Question

Benzaldehyde condenses with N, N-dimethylaniline in presence of anhydrous \[ZnC{l_2}\] to give
A. Michler’s ketone
B. Azo dye
C. Malachite green
D. Buffer yellow

Answer 1

Hint: The chemical formula of benzaldehyde is \[{C_6}{H_5}CH = O\]. In N, N-dimethylaniline and hydrogen of the amine group are replaced by two methyl groups. In the given reaction dehydration takes place as a water molecule is removed. It is a type of condensation reaction.

Complete Step by Step Solution:
Benzaldehyde in reaction with N,N-dimethyl aniline in presence of anhydrous zinc chloride gives malachite green. Anhydrous means free from water. Here, the two hydrogen atoms from two molecules of N, N-dimethylaniline are removed along with one oxygen atom of benzaldehyde forming a malachite green and removing the water molecule.

The complete reaction between benzaldehyde and N, N-dimethylaniline in presence of anhydrous \[ZnC{l_2}\]is shown below.

Image: Reaction of benzaldehyde and N, N-Dimethylaniline

Therefore, the correct option is C.

Additional information: Hermann Fisher was the first scientist to prepare Leuco malachite green in the year 1877. Malachite green is generally used in the dyestuff industry as triarylmethane and in the pigment industry. The term malachite green is used due to its coloured green cation. Malachite green is very hard to detect as it does not give any fluorescence in an aqueous solution.

Note: It should be noted that two molecules of N, N-dimethylaniline are reacted not one molecule with benzaldehyde to give malachite green. First when benzaldehyde reacts with N, N-dimethylaniline it gives leuco malachite green which is a colourless compound which further on reacting gives hydrogen chloride and oxidation forms malachite green.