CBSE Class 12 Chemistry Notes: Haloarenes – Chemical Reactions

CHEMICAL REACTIONS OF HALOARENES:-

Haloarenes are extremely less reactive than haloalkanes towards Nucleophilc Substitution Reactionsdue to following reasons.

 

REASONS HALOARENES DO NOT SHOW NUCLEOPHILIC SUBSTITUTION:-

i.  Resonance effect:-

In haloarenes the lone pair of electrons on halogen atom is delocalized on the benzenering.

 

Since aryl halides are stabilizes by resonance hence the energy of activation for displacement of halogen from aryl halide is much greater than alkyl halides thus they not under go Nucleophilic Substitution reaction.

ii. Difference of hybridization of carbon atom in C – X bond:-

 

Since in haloarenes the C atom to which the X group is attached is SP² hybridized thus it is become difficult to replace it by the Nucleophile.

iii. Electronic Repulsion:-

Since arenes and Vinyl halides are electron rich molecules due to presenceof n bonds, they repel Nucleophile attacking them.

 

NUCLEOPHILIC SUBSTITUTION FOR HALOARENES:-

Due to low reactivity Nucleophilic substitution reactions in arylhalides occurs under drastic conditions.

i. Replacement by (- OH group):-

The presence of electron withdrawing groups such as –NO₂, –CN at O and P^– positions of haloarenes, w.r.t halogen greatly activates the halogen towards Nucleophilic Substitution

 

 

ii. Mechanism and explanation:-

The presence of NO₂ groups at O – and P positions withdraws electrons from benzene ring and Facilitates the attack of Nucleophile on haloarenes.

Thus carbocation is stabilized by resonance

Attack at o-position

 

 

i).  II^nd structure and V structure respectivety in cases of P – chloronitrobenzene and O –chloronibenzenecarries negative change on carbon atom bearing NO₂ group.

• There for the carbocations are stabilized by NO₂ groups as well as by pe^q of the benzene ring thus. Facilitate chlorobenzene towards Nucleophilic attack.

 

CHEMICAL REACTIONS OF HALOARENES-III

 ELECTRO PHILIC SUBSTITUTAION REACTIONS:-

The halogen group is deactivating the haloarenes Facillitakes the further substitution at the ortho and para positions.

1. Halogenations Reaction:-

 

2. Nitration Reaction:-

 

3. Sulphonation:-

 

4. Friedul Craft’s Reactions:-

 

 

5. Reaction With Sodium (Wurtz Fitting Reaction):-

 

 

 

CHEMICAL REACTIONS OF HALOARENES-IV

 6. Fitting Reaction:-

 

 

7. Reaction With Magnesium:-

8. Reduction:-

 

 

Questions:-

1. What will be the major product of following reaction?

Ans

 

 

2. Haloalkanes reacts with KCN to form alkyl cynides as main product but AgCN forms isocynide as chief product explain?

Ans.     Please See the theory of Nucleophilic substitution reaction.

3. Write the product of given reactions

 

i.

 

ii.

iii.

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