Benzoic acid-definition-properties-preparation from benzene and phenol.

                                Benzoic acid definition in chemistry 

The most simplest aromatic mono carboxylic acid with molecular formula C₆H₅COOH, is called benzoic acid.This is also known as benzene carboxylic acid.

Benzoic acid is a colourless or white crystalline solid with pleasant odor . Benzoic acid occurs naturally in many plants, in certain resins, particularly gum-benzoin . 

It also present in balsams, in the urine of horses as hippuric acid.

Benzoic acid properties 

In general, benzoic acid is slightly stronger acid than aliphatic , [ except formic acid ] less soluble in water and less volatile. The melting point of benzoic acid is 395 K  and the boiling point is 523 K.

The benzoic acid is slightly soluble in water. But ,it more soluble in acetone ,benzene , carbon tetra chloride,chloroform ,alcohol, ethyl ether, hexane and liquid ammonia etc.

The reactions of the carboxylic group are very similar to those in the aliphatic acids , but apparently they differ in that they do not undergo coupling by the Kolbe electrolytic synthesis .

However, according to Fichter, the electrolysis of a solution of benzoic acid in pyridine gave a number of products, among which was biphenyl. 

Benzoic acid from benzene

Benzoic acid can be prepared from benzene in two step . In first step, Friedel craft’s alkylation of benzene produced mono alkyl benzene , such as toluene.

Then , in the second step, oxidation of this mono alkyl benzene or toluene in presence potassium dichromate and concentrated sulfuric acid gives the expected benzoic acid.
  

Benzoic acid from phenol 

There are different method for preparation of benzoic acid from phenol. Among them , one important method for preparation of benzoic acid from phenol via diazotization reaction .

In this method phenol is react first with ammonia in presence of anhydrous ZnCl₂ Lewis acid .The product obtained is aniline.

In second step, aniline under goes diazotization reaction in presence of sodium nitrite and dilute HCl  at very low temperature ( 0ᵒ – 5ᵒ C ) , the product obtained is benzene 

diazonium chloride salt , which , on sandmeyer reaction  with CuCN / KCN  forms cyano benzene.

Finally , on acidic hydrolysis of cyano benzene , the expected product , benzoic acid is obtained.

The stepwise reactions for this method is shown above .

Summary

                               

• Benzoic acid definition in chemistry 
• Benzoic acid properties 
• Benzoic acid from benzene 
• Benzoic acid from phenol 

Why phenol is acidic in nature ?