Question

Organic compounds containing carbonyl group as the functional group are aldehydes, ketones and carboxylic acids. The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen as compared to carbon. Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. They react with HCN, Sodium bisulphite, Grignard reagent and a number of ammonia derivatives like Hydroxylamine, Hydrazine, Semicarbazide, etc. Aldehydes and ketones undergo various reactions due to the acidic nature of \( \alpha \)-hydrogen. Aldehydes and ketones having at least one \( \alpha \)-hydrogen atom undergo a reaction in the presence of dilute alkali as a catalyst to form \( \beta \)-hydroxy aldehydes or \( \beta \)-hydroxy ketones. On the other hand, aldehydes which do not possess an \( \alpha \)-hydrogen undergo self-oxidation and reduction in the presence of concentrated alkali.

Draw the structure of the semicarbazone of cyclohexanone.

Hint:

Replace the C=O of cyclohexanone by C=N-NH-CO-NH2 (loss of one water molecule). The product is a ring bearing the semicarbazone side chain.

Answer 1

Step 1: Recall the reaction:
Aldehydes and ketones react with semicarbazide, \( \mathrm{H_2N-NH-CO-NH_2} \). The \( \mathrm{-NH_2} \) group attached to the other N (the hydrazine end) attacks the carbonyl carbon.
Water is lost, and the C=O group is replaced by C=N. The product is a semicarbazone.

Step 2: Write the reaction for cyclohexanone:
\[ \mathrm{C_6H_{10}O + H_2N-NH-CO-NH_2 \rightarrow C_6H_{10}=N-NH-CO-NH_2 + H_2O} \]

Step 3: Describe the structure:
It is a six-membered saturated ring of carbon atoms.
One ring carbon (the former carbonyl carbon) is double bonded to a nitrogen atom.
That nitrogen is single bonded to NH, which is bonded to a carbonyl group, C(=O), and then to \( \mathrm{NH_2} \).
The side chain is \( \mathrm{=N-NH-C(=O)-NH_2} \). The molecular formula is \( \mathrm{C_7H_{13}N_3O} \).

Final Answer:
Cyclohexanone semicarbazone (2-cyclohexylidenehydrazine-1-carboxamide): a cyclohexane ring with \( \mathrm{=N-NH-CO-NH_2} \) at one ring carbon.