Question

The molecular formula of acetaldehyde semicarbazone is:
A) $C{H_3} - CH = N - CO - NH - N{H_2}$
B) $C{H_3} - CH = N - NH - CON{H_2}$
C) $C{H_3} - CH = N - OH$
D) $C{H_3} - CH = N - N{H_2}$

Answer 1

Hint: Semicarbazones are derivatives of imines and formed by the condensation reaction between an aldehyde or ketone and semicarbazide. Chemical formula of semicarbazide is $N{H_2}(CONH)N{H_2}$. Do the condensation reaction between acetaldehyde ($C{H_3}CHO$) and semicarbazide to get the molecular formula of acetaldehyde semicarbazone.

Complete step by step answer:
 Acetaldehyde semicarbazone belongs to the group of semicarbazones. Semicarbazones are formed by the condensation reaction between an aldehyde or ketone and semicarbazide. A condensation reaction proceeds with the loss of a water molecule. So, to form acetaldehyde semicarbazone, we need to do the condensation reaction of aldehyde, which is acetaldehyde here, with semicarbazide.
Semicarbazide is a derivative of urea having chemical formula as: $N{H_2}(CONH)N{H_2}$
Reaction of acetaldehyde with semicarbazide:
So, there is an elimination of water molecules in the above reaction to form acetaldehyde semicarbazone. Hence, we get molecular formula of acetaldehyde semicarbazone as: $C{H_3} - CH = N - CO - NH - N{H_2}$

Additional Information:
Ketones also reacts with semicarbazide to form semicarbazones. A general reaction to form semicarbazones of ketones is:
$R - CO - R' + N{H_2} - CO - NH - N{H_2} \to RR'C = N - CO - NH - N{H_2}$
For an example, molecular formula of semicarbazone of ketone will be:
${(C{H_3})_2}C = N - CO - NH - N{H_2}$

Some semicarbazones have antiseptic properties. Nitrofurazone (trade name as Furacin) is such an example. Thiosemicarbazone is a semicarbazone which contains sulphur atom (thio group) in the place of oxygen atom. It possesses anti-viral, anti-malarial and anti-cancer activities.
So, the correct answer is “Option A”.

Note: Semicarbazones are crystalline solids and they are very useful for the identification of aldehydes and ketones by the melting point analysis method. Molecular weight of acetaldehyde semicarbazone is 101 g/mol. Its molecular formula can also be written as: ${C_3}{H_7}{N_3}O$ . Its IUPAC name is [(E)-ethyleneamines]urea.