Why is chlorine, ortho and para directing and Why ‘Cl’-atom can not form H-bond but ‘N’-atom can ?

Why is chlorine, ortho and  para directing  in spite of deactivating group ?

Most of the reaction of chloro benzene [such as nitration, sulphonation, halogenations  and Friedel craft reaction etc ] are electrophilic reaction. Now, –Cl group is a deactivating group.

Due to – I effect, chlorine atom attract electron from benzene nucleus towards itself .

But, due to + R effect ,the lone pair of ‘Cl’-atom participate in resonance with the π electron of benzene nucleus.

As a result ,the density of electron increases on ortho and para carbon atom of benzene ring .

Consequently, the electrophile attack on the ortho and para carbon atom of benzene nucleus .

Therefore , in spite of deactivating group chlorine atom acts as a ortho and para directing.

Although ,the para product is greater than ortho product due to more steric hindrance in ortho position .

For example, nitration of chloro benzene forms ortho chloro nitro benzene and para chloro nitro benzene .

Aromatic nitration reaction is an electrophilic substitution reaction . Mechanism of aromatic nitration reaction for preparation of ortho product is shown below, 

Which one is more active toward basic hydrolysis between chloro benzene and 1-chloro-2,4-dinitro chloro benzene ?

The hydrolysis of chloro benzene and 1-chloro-2,4-dinitro chloro benzene are example of aromatic nucleophilic substitution reaction . Now, –NO₂ group strong electron withdrawing group . 

So, due to  – R effect of two nitro group , the density of electron of benzene nucleus decreases .

As a result, the benzene nucleus becomes more active and easily attacked by the nucleophile .

Besides this, the inter mediate carbo anions is produced by the attack of nucleophile , gains extra stability through the resonance of two –NO₂ groups.

But, in case of chloro benzene, no such type of electron withdrawing groups are present . Hence, the carbo anions produced is less stable with respect to carbanions produced from 1-chloro-2,4-dinitro chloro benzene.

Consequently, 1-chloro-2,4-dinitro chloro benzene is more effective towards basic hydrolysis than chloro benzene.

What is the condition for the formation of hydrogen bond ?

There are two main condition for the formation of hydrogen bond of any molecule. 

(I ) The molecule which participate in the formation of hydrogen bond must be contain a specific hydrogen atom that is attached with a strong electronegative atom ( such as F, N , O etc ) and ,

(II)The atom of the electronegative molecule should be small in size.

Why ‘Cl’-atom can not form H-bond but ‘N’-atom can ?

Fluorine,oxygen and nitrogen atoms are effective for the formation of hydrogen bond . Because, only these three atoms can fulfill the sufficient condition for the formation of hydrogen bond .

Now, both chlorine and nitrogen atom are equally electronegative. That is their electronegativity value in pauling scale is 3. 

Yet chlorine atom can not form hydrogen bond ,while nitrogen atom can form hydrogen bond.

This is because of that ,the atomic radius of chlorine ( 0.99 A^ᵒ ) is larger than the atomic radius of nitrogen ( 0.70 A^ᵒ ), which does not permit the chlorine atom for the formation of stable hydrogen bond.

Summary

• Why is chlorine, ortho and  para directing  in spite of deactivating group ?
• Which one is more active toward basic hydrolysis between chloro benzene and 1-chloro-2,4-dinitro chloro benzene ?
• What is the condition for the formation of hydrogen bond ?
• Why ‘Cl’-atom can not form H-bond but ‘N’-atom can ?

Why nucleophilic substitution reaction of 1-chloro-2,4,6-tri nitro benzene occurs very easily ?