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Question: The molecular formula of acetaldehyde semicarbazone is: A) \(C{H_3} - CH = N - CO - NH - N{H_2}\) ...

The molecular formula of acetaldehyde semicarbazone is:
A) CH3−CH=N−CO−NH−NH2C{H_3} - CH = N - CO - NH - N{H_2}
B) CH3−CH=N−NH−CONH2C{H_3} - CH = N - NH - CON{H_2}
C) CH3−CH=N−OHC{H_3} - CH = N - OH
D) CH3−CH=N−NH2C{H_3} - CH = N - N{H_2}

Explanation

Solution

Semicarbazones are derivatives of imines and formed by the condensation reaction between an aldehyde or ketone and semicarbazide. Chemical formula of semicarbazide is NH2(CONH)NH2N{H_2}(CONH)N{H_2}. Do the condensation reaction between acetaldehyde (CH3CHOC{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: NH2(CONH)NH2N{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: CH3−CH=N−CO−NH−NH2C{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′+NH2−CO−NH−NH2→RR′C=N−CO−NH−NH2R - 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:
(CH3)2C=N−CO−NH−NH2{(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: C3H7N3O{C_3}{H_7}{N_3}O . Its IUPAC name is [(E)-ethyleneamines]urea.