Question

Which of the following compounds will give a positive iodoform test?
(A) Isopropyl alcohol
(B) Propionaldehyde
(C) Ethyl phenyl ketone
(D) Benzyl alcohol

Answer 1

Hint: The organic iodine compound of the formula CHI3 is iodoform (also called triiodomethane and, inaccurately, carbon triiodide). It is a pale, yellow, crystalline, volatile substance with a penetrating and distinctive aroma and a chloroform analogy, sweet-tasting aroma (occasionally called to as that of hospital, where it is still commonly used in older chemical texts). It can also be used as a disinfectant.

Complete step by step solution:
The iodoform tests are a test of the presence of $RCOC{H_3}$ structure carbonyl compounds and $RCH\left( {OH} \right)C{H_3}$ structure alcohols.
The Iodoform Test is employed to test the presence in a given unknown substance of carbonyl compounds of the $R - CO - C{H_3}$ structure or of $R - CH\left( {OH} \right) - C{H_3}$ structural alcohols. A yellow precipitation with an "antiseptic' smell results from the reaction of iodine, a base and a methyl ketone. It also tests for a few secondary alcohols that have alpha positions containing at least one methyl group.
A solution from ${I_2}$, followed by only enough NaOH to remove the colour, is added into a small amount of your unknown. The formation (with a characteristic antiseptic smell) of a pale-yellow precipitate of iodoform is a good result.
A positive iodoform test for isopropyl alcohol ${\left( {C{H_3}} \right)_2}CHOH$ is possible because acetone $C{H_3}COC{H_3}$-methyl ketone can easily be oxidised.
The positive iodoform test is provided by methyl ketone.
Hence, we can say that option A is the correct option.

Note: The compound is used as a disinfectant on a small scale. In medicine it was used as an antiseptic and a medical cure for wounds and worms at the beginning of the 20th century, although superior antiseptics have now superseded this application. It is an active ingredient for dogs and cats in many ear powders together with zinc oxide and propionic acid used in the prevention of infection and the removal of hair.