Question

Which of the following alcohol gives the Iodoform test?
A) methanol
B) propan-2-ol
C) pentane-3-ol
D)Carbinol

Answer 1

Hint: The Iodoform test is given by the compounds containing the methyl group at the alpha position. So, let us check the structure of the following option and the structure which contains the alpha-methyl group will be the correct option.

Complete step by step answer:
The alpha position is the position next to the carbon attached to the functional group. And alpha hydrogen is that hydrogen which is attached to alpha carbon.
We can show the chemical representation of the Iodoform test given by the secondary alcohol-containing methyl group at the alpha positions as follows:
\[{\text{R - CH(OH) - C}}{{\text{H}}_{\text{3}}}\xrightarrow[{{\text{NaOH}}}]{{{{\text{I}}_{\text{2}}}}}{\text{RCOONa + CH}}{{\text{I}}_{\text{3}}}\]
As we know that the methanol and carbinol are primary alcohols, it is confirmed that option A and option D will not give the Iodoform test because primary alcohol will never contain methyl group at the alpha position.
Now, we know that both propan-2-ol and pentan-3-ol are secondary alcohol but we have to check which of the following contains the alpha-methyl group.
Let us see the structure of the following compound.
Propan-2-ol: \[{\text{C}}{{\text{H}}_{\text{3}}}{\text{ - CH(OH) - C}}{{\text{H}}_{\text{3}}}\]
Pentane-3-ol: \[{\text{C}}{{\text{H}}_{\text{3}}}{\text{ - C}}{{\text{H}}_{\text{2}}}{\text{ - CH(OH) - C}}{{\text{H}}_{\text{2}}}{\text{ - C}}{{\text{H}}_{\text{3}}}\]
By seeing the structure, we can confirm that propan-2-ol has two alpha-methyl groups and pentane-3-ol has zero alpha methyl group.
Let us see the chemical reaction of propan-2-ol giving Iodoform test.
The reaction is shown below.
\[{\text{C}}{{\text{H}}_{\text{3}}}{\text{ - CH(OH) - C}}{{\text{H}}_{\text{3}}}\xrightarrow[{{\text{NaOH}}}]{{{{\text{I}}_{\text{2}}}}}{\text{C}}{{\text{H}}_3}{\text{COONa + CH}}{{\text{I}}_{\text{3}}}\]

Therefore, we can conclude that the correct option for this question is option B.

Note:
We can get confused between Propan-2-ol and Pentane-3-ol as both the alcohols contain hydroxyl groups at the secondary position. But here we should focus on the alpha-methyl group containing secondary alcohol.