Draw the structure of the semicarbazone of acetone.
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Solution and Explanation
Aldehydes and ketones undergo nucleophilic addition reactions at the carbonyl carbon. Their reactivity depends on steric hindrance and electronic effects. Reduction with \(Zn(Hg)/conc.\;HCl\) is Clemmensen reduction.
Step 1: Structure of semicarbazone of acetone.
Acetone is: \[ CH_3COCH_3 \] Semicarbazide is: \[ NH_2NHCONH_2 \] Acetone reacts with semicarbazide to form semicarbazone. \[ CH_3COCH_3+NH_2NHCONH_2 \rightarrow (CH_3)_2C=NNHCONH_2+H_2O \] So the structure is: \[ (CH_3)_2C=NNHCONH_2 \]
Step 2: Acidity of alpha hydrogens.
The hydrogen atoms attached to the carbon next to the carbonyl group are called \(\alpha\)-hydrogens. When an \(\alpha\)-hydrogen is removed, an enolate ion is formed. \[ RCOCH_2R' \rightarrow RCOCHR'^-+H^+ \] The enolate ion is resonance stabilised: \[ RCOCHR'^- \leftrightarrow RCO^- = CHR' \] Because the conjugate base is stabilised by resonance, \(\alpha\)-hydrogens are acidic.
Step 3: Reactivity towards HCN.
HCN adds to carbonyl compounds by nucleophilic addition. Aldehydes are more reactive than ketones because they have less steric hindrance and less electron donation. Bulky ketones are least reactive. Therefore: \[ (CH_3)_3C-CO-CH_3\lt CH_3COCH_3\lt CH_3CHO \]
Step 4: Etard reaction.
In Etard reaction, toluene is oxidised to benzaldehyde using chromyl chloride. \[ C_6H_5CH_3 \xrightarrow{CrO_2Cl_2/CS_2} C_6H_5CHO \]
Step 5: Clemmensen reduction of cyclohexanecarbaldehyde.
Cyclohexanecarbaldehyde contains the group: \[ -CHO \] With \(Zn(Hg)/conc.\;HCl\), aldehyde group is reduced to methyl group. \[ C_6H_{11}CHO \xrightarrow{Zn(Hg)/conc.\;HCl} C_6H_{11}CH_3 \] The product is methylcyclohexane. Hence: \[ (CH_3)_2C=NNHCONH_2 \] \[ \alpha\text{-hydrogens are acidic due to resonance stabilisation of enolate ion.} \] \[ (CH_3)_3C-CO-CH_3\lt CH_3COCH_3\lt CH_3CHO \] \[ C_6H_5CH_3 \xrightarrow{CrO_2Cl_2/CS_2} C_6H_5CHO \] \[ C_6H_{11}CHO \xrightarrow{Zn(Hg)/conc.\;HCl} C_6H_{11}CH_3 \]
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