CBSE Class 12 Chemistry Notes: Haloalkanes – Chemical Reactions

CHEMICAL REACTIONS OF HALOALKANES:-

The reactions may be divided in 3 categories:-

1. Nucleophilic substitution reaction.
2. Elimination reaction.
3. Reaction with metal.

Nucleophilic substitution reaction:-

Reactions in which stronger nucleophile replaces a weaker nucleophile are termed as the Nucleophilic substitutions reaction.

 

The presence of the small +ve charge on the carbon atom makes it susceptible to attack byNucleophilic reagents.

 

Amongst the halide ions the order in which leaving group departs and have order:-

 

i. Substitution of Halogen by Hydroxyl group:-

 

 

ii.  Substitution of Halogen by Alkony Group:-

This Reaction is called Williamson Synthesis

iii. Substitution of Halogen by Cyano Group:-

 

Alkylcynides are very useful compounds:-

They can be converted into, amides, carboxylic acids, primary amines etc

iv. Substitution of Halogen by Isocynide Group:-

Isocynide on reduction gives secondary amines

v. Ambident Nucleophiles:-
Such Nucleophiles which have more than one site through which the reaction can occur are called Ambident Nucleophiles.

 

vi. Substitution of Halogens by Amino Group:-

This reaction is called Hofmann Amonolysis
vii. Substitution of Chlorine and Bromine by Iodine:-

This reaction is called Finkelstein reaction

MECHANISM OF NUCLEOPHILIC SUBSTITUTION REACTION:-

This Reaction May Proceed By Two Ways:-

i. SN¹ Mechanism (Substitution, Nucleophilic, Unimolecular)
ii. SN² Mechanism (Nucleophilic, Substitution, Bimolecular)

1. Substitution Bimolecular SN²:-.

i. This reaction mechanism follows second order kinetics

ii. The rate of reaction depends upon concentration of both reactants.

iii. The incoming nucleophile attacks alkyl halide, the carbon halogen bond starts breaking and new C – OH bond is formed

iv. The two process takes place in single step and intermediate is get formed.

For Example:-

v.  SN² reactions are always accompanied by inversion of configuration just in the same way as umbrella turns inside in strong wind this is called as Walden Inversion

• 2.      Substitution Nucleohilic (SN¹)

              i.  These reactions are carried out in polarprotic solvents.

 

              ii.  They follows the first order kinetics. The rate of reaction is depends only upon alkyl halide.

Rate = K [alkyl halide]

iii.  Reaction is proceed in two steps

              iv. Production of alkyl carbocation

              v.   The carbocation being a reactive species is attacked

              vi.  The SN² mechanism follows racemisation optical isomerism immediatey by the OH^– or any other Nucleophilie.

Example:-

 

Step I:-
Formation of Carbocation

This stepis slow stephence is the rate determining step
Step II:-

Tert – Butyl Carbocation

CHEMICAL REACTIONS OF HALOALKANES- III

Elimination Reactions:- [Dehydrohalogenation]:-

When a haloalkane is heated with a concentrated alcoholic solution of potassium hydroxide a molecule of hydrogen halide is eliminated and alkene is formed.

This is also b – elimination reaction

Sytzeff Elimination Rule:-

According to this rule in case of elimination, the more highlysubstitutedalkene (having lesser number of hydrogenatoms on the doublly bonded carbon atom) is the major product of dehydrohalogenation.

Example:-

C is – trans Isomerism of SytzeffProducts:-

Reaction With Metals of Haloalkanes:-

Most of organic halides reacts with certain metals to form compounds containing carbon – metal bond. such compounds are known as the organometallic compounds. also known as Grighard Reagents.

Reaction Wurtz Reaction:-

 

 

 

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