Question

Which of the following does not give the iodoform test?
A. ${{\text{C}}_{2}}{{\text{H}}_{5}}\text{OH}$
B. ${{\text{C}}_{6}}{{\text{H}}_{5}}\text{OH}$
C. ${{\text{(C}{{\text{H}}_{3}}\text{)}}_{2}}\text{CHOH}$
D. All of the above

Answer 1

Iodoform test is used to test whether the carbonyl group or alcohol group is present or not in the molecule. The compound with ketone and alcohol groups will give a positive test for the iodoform. Iodine along with a base is used as a reactant.

Complete answer:
- In the given question, we have to identify the molecule which will not give the positive test for iodine.
- Iodoform test is used to identify the presence of a carbonyl such as $\text{R - CO - C}{{\text{H}}_{3}}\text{ }$ or alcohol group such as $\text{R - CH(OH)-}$.
- When iodine, a base such as sodium hydroxide is combined with the compound containing methyl ketone or secondary alcohol which has a methyl group.
- Then they form a precipitate of yellow coloured triiodomethane which confirms the presence of the carbonyl group.
- With ethyl alcohol:
$\text{C}{{\text{H}}_{3}}\text{CHOH + }{{\text{I}}_{2}}\text{ + NaOH }\to \text{ CH}{{\text{I}}_{3}}\text{ + COONa}$
- With ${{\text{(C}{{\text{H}}_{3}}\text{)}}_{2}}\text{CHOH}$
${{(\text{C}{{\text{H}}_{3}})}_{2}}\text{CHOH + }{{\text{I}}_{2}}\text{ }+\text{ NaOH }\to \text{ CH}{{\text{I}}_{3}}+\text{ C}{{\text{H}}_{3}}\text{COONa}$
- whereas not only ketogenic or alcoholic groups but the aldehyde such as acetaldehyde can also give the positive iodoform test because it consists of a methyl group attached to the carbonyl, $\text{C}{{\text{H}}_{3}}\text{ - C=O}$.
- So, among the given options only option B will not give the positive iodoform test because there is no presence of the methyl group in the molecule.
- Whereas in option A and B, a methyl group is present as we can see
$\text{C}{{\text{H}}_{3}}\text{CHOH and C}{{\text{H}}_{3}}\text{C = O}$.

Therefore, option B is the correct answer.

Note:
As we know that iodine has antiseptic properties. So, in ancient times the iodoform was used as a disinfectant to kill the bacteria and an antiseptic to heal the wounds. Moreover, iodoform has a saffron-like structure with hexagonal structure.