Distinguishing between hexanal and 2-hexanone

Hexanal and 2-hexanone are both six-carbon compounds with a carbonyl group (C=O) attached to the second carbon atom. They differ only in the position of the carbonyl group, which is at the end of the chain in hexanal and in the middle of the chain in 2-hexanone. Therefore, they have similar physical and chemical properties and can be difficult to distinguish.

Reagents for distinguishing between hexanal and 2-hexanone

There are several reagents that can be used to distinguish between hexanal and 2-hexanone based on their different chemical properties:

1. Tollens reagent: This reagent contains silver ions in a basic solution. It is used to distinguish between aldehydes and ketones. Aldehydes (such as hexanal) are oxidized by Tollens reagent to form carboxylic acids, which react with silver ions to produce a silver mirror on the inside of the test tube. Ketones (such as 2-hexanone) do not react with Tollens reagent and do not produce a silver mirror.

2. Br in CCl4: This reagent is used to distinguish between alkenes and alkanes. Alkenes (such as hexene) react with bromine to form a dibromo compound, which turns the reddish-brown color of the bromine solution to colorless. Alkanes (such as hexane) do not react with bromine and do not change the color of the bromine solution. This reagent is not useful for distinguishing between hexanal and 2-hexanone because both compounds are saturated and do not contain a carbon-carbon double bond.

3. I in NaOH: This reagent is used to distinguish between primary, secondary, and tertiary alcohols. Primary alcohols (such as hexanol) are oxidized by iodine in the presence of sodium hydroxide to form carboxylic acids, which give a yellow precipitate of iodoform upon treatment with excess iodine and sodium hydroxide. Secondary alcohols (such as 2-hexanol) are oxidized by iodine to form ketones, which do not give the iodoform test. Tertiary alcohols (such as 2-methyl-2-hexanol) do not react with iodine and do not undergo oxidation. This reagent is not useful for distinguishing between hexanal and 2-hexanone because both compounds are carbonyl compounds and do not contain an alcohol group.

Answer

Therefore, the correct answer is option B (Br in CCl4) because this reagent is not useful for distinguishing between hexanal and 2-hexanone.