Question

Structural isomerism only should be considered when answering this question. All the isomeric alcohols with the molecular formula ${{\text{C}}_5}{{\text{H}}_{12}}{\text{O}}$ are added separately to warm alkaline aqueous iodine.
How many of the isomers give yellow precipitate?
A. $0$
B. $1$
C. $2$
D. $3$

Answer 1

Alcohols are the organic compounds having hydroxyl functional groups. Isomers are compounds having the same chemical formula but different structures. Iodoform test is used to check the presence of carbonyl compounds or certain alcohols in an unknown compound.

Complete step by step answer:
Structural isomers are the compounds having the same molecular formula but different structural formula. They are of five types.
There are eight isomeric alcohols for ${{\text{C}}_5}{{\text{H}}_{12}}{\text{O}}$ molecule. The compounds having five carbon have prefix pent. The constitutional isomers of alcohols having molecular formula ${{\text{C}}_5}{{\text{H}}_{12}}{\text{O}}$ are pentan-1-ol, pentan-2-ol, pentan-3-ol, 2-methyl butan-1-ol, 3-methyl butan-1-ol, 2-methyl butan-2-ol, 3-methyl butan-2-ol and 2,2-dimethyl propanol.
Haloform reaction is a chemical reaction where a haloform is produced by the exhaustive halogenation of aldehyde or ketone or certain alcohol in the presence of a base. The reaction can be used to transform acetyl groups into carboxyl groups or to produce haloform. In iodoform test, triiodo methane is formed as a product.
Since we are considering only isomeric alcohols, we can tell that the iodoform test is used only for secondary alcohols with a methyl group in alpha position and ethanol.
Initially the alcohols are converted to aldehyde, then it is iodinated forming a yellow precipitate. Tertiary alcohols cannot be oxidized because it does not have any hydrogen atom attached to the carbon. Moreover only ethanol gives positive results for iodoform test among primary alcohols. Secondary alcohols can be oxidized to ketones, thereby giving results for iodoform test.
Simple secondary alcohols with hydroxyl groups in the second carbon give positive results for iodoform test.
Thus we can tell that 3-methyl butan-2-ol and pentan-2-ol give positive results for iodoform test.
Hence only two isomeric alcohols of ${{\text{C}}_5}{{\text{H}}_{12}}{\text{O}}$ give positive result for iodoform test.
Hence option C is correct.

Note:
Positive iodoform test is given by the following compounds:
-Acetaldehyde
-Methyl ketones
-Ethanol
-Secondary alcohols with methyl groups in alpha position