Question

Compound 'A' (molecular formula \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}\) ) is treated with acidified potassium dichromate to form a product 'B' (molecular formula \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\) ). 'B' forms a shining silver mirror on warming with ammonical silver nitrate. 'B' when treated with an aqueous solution of \(\mathrm{H}_{2} \mathrm{NCONHNH}_{2}\) HCl and sodium acetate gives a product 'C'. Identify the structure of ' \(\mathrm{C}^{\prime}\). (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{NNHCONH}_{2}\) (b) \(\mathrm{CH}_{3}-\mathrm{C}=\mathrm{NNHCONH}_{2}\)

Short answer

The structure of compound 'C' is (a) \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH} = \mathrm{NNHCONH}_{2}\).

Step-by-step solution

Identify Compound A

Given that the molecular formula of compound 'A' is \( \mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O} \), this suggests that 'A' could be either an alcohol or an ether. Since it reacts with acidified potassium dichromate, which is a common oxidizing agent for alcohols, we conclude that 'A' is an alcohol.

Oxidation to Form B

Compound 'B' has the molecular formula \( \mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O} \), which indicates either an aldehyde or a ketone. Since 'A' is oxidized to 'B' using potassium dichromate, this suggests that 'B' is likely a carbonyl compound.

Identify the Carbonyl Compound B

'B' forms a silver mirror with ammoniacal silver nitrate, which is Tollens' reagent. This indicates that 'B' is an aldehyde because only aldehydes react with Tollens' reagent. Thus, 'B' is propanal (\( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CHO} \)).

Reaction with Hydrazine Derivative to form C

When 'B' is treated with \( \mathrm{H}_{2}\mathrm{NCONHNH}_{2} \), it forms a hydrazone. This reaction typically involves the carbonyl carbon of the aldehyde or ketone forming a double bond with the nitrogen atom of the hydrazine derivative. Therefore, the structure of compound 'C' is \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} = \mathrm{NNHCONH}_{2} \).

Identify the Correct Answer

Compare the structure obtained for compound 'C' with the options provided. The structure \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} = \mathrm{NNHCONH}_{2} \) matches option (a). Therefore, option (a) is the correct answer.

Key concepts

Oxidation reactions

Oxidation reactions play a crucial role in organic chemistry, particularly in the transformation of alcohols to aldehydes or ketones. These reactions involve the increase in oxidation state of a molecule through the gain of oxygen or loss of hydrogen.
One of the most commonly used oxidizing agents for alcohols is potassium dichromate (\(K_2Cr_2O_7\) ), especially in acidic conditions. This agent targets the hydroxyl group in alcohols, removing hydrogen atoms and introducing a double-bonded oxygen, resulting in either an aldehyde or a ketone, depending on the original alcohol's structure. For primary alcohols like propanol, the oxidation typically yields an aldehyde.

• Example: Ethanol (\(CH_3CH_2OH\) ) getting oxidized to acetaldehyde (\(CH_3CHO\) ).

In oxidation reactions, care must be taken to ensure optimal conditions since different settings might lead to over-oxidation or undesirable side reactions. These reactions are foundational to understanding organic transformations and synthesis.

Aldehydes

Aldehydes are a class of organic compounds containing a carbonyl group (C=O), where the carbon atom is bonded to a hydrogen atom and a variable R group. This functional group imparts distinctive chemical properties to aldehydes, making them highly reactive.
One notable reaction that involves aldehydes is their ability to form a silver mirror with Tollens' reagent, indicating that aldehydes can be easily oxidized. This property differentiates them from ketones, which do not react with Tollens' reagent.

• Indicator: Ammoniacal silver nitrate reacts with aldehydes like formaldehyde to form a bright silver deposit.

Aldehydes can be found naturally in many flavors and fragrances, such as vanillin and cinnamaldehyde in cinnamon. In synthetic chemistry, they are pivotal intermediates for further chemical transformations, including the formation of other functional groups like alcohols and acids.

Hydrazone formation

Hydrazone formation is a significant reaction in organic chemistry, utilized for its ability to convert aldehydes or ketones into more stable compounds. The reaction involves a hydrazine derivative, such as semicarbazide, reacting with the carbonyl group of aldehydes or ketones, forming a double bond with nitrogen.
This process begins with the nucleophilic attack of the hydrazine group on the electrophilic carbonyl carbon, leading to a rearrangement that results in a hydrazone.

• Step example: The reaction of acetone with phenylhydrazine forms acetone phenylhydrazone.

The resulting hydrazones are often crystalline solids, making them easy to purify and characterize, useful for identifying unknown carbonyl compounds. This reaction provides important insights into molecular structures and is crucial for various applications, including pharmaceuticals and material sciences.