Question

Examine the structural formulas of compounds given below and identify number of compounds which show positive iodoform test on vigorous reaction with excess of alkali and I₂

Answer 1

5

Answer 2

The iodoform test is given by compounds containing a methyl ketone group (R-CO-CH₃) or a methyl carbinol group (R-CH(OH)-CH₃). Under the given conditions of "vigorous reaction with excess of alkali and I₂", compounds that can be converted into these functional groups through hydrolysis or oxidation will also give a positive test.

Let's examine each compound:

1. 3-Pentanone (CH₃CH₂COCH₂CH₃): This is a ketone, but it does not have a methyl group directly attached to the carbonyl carbon. It has -CH₂CH₃ groups on both sides. Therefore, it will not give a positive iodoform test.

2. 2-Butanone (CH₃COCH₂CH₃): This is a methyl ketone, as it has a -COCH₃ group. Therefore, it will give a positive iodoform test.

3. Acetaldehyde (CH₃CHO): This is a methyl aldehyde, as it has a -COCH₃ group (where R=H). Therefore, it will give a positive iodoform test.

4. Propiophenone (C₆H₅COCH₂CH₃): This is a ketone, but it does not have a methyl group directly attached to the carbonyl carbon. It has a -CH₂CH₃ group. Therefore, it will not give a positive iodoform test.

5. 2,2-Dibromobutane (CH₃-CH₂-C(Br)₂-CH₃): This is a geminal dihalide. Under vigorous reaction with excess alkali (hydrolysis), gem-dihalides on the same carbon atom are hydrolyzed to form a ketone.

   CH₃-CH₂-C(Br)₂-CH₃ + 2OH⁻ → CH₃-CH₂-CO-CH₃ (2-Butanone) + 2Br⁻ + H₂O

   Since 2-Butanone is a methyl ketone, it will give a positive iodoform test. Therefore, 2,2-Dibromobutane will give a positive iodoform test.

6. Acetophenone (C₆H₅COCH₃): This is a methyl ketone, as it has a -COCH₃ group. Therefore, it will give a positive iodoform test.

7. Cyclohexane-1,4-dione: This compound has two carbonyl groups. The carbons adjacent to the carbonyl groups are -CH₂- (methylene) groups. There are no methyl groups adjacent to any carbonyl carbon. Therefore, it will not give a positive iodoform test.

8. 1,3-Diphenylpropane-1,3-dione (Dibenzoylmethane) (C₆H₅COCH₂COC₆H₅): This is a β-diketone. The carbons adjacent to the carbonyl groups are a -CH₂- group and phenyl groups. There are no methyl groups directly attached to any carbonyl carbon. Therefore, it will not give a positive iodoform test.

9. Cyclopentanone: The carbons adjacent to the carbonyl group are -CH₂- (methylene) groups. There are no methyl groups adjacent to the carbonyl carbon. Therefore, it will not give a positive iodoform test.

10. 2,3-Dichlorobutane (CH₃-CH(Cl)-CH(Cl)-CH₃): This is a vicinal dihalide. Under vigorous reaction with excess alkali (hydrolysis), vicinal dihalides are hydrolyzed to form diols.

    CH₃-CH(Cl)-CH(Cl)-CH₃ + 2OH⁻ → CH₃-CH(OH)-CH(OH)-CH₃ (2,3-Butanediol) + 2Cl⁻

    2,3-Butanediol contains two methyl carbinol groups (CH₃-CH(OH)- and -CH(OH)-CH₃). These secondary alcohol groups are oxidized by I₂/OH⁻ to form carbonyl groups, leading to butane-2,3-dione (CH₃-CO-CO-CH₃), which contains methyl ketone groups. Therefore, 2,3-Dichlorobutane will give a positive iodoform test.

The compounds that show a positive iodoform test are:

1. 2-Butanone
2. Acetaldehyde
3. 2,2-Dibromobutane
4. Acetophenone
5. 2,3-Dichlorobutane

Thus, a total of 5 compounds show a positive iodoform test.