Question

A compound that gives a positive iodoform test is:

$$A.\;\;\;\;\;2 - \;pentanone \\\ B.\;\;\;\;\;3 - \;pentanone \\\ C.\;\;\;\;\;3 - \;pentanol \\\ D.\;\;\;\;\;1 - \;pentanol \\\$$

Answer 1

Hint : We must know that a compound that contains $R - CO - C{H_3}$or $R - CH\left( {OH} \right) - C{H_3}$can only give a positive iodoform test.

Complete step by step solution :
We must remember that the iodoform test is carried out to determine the presence of aldehyde $\left( { - CHO} \right)$ and ketone $\left( { - C = O} \right)$ having $C{H_3}_ - CO$ group. Positive iodoform test is given by compounds containing methyl keto group which is $C{H_3} - CO$ group.
Following are the compounds that give positive iodoform test:
Acetaldehyde $\left( {H - C = O} \right)$
Methyl Ketones $\left( { - C = O} \right)$
Ethanol $\left( { - C - OH} \right)$
Secondary Alcohols that contain Methyl Groups in Alpha Position
Iodoform test identifies the presence of carbonyl compounds with the structure $R - CO - C{H_3}$or alcohols with the structure$R - CH{\text{ }}\left( {OH} \right) - C{H_3}$. In the iodoform test, iodine solution, a base ($NaOH$or$KOH$) and a methyl ketone reacts in such a way that gives a yellow precipitate along with a characteristic antiseptic smell.

Following is a structure of $2 - \;pentanone$

Following is a structure of$3 - \;pentanone$

Following is a structure of$3 - \;pentanol$. It is a secondary alcohol where pentane has a substitution at position 3 by a hydroxyl group.

Following is a structure of $1 - \;pentanol$

As we can see in the above chemical structures, among all options, $2 - pentanone$is the only compound containing the $C{H_3}CO\;$group.
Hence, we can conclude that the option B is correct.
Additional information:
Positive iodoform test forms the pale yellow precipitate of iodoform which can be easily identified by its characteristic “antiseptic” smell.

Note : We must know that not all the alcohols give an iodoform test, only Secondary Alcohols containing Methyl Groups at Alpha Position will give a positive iodoform test.