Question

Cacodyl oxide test is given by _________ to give an obnoxious smelling compound.
(a) Acetic acid
(b) Formic acid
(c) Both a and b
(d) None of the above

Answer 1

Cacodyl oxide is an oxide of Cacodyl which is also called dicacodyl, and is an organometallic compound where the metal part is Arsenic metal. Cacodyl oxide test is conducted to identify its formation through its strong odour.

Complete step by step answer:
(1) Organometallic compounds are the compounds of Carbon with a metal, they are denoted by ${\text{R - M}}$, where ${\text{R}}$ is for carbon compound, and ${\text{M}}$ is for the metal. They are a source of nucleophilic carbon, so, they can be represented as ${R^ - } - {M^ + }$.

(2) Cacodyl oxide has a chemical formula ${\left[ {{{\left( {C{H_3}} \right)}_2}As} \right]_2}O$. Its IUPAC name is Dimethyl arsenious anhydride. Therefore, it is also called an organo-arsenic compound. It is a major part of Cadet’s fuming liquid. This is famous because it was the first organometallic compound which was synthesized in purely relative form.

(3) Cacodyl oxide is prepared by a thermal reaction where Potassium acetate is heated with arsenic trioxide. The reaction for its preparation is as follows:
$A{S_2}O + 4K{C_2}{H_2}{O_2} \to {\left[ {{{\left( {C{H_3}} \right)}_2}As} \right]_2}O + 2C{O_2} + 2{K_2}C{O_3}$
(4) In the Cacodyl test, Arsenic acid is mixed, and evaporated in a solution of acetic acid, and then neutralized by Potassium dioxide. This leads to the formation of a strong odour compound called Cacodyl oxide, ${\left[ {{{\left( {C{H_3}} \right)}_2}As} \right]_2}O$. Formic acid cannot be used in place of Acetic acid.

Hence, option (a) Acetic acid is the correct answer.

Note:
During the test for Cacodyl oxide, students are advised not to smell the product formed because Cacodyl oxide is a toxic compound, and also very unpleasant to smell which is why it is termed as an obnoxious smelling compound in the question.