Question

What is meant by the semicarbazone? Give an example of reaction (with mechanism) showing its formation.

Answer 1

Hint: Nucleophiles such as $N{H_3}$ or its derivatives can add to the carbonyl group of ketones and aldehydes to give nitrogen-substituted derivatives.

Complete step by step answer:
We know that aldehydes and ketones are carbonyl compounds as they contain $ > C = O$ group. Due to the difference in the electronegativity, $ > C = O$ bond can be shown as $ > {C^{\delta + }} = {O^{\delta - }}$. We know that electrophiles are those which can accept electrons as carbon centres in $ > {C^{\delta + }} = {O^{\delta - }}$. This electrophilic centre can be attacked by a variety of nucleophiles such as cyanide ion or ammonia. The addition of nucleophiles changes the hybridization of the carbon centre from $s{p^2}$ to $s{p^3}$ and an intermediate is formed which gets neutralized further to give the product.
Here, we will take the nucleophilic addition of ammonia to a carbonyl group which can be explained as follows:
\[ > {C^{\delta + }} = {O^{\delta - }} + N{H_3} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\rightarrow\over
{\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} \left[ { > C\left( {OH} \right)N{H_2}} \right]\]
Initially, ammonia being a nucleophile attacks the electrophilic carbon centre and forms an intermediate. This is a reversible step and is being catalyzed by acid. Now, this tetrahedral intermediate undergoes dehydration to give the final product, which is imine in this case as follows:
\[\left[ { > C\left( {OH} \right)N{H_2}} \right] \to \left( { > C = NH} \right) + {H_2}O\]
Derivatives of ammonia can also be used in place of ammonia to give the corresponding nitrogen substituted derivatives. For example, hydroxylamine can give oxime and hydrazine can give a hydrazone.
When a semicarbazide is added to the carbonyl group of ketones or aldehydes, the product is called semicarbazone.
Let’s have a look at the semicarbazide addition to cyclobutanone:

As we can see that firstly, electrophilic centres are created at carbon by protonation of oxygen atoms. The lone pair of electrons on nitrogen in semicarbazide is accepted by the electrophilic carbon of cyclobutanone giving a positive charge to the nitrogen. The neutrality is gained by removal of a proton to give the addition intermediate. This intermediate readily loses water to give the final product which is called semicarbazone.

Note:
A semicarbazone is a derivative of imines formed by a condensation reaction between a ketone or aldehyde and semicarbazide.