on acidic strength of phenol:
It must be noted that the presence of electron withdrawing groups (-NO2, -CHO etc) and electron releasing groups(-R, -NH2, etc) also affects the acidic strength. In general,
i) The electron withdrawing groups (-X) increase the acidic strength by stabilizing the phenoxide ion by dispersal of negative charge.
ii) The electron releasing groups (Y) decrease the acidic strength by destabilizing the phenoxide ion by concentrating the negative charge.
iii) The electron releasing or electron attracting effect is generally more pronounced if the substituent is present at the ortho or para positions.
iv) The increase in the number of substituents further increases or decreases the acid strength.
Electrophilic Substitution Reaction
In phenols, – OH group activates the benzene ring towards electrophilic substitution reactions and directs the substitution at ortho and para positions.
1) Halogenation: Phenols reacts with Bromine in presence of CS2 or CCl4 give a mixture of ortho and para bromophenol.
However, Phenol reacts with bromine water to give a white precipitates of 2,4,6-tribromophenol.
2) Nitration: Phenol reacts with dilute nitric acid to give a mixture of o- and p-nitrophenol, however, with conc. nitric acid it gives picric acid.
3) Nitrosation: When phenol is treated with ice cold solution of nitrous acid (HNO2) to form p-nitrosophenol. Which can be further oxidized with dilute HNO3 to form p-nitrophenol.
4) Sulphonation: When phenol is treated with conc. sulphuric acid at 20ºc, o-phenolsulphonic acid (2-Hydroxy benzene sulphonic acid) and at 100ºc, p-phenolsulphonic acid (4-hydroxy benzene sulphonic acid) is the main product.
5) Friedel Craft Alkylation: When phenol reacts with alkyl halides in the presence of anhydrous AlCl3 to form mainly p-derivatives with small quantity of ortho-isomer.
6) Coupling Reaction: Phenols react with diazonium salts in an alkaline solution to form p-hydroxyazobenzene, called azo dyes.
7) Reimer-Tiemann Reaction: The treatment of phenol with chloroform in the presence of aqueous solution sodium hydroxide at 340k, results in the formation of o-hydroxybenzaldehyde.
8) Kolbe’s Reaction: when sodium phenoxide is heated with CO2 at 400 K and at a pressure of 4-7 atm., sodium salicylate is formed as the major product, which on acidification yields salicylic acid.
9) Reaction with phthalic anhydride: Phenol reacts with phthalic anhydride in presence of conc. H2SO4 to form phenolphthalein.
