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.