14.25:
Preparation of Nitriles
Nitriles, also classified as carboxylic acid derivatives, are commonly prepared by dehydration of amides.
The reaction requires strong dehydrating agents like phosphorous pentoxide or boiling acetic anhydride.
Similarly, thionyl chloride is another important dehydrating agent used for preparing nitriles.
The mechanism begins with a nucleophilic attack by the amide on the thionyl chloride, forming an intermediate.
Next, the electron pairs are rearranged with the departure of a chloride ion as a leaving group.
Further deprotonation removes the positive charge on the nitrogen atom.
Lastly, a second deprotonation releases sulfur dioxide and chloride ion, giving nitrile as the final product.
Notably, nitriles are also prepared by reacting alkyl halides with a cyanide ion, forming a new carbon–carbon bond.
The reaction follows the SN2 mechanism, thereby requiring primary or unhindered secondary alkyl halides, but tertiary alkyl halides cannot be used.
14.25:
Preparation of Nitriles
One of the common methods to prepare nitriles is the dehydration of amides. This method requires strong dehydrating agents like phosphorous pentoxide or boiling acetic anhydride for converting amides to nitriles. Another reagent namely, thionyl chloride also accomplishes the dehydration of amides, where amide acts as a nucleophile. The first step of the mechanism involves the nucleophilic attack by the amide on the thionyl chloride to form an intermediate. In the next step, the electron pairs are rearranged by eliminating a chloride ion as a leaving group. In the subsequent deprotonation step, the positive charge on the nitrogen atom is removed. Finally, the second deprotonation releases sulfur dioxide and a chloride ion to yield a nitrile as the final product.
An alternative method for preparing nitriles is the reaction of a primary or unhindered alkyl halide with a cyanide ion via an SN2 mechanism to form a new carbon–carbon bond.
Additionally, aryl cyanides are prepared via a Sandmeyer reaction involving the treatment of aryldiazonium salt with cuprous cyanide.