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Haloalkanes
Nucleophilic Substitution Reactions

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Nucleophilic Substitution Reaction

In nucleophilic substitution reaction, a stronger nucleophile (electron rich species) displaces a weaker nucleophile. In case of haloalkanes, the halide ion gets displaced from alkyl halide.

Nucleophilic substitution mechanism

Some important concepts of nucleophilic substitution reactions such as SN1 and SN2 mechanisms are discussed in chemical properties of haloalkanes. If you do not know what they are, please read them first. Let us study the reactions now.

Reaction of Haloalkanes with Aqueous Alkali

Haloalkanes react with boiling aqueous alkalies to produce alcohols. The general reaction and an example are given below :

General reaction

General reaction of haloalkanes with aqueous alkali

Example

reaction of chloroethane with aqueous KOH

Williamson's Synthesis

In williamson's synthesis, primary and secondary alkyl halides react with R'ONa (Sodium alkoxide) or R'OK (Potassium alkoxide) to produce ethers.

Williamson's synthesis
Williamson's synthesis : formation of 2-ethoxypropane

In Williamson's synthesis, the alkyl halide should not be 3° as elimination takes place with 3° alkyl halides because sodium alkoxide is basic.

Williamson's synthesis : tertiary alkyl halide undergoes elimination reaction

Reaction of Haloalkanes with alcoholic KCN

Alkyl halides react with alcoholic KCN (potassium cyanide) to form alkyl cyanides (alkanenitriles).

General reaction

General reaction of haloalkanes with KCN

Example

Reaction of alkyl iodide with potassium cyanide

The reaction of alkyl halides with potassium cyanide is very useful when we want to increase the length of the carbon chain. For example, CH3CH2I contains two carbon atoms while CH3CH2CN contains three carbon atoms.

Reaction of Haloalkanes with AgCN

The aqueous ethanolic solution of a haloalkane forms alkyl isocyanides (isonitriles) when heated with silver cyanide (AgCN)

General reaction

General reaction of alkyl halides with silver cyanides

Example

Reaction of iodoethane with AgCN

Ambident nucleophiles

In cyanide ion, both C and N atoms are electron donors. Such nucleophiles which have more than one site through which reaction can take place are known as ambident nucleophiles.


Why do alkyl halides give alkyl cyanides with KCN and alkyl isocyanides with AgCN as major products?

Reason : Alkyl halides produce alkyl cyanides with potassium cyanide (KCN) but alkyl isocyanides with silver cyanide (AgCN) because KCN is predominantly ionic and provides cyanide ion in solution.

K+ [:C☰N:]

Although both carbon and nitrogen atoms can donate electrons, attack takes place through carbon atom since C−C bonds are stronger than C−N bonds.

alkyl halides react with KCN to give cyanides : mechanism

On the other hand, AgCN is predominantly covalent and CN ion is not free; hence, attack takes place through nitrogen to give alkyl isocyanides.

haloalkanes react with AgCN to give isocyanides : mechanism

Reaction of Haloalkanes with Sodium or Potassium Nitrite

Haloalkanes react with sodium or potassium nitrite (NaNO2 or KNO2) to form alkyl nitrites.

General reaction of alkyl halides with potassium nitrite

Reaction of Haloalkanes with Silver Nitrite

Aqueous ethanolic solution of a haloalkane is heated with silver nitrite to form nitroalkanes.

General reaction of alkyl halides with silver nitrite

Nitrite ion is an ambident nucleophile.

Haloalkanes give R-O-N=O with potassium nitrite but R-NO2 with silver nitrite because KNO2 being ionic in nature produces nitrite ion (ONO) which attacks from O on C-atom but AgNO2 being covalent in nature does not produce nitrite ion; therefore, attack takes place through nitrogen atom.

Reaction of Haloalkanes with Carboxylate Group

Haloalkanes form esters when heated with an ethanolic solution of silver salt.

Reaction of haloalkanes with carboxylate group yields ester

Questions

Question : What are amibident nucleophiles? Explain with an example.

Answer : Nucleophiles which can attack another molecule through more than one sites are called ambident nucleophiles. Example: Cyanide ion.

Resonance in cyanide ion

Cyanide ion can attack through carbon to form cyanides and through nitrogen to form isocyanides.

Question : Out of C6H5CH2Cl and C6H5CHClC6H5 which is more easily hydrolysed by aqueous KOH?

Answer : The reaction of alkyl halides with aqueous KOH is nucleophilic substitution reaction. The reaction can undergo either by SN1 mechanism or SN2 mechanism.

Under SN1 condition : In SN1 reaction, the reactivity increases as the stability of intermediate carbocation increases. The carbocation of C6H5CHClC6H5, which is C6H5CH+C6H5, is more stable than the carbocation of C6H5CH2Cl, which is C6H5CH2+, because C6H5CH+C6H5 is a 2° carbocation stabilised by delocalisation of two C6H5 rings whereas C6H5CH2+ is a 1° alkyl halide stabilised by delocalisation of one C6H5 ring.

carbocation C6H5-CH+-C6H5

C6H5CH+C6H5 carbocation

carbocation C6H5-CH2+

C6H5CH2+ carbocation

Therefore, C6H5CHClC6H5 gets hydrolysed more easily than C6H5CH2Cl under SN1 conditions.

Under SN2 condition : In SN2 reaction, steric hindrance reduces the reactivity; therefore, C6H5CH2Cl being primary alkyl halide gets hydrolysed more easily than C6H5CHClC6H5, which is a secondary alkyl halide, under SN2 conditions.

Conversions

You should be able to solve the following conversions provided you have studied previous tutorials.

Convert propene to 1-nitropropane.

Convert toluene to benzyl alcohol.

Convert ethanol to propanenitrile.

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