What is Dow process and Compare the boiling point of ortho - meta and para dichloro benzene .

What is Dow process ? Give example .

When the mixture of chloro benzene and 6 – 8 % solution of sodium hydroxide is heated at 350 ᵒC temperature under 300 atmosphere pressure , sodium phenoxide is obtained .

 Now, on acidic hydrolysis of sodium phenoxide , gives phenol . This process of preparation of phenol from chloro benzene is known as Dow process . 

This is an example of aromatic nucleophilic substitution reaction . 

The reaction is carried out through active benzyne inter mediate formation .

Example of Dow process is as follows,

The reaction mechanism of Dow process is shown below .

Why the hydrolysis of o-nitro chloro benzene occurs easily than chloro benzene in basic medium ?

The hydrolysis of o-nitro chloro benzene and chloro benzene is an example of nucleophilic substitution reaction .

The nucleophilic substitution reaction is facilitated ,if the compound contain electron withdrawing group.  

Now, in ortho nitro chloro benzene , the –NO₂ group is an electron withdrawing group. 

 Due to – R effect of –NO₂ group , the density of electron of benzene nucleus decreases . 
As a result , the nucleophile can attack the targeted carbon atom easily . 

Besides, the inter mediate carbanions produced , gains extra stability through resonance with the π electrons of benzene ring and –NO₂ group .

Consequently, the hydrolysis of o-nitro chloro benzene occurs easily than chloro benzene in basic medium .

 

Compare the boiling point of ortho, meta and para dichloro benzene .

The boiling point of ortho, meta and para dichloro benzene are close to each other . 

But ,among this three isomers the boiling point ortho isomer is greater than meta and para isomers .

This can be explained with their dipole moment value ,that is force of dipole –dipole attraction between the molecules . 

The experimentally dipole moment value of ortho, meta and para dichloro benzene are 2.30 D , 1.48 D and 0 D respectively. 

So, the boiling point of the above three isomers should be in the following order,

o-dichloro benzene > m-dichloro benzene > p-dichloro brenzene .

But the actual order is o-dichloro benzene > p-dichloro brenzene > m-dichloro benzene.  

That is ,the boiling point ofpara isomer is slightly higher than meta isomer . Although the dipole moment of para dichloro benzene is  0 D .

Because the p-dichloro brenzene molecules is more symmetrical . So , in crystal lattice of p-dichloro brenzene , the molecules are close to each other . 

Hence, to separate the molecules , high energy is required .

Consequently, the boiling point of para isomer is slightly higher than meta isomer.

 

 Why fluoro arene or aryl fluoride can not prepared by direct fluorination of aromatic hydrocarbons ? 

Fluoro-arene or aryl fluoride can not prepared by direct fluorination of aromatic hydrocarbons, because fluorine is highly reactive in chemical reaction . 

On reaction between fluorine and aromatic hydrocarbon an explosion occurs which can not be controlled .  

 

What is the best method for the preparation fluoro arene or aryl fluoride ?

The best method for the preparation fluoro arene or aryl fluoride is known as shimaan reaction . 

In this reaction , fluoro boric acid is added to the solution of benzene diazonium chloride salt . 

As a result , a precipitate of insoluble diazonium fluoro borate is appeared . It is dried after separating this precipitate from the solution . 

Then ,on heating this dry sample gives fluoro arene or aryl fluoride .  

Summary :

What is Dow process ?  Give example .

Why the hydrolysis of o-nitro chloro benzene occurs easily than chloro benzene in basic medium ?

Compare the boiling point of ortho, meta and para dichloro benzene .

Why fluoro arene or aryl fluoride can not prepared by direct fluorination of aromatic hydrocarbons ? 

What is the best method for the preparation fluoro arene or aryl fluoride ?

What is aFinkelstein reaction ? click here .