Tuesday, February 5, 2008

Phenols - Chemical Properties

Can be classified into three groups

A. Reactions of phenolic group (-OH group)
B. Reactions of benzene ring
C. Special reactions

A. Reactions of phenolic group (_OH group)

1.Acidic character:
They turn blue litmus red and react with alkali metals and alkalies to form their salts.

Phenol is weaker acid than carboxylic acid and stronger acid compared to alcohols.

2. Action with zinc dust
When heated with zinc dust, phenol is reduced to benzene.

3. Action with ammonia
Phenol reacts with ammonia in the presenceof anhydrous zinc chloride to give aniline.

4. Action with acid chlorides and acid anhydrides
Esters are formed.

5. Action with benzyl chloride
Phneol benzoate is formed. The reaction occurs in the presence of aqueous NaOH.
Reaction is named Schotten Baumann reaction

B. Reactions of benzene ring (Substitution)

Phenols undergo electrophilic aromatic substitution reactions. OH group is an activating group. Therefore the reactions with phenol occur at a faster rate than reaction with benzene. OH group directs the substituents to ortho and para positions.


1a. Bromine Br in ortho and para positions 2,4,6-tribromophenol
1b. If the reaction is carried out in CS2 or CCl4, a mixture of ortho and para bromophenol is obtained.

2a. NO2 - Reaction of phenol with concentrated HNO3 in the presence of H2SO4 gives 2,4,6-trinitrophenol.

2,4,6-trinitrophenol is also termed picric acid.
2b. With dilute nitric acid phenol reacts at low temperature (293K) and gives a mixture of ortho and para nitrophenol.

3. NO – reaction with nitrous acid (NaNO2 + HCL) at low temperature (280 K) gives p-nitrosophenol (p indicates that NO is in para position to OH)). This reaction is called nitrosation.

4. SO3H – Phenol reacts with concentrated sulphuric acid to form a mixture of 0- and p-phenol sulphonic acid.

5. CH3 – When phenol is heated with alkyl halides in the presence of anhydrous chloride, CH3 becomes a substituent. The phenol with CH3 as a substituent is termed Cresol. O-cresol and p-cresol are formed. The reaction is called Friedel Craft reaction.

Special reactions of Phenol

1. Kolbe's reaction:
It is a reaction of sodium phenoxide.
When sodium phenoxide is heated with carbon dioxide at about 400 K and under 4 to 7 atm, sodium salicylate is formed as a major product. This on acidification gives salicylic acid.

2. Reimer-Tiemann reaction:
When phenol is refluxed with chloroform in the presence of acqueous sodium hydroxide at 340 K followed by hydrolysis, an aldehydic group (HC=O) gets introduced in the ring at a position ortho to the phenolic group. So ortho hydroxy bezaldehyde is formed. It is also called salicylaldehyde.

3. Coupling reaction
In the alkaline medium, an ice cold solution of phenol combines with an ice cold solution of benzene diazonium chloride to form azo dye.

4. Reaction with pthalic anhydride
Phenol reacts with phthalic anhydride in the presence of conc. H2So4 to give phenolphthalein.

5. Condensation with formaldehyde
Phenol condenses with formaldehyde in the presence of dil. Acid or alkali as catalyst to give a polymer called bakelite.

6. Hydrogenation
Phenol can be converted to cyclohexagonal in the presence of nickel catalyst at 430 K.

7. Oxidation
On exposure to air or chromic acid, phenol undergoes oxidation to p-benzoquinone. It is a pink coloured compound.

8. Reaction with ferric chloride
Phenols react with neutral ferric chloride to form coloured water soluble complex compounds.

9. Libermann's test
On warming with conc. H2SO4 and sodium nitrite, phenols give red or brown colouration. The colour changes to blue or green by the addition of aqueous NaOH.

No comments: