Question

Convert Cyanide to Aldehyde and Ketone.

Answer 1

Hint: The formation of aldehyde and ketone from cyanide needs a strong reducing agent. An electrophilic reducing agent for obtaining an aldehyde. In contrast, a nucleophilic reducing agent with the ability to form new carbon-carbon bonds, to form a ketone.

Complete step by step answer:
A compound with cyanide is represented by $R-C\equiv N$ or $RCN$, aldehyde is of the type $R-CHO$ and a ketone is of the type $$R-CO-R^{\prime }$$ where $R$ and $R^{\prime }$ are any carbon group.
To convert cyanide into an aldehyde, we can use a reagent called DIBAL-H. It is a reducing agent. The reaction is as follows.
$$RCN\underset{H_{2}O}{\overset{DIBAL-H}{\to }}R-CHO$$
Di-isobutyl aluminum hydride or DIBAL-H has been used to reduce nitriles and esters into aldehydes. The mechanism of reduction is, DIBAL-H has an aluminum atom which functions as a Lewis acid. A lewis acid accepts electron pairs. This electron pair is donated by the nitrile. This paves the way for reduction. The hydride ion present in DIBAL-H leaves the compound and gets attached to the carbon atom of cyanide which results in the formation of imine. Now water comes into play and hydrolysis the imine to form an aldehyde.
To convert cyanide into a ketone, we will use Grignard reagent. The reaction is as follows.
$$RCN\underset{H_{3}O}{\overset{R^{\prime }-MgX}{\to }}R-CO-R^{\prime }$$
Grignard reagent is a very important reagent which forms carbon-carbon bonds using organometallic reaction. The mechanism of the reaction is, the Grignard reagents inserts a new carbon into the cyanide, leading to the formation of a new carbon-carbon bond. When this compound undergoes hydrolysis, we get an imine. This imine has nitrogen, which undergoes protonation and becomes iminium ion. Hydrolysis of the ions results in the elimination of the ammonia. Then, the oxygen undergoes deprotonation, leaving us with a ketone.

Note:
The intermediate formed here, imine is a compound of carbon in which the carbon forms a double bond with nitrogen. Carbon is attached to 2 other carbon groups whereas nitrogen is attached to one other carbon group. The iminium ions formed here are also similar to imine but the difference is that nitrogen now possesses a positive charge and now has two carbon groups attached instead of one. Both of these intermediates are formed in the formation of aldehydes and ketones. The hydrolysis mentioned here, is the reaction of compound with water.