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Q6.19(vii):
How the following conversions can be carried out? (vii) Ethanol to propanenitrile

Solution :

Step 1: Chemical Transformation Strategy

To convert Ethanol ($CH_3CH_2OH$) to Propanenitrile ($CH_3CH_2CN$), we must increase the carbon chain length by one atom and introduce a nitrile (cyano) functional group. The conversion is performed in two primary stages:

  1. Activation of the Hydroxyl Group: Conversion of the alcohol into a suitable leaving group (a haloalkane).
  2. Nucleophilic Substitution: Reaction of the haloalkane with a cyanide ion ($CN^-$) to facilitate carbon-carbon bond formation.

Step 2: Step-by-Step Reaction Mechanism

Part A: Conversion of Ethanol to Chloroethane

Ethanol is reacted with Thionyl chloride ($SOCl_2$) in the presence of pyridine. Thionyl chloride is preferred as the by-products ($SO_2$ and $HCl$) are gases, leaving the pure alkyl halide.

$CH_3CH_2OH + SOCl_2 \xrightarrow{Pyridine} CH_3CH_2Cl + SO_2\uparrow + HCl\uparrow$

[Justification: Nucleophilic substitution of the hydroxyl group by the chloride ion via the $S_N2$ mechanism involving a chlorosulphite intermediate.]

Part B: Conversion of Chloroethane to Propanenitrile

The resulting Chloroethane is reacted with Potassium Cyanide ($KCN$) in an aqueous-ethanolic solution. The cyanide ion acts as an ambident nucleophile, attacking the electrophilic carbon of the ethyl chloride.

$CH_3CH_2Cl + KCN_{(alc)} \rightarrow CH_3CH_2CN + KCl$

[Justification: This is a bimolecular nucleophilic substitution ($S_N2$) reaction. The $CN^-$ ion attacks the carbon attached to the chlorine atom from the backside, leading to the inversion of configuration and the formation of a new $C-C$ bond.]

Reaction Pathway Summary

$CH_3CH_2OH$ $SOCl_2$ $CH_3CH_2Cl$ $KCN$ $CH_3CH_2CN$ (Ethanol) (Chloroethane) (Propanenitrile)

Analytical Summary Table

Reagent Intermediate/Product Mechanism
$SOCl_2$ $CH_3CH_2Cl$ $S_N2$ (Substitution)
$KCN(alc)$ $CH_3CH_2CN$ $S_N2$ (Nucleophilic displacement)

Final Solution: The conversion of Ethanol to Propanenitrile is achieved by first converting ethanol to chloroethane using $SOCl_2$, followed by a nucleophilic substitution reaction with alcoholic $KCN$ to yield $CH_3CH_2CN$.


More Questions from Class 12 Chemistry Haloalkanes and Haloarenes Exercises


CBSE Solutions for Class 12 Chemistry Haloalkanes and Haloarenes


Chapters in CBSE - Class 12 Chemistry


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