Haloalkanes
Nucleophilic Substitution Reactions
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.
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
Example
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.
In Williamson's synthesis, the alkyl halide should not be 3° as elimination takes place with 3° alkyl halides because sodium alkoxide is basic.
Reaction of Haloalkanes with alcoholic KCN
Alkyl halides react with alcoholic KCN (potassium cyanide) to form alkyl cyanides (alkanenitriles).
General reaction
Example
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
Example
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.
On the other hand, AgCN is predominantly covalent and CN ion is not free; hence, attack takes place through nitrogen to give alkyl isocyanides.
Reaction of Haloalkanes with Sodium or Potassium Nitrite
Haloalkanes react with sodium or potassium nitrite (NaNO2 or KNO2) to form alkyl nitrites.
Reaction of Haloalkanes with Silver Nitrite
Aqueous ethanolic solution of a haloalkane is heated with silver nitrite to form nitroalkanes.
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.
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.
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.
C6H5CH+C6H5 carbocation
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.
