Question

\[R - NH - COH\, \overset{POCl_{3},Pyridine}{\rightarrow} \, Product\]
In the given reaction what will be the product?
A. \[R - N = C = O\]
B. \[R - \mathop N\limits^ + \equiv \mathop C\limits^ - \]
C. \[R - C \equiv N\]
D. None of the above

Answer 1

Hint: Reaction of amides with \[POC{l_3}\] i.e., phosphorus oxychloride gives nitriles. This is a dehydration reaction as net elimination is of water. Pyridine is a basic solvent used as a solvent in this reaction.

Complete Step by Step Solution:
In this reaction, \[POC{l_3}\] is a dehydrating agent and pyridine is used as a solvent.
The mechanism of the dehydration with phosphorus oxychloride is as follows:
There is lone pair present in the Nitrogen of amide bond of reactant which is delocalized in the amide group and the negative generates on the oxygen of amide group which attacks the phosphorus of \[POC{l_3}\]as phosphorus has empty d-orbitals, then the compound forms which has a positive charge on nitrogen. To neutralise the charge, hydrogen from the nitrogen of the compound gets removed. Then a lone pair of nitrogen is again delocalized to form a triple bond between nitrogen and carbon which results in cleavage of carbon and oxygen bond as carbon valency is four. Now the resultant \[R - \mathop N\limits^ + \equiv C - H\] has acidic proton which will get removed to give the final product \[R - \mathop N\limits^ + \equiv \mathop C\limits^ - \].

The reaction of a compound having an amide functional group with \[POC{l_3}\] and pyridine gives \[R - \mathop N\limits^ + \equiv \mathop C\limits^ - \] product which is option B by the process called dehydration.

Image: Reaction of a compound containing amide bond with phosphorus oxychloride

So, option B is correct.

Note: Phosphorus is from the third period of the p-block element, that’s why it has empty d -orbitals to accept electrons from the surrounding element. Carbon has four valencies, so to maintain its stability, it cleaves the bond with oxygen as \[C - O\]bond is more polar than\[C - H\].