Which of the following will give a positive Tollens' test?
a. propanal
b. ethanol
c. ethyl methyl ether

Verified step by step guidance

1. Understand the Tollens' test: The Tollens' test is a chemical test used to identify aldehydes. It involves the use of Tollens' reagent, which contains silver ions (Ag⁺) in an ammonia solution. Aldehydes are oxidized to carboxylic acids, and the silver ions are reduced to metallic silver, forming a 'silver mirror' on the test tube.
2. Analyze the first compound, propanal: Propanal (CH₃CH₂CHO) is an aldehyde because it contains a carbonyl group (C=O) at the end of the carbon chain. Aldehydes give a positive Tollens' test, so propanal will give a positive result.
3. Analyze the second compound, ethanol: Ethanol (CH₃CH₂OH) is a primary alcohol, not an aldehyde. Alcohols do not react with Tollens' reagent, so ethanol will not give a positive Tollens' test.
4. Analyze the third compound, ethyl methyl ether: Ethyl methyl ether (CH₃OCH₂CH₃) is an ether, which contains an oxygen atom bonded to two alkyl groups. Ethers do not have a carbonyl group and do not react with Tollens' reagent, so ethyl methyl ether will not give a positive Tollens' test.
5. Conclude the results: Among the given compounds, only propanal will give a positive Tollens' test because it is the only compound that contains an aldehyde functional group.

Key Concepts

Tollens' Test

Tollens' test is a qualitative test used to identify aldehydes and certain ketones. It involves the reaction of the carbonyl compound with Tollens' reagent, which contains silver ions. A positive result is indicated by the formation of a silver mirror on the test tube's surface, signifying the reduction of silver ions to metallic silver.

Aldehydes vs. Ketones

Aldehydes are organic compounds characterized by the presence of a carbonyl group (C=O) at the end of the carbon chain, while ketones have the carbonyl group located within the chain. This structural difference is crucial because only aldehydes typically give a positive Tollens' test, as they can be oxidized to carboxylic acids, whereas most ketones do not react in this manner.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In the context of the question, the presence of the aldehyde functional group (-CHO) in propanal is what allows it to react positively in the Tollens' test, while ethanol (an alcohol) and ethyl methyl ether (an ether) do not contain this reactive group.