Question

What is the end product (B) in the following reaction sequence?

(A) $n - butylamine$
(B) $n - propyl cyanide$
(C) $propyl\;isocyanide$
(D) $n - propylamine$

Answer 1

Above reactions suggest that butanamide $C{H_3}C{H_2}C{H_2}CON{H_2}$ reacts with Phosphorus pentoxide ${{\text{P}}_2}{{\text{O}}_5}$ to give product A and when Product A is treated with Lithium Aluminium hydride $LiAl{H_4}$ it gives a product B. Phosphorus pentoxide is used for dehydration reaction to convert amides into nitriles. Lithium Aluminum hydride is a reducing agent used to convert cyanides into amines.

Complete answer:
Phosphorus pentoxide is a dehydrating agent, it is used for the conversion of nitriles from primary amides. Since butanamide is a primary amide, it will undergo dehydration to form cyanide and water the reaction taking place will be:

From the reaction we can see the product formed is ${\text{n}} - propyl cyanide$ . According to the question the ${\text{n}} - propyl cyanide$ is the product A.
Now, product A or ${\text{n}} - propyl cyanide$ is treated with Lithium Aluminum hydride which is a reducing agent used to convert cyanides or nitriles into amines to get product B, the reaction taking place will be:

From the reaction we can see the product formed is ${\text{n}} - butylamine$ . According to the question the ${\text{n}} - butylamine$ must be the product B.
So, option (A) is the correct answer.

Note:
Phosphorus pentoxide is a very good dehydrating agent it is used in different areas as a part of Onodera reagent which is used for the oxidation of alcohols, used for converting some mineral acids into anhydrides. Lithium Aluminum hydride is a good reducing agent it is used to reduce polar bonds. It has many uses: it is used to convert aldehyde and ketone to primary and secondary alcohols respectively, carboxylic acids to primary alcohols, amides and nitrile to amines.