Question

Which of the following reagents would react with butan-2-one to produce propanoic acid after acidification? (A) Fehling's solution (B) Tollen's reagent (C) \(\mathrm{NaI}\) in \(\mathrm{NaOH}\) (D) \(\mathrm{I}_{2}\) in \(\mathrm{NaOH}\)

Short answer

The correct reagent to react with butan-2-one and produce propanoic acid after acidification is \(I_2\) in \(NaOH\) (option D). The presence of hypoiodite ion (OI-) in the reagent mixture can perform oxidative cleavage, and after acidification, it will produce the desired product, propanoic acid.

Step-by-step solution

Analyze the structures of the starting material and desired product

First, to know about the reactions and reagents selections, we need to analyze the structure of the starting material, butan-2-one, and the desired product, propanoic acid. Butan-2-one (CH3C(O)CH2CH3) is a ketone with four carbon atoms. Propanoic acid (CH3CH2COOH) is a carboxylic acid with three carbon atoms. So, we need a transformation from a ketone to a carboxylic acid with one less carbon atom.

Analyze the given reagents

Now let's analyze each given reagent to determine if it will produce the desired reaction: (A) Fehling's solution is mainly used for the detection of aldehydes, not for the transformation of ketones to carboxylic acids. It is not suitable for this reaction. (B) Tollen's reagent is also used for the detection of aldehydes, specifically it oxidizes the aldehyde to a carboxylic acid. Ketones are not typically oxidized by Tollen's reagent, so it is also unsuitable for this reaction. (C) NaI in NaOH is a combination of sodium iodide and sodium hydroxide. This mixture can generate the iodine ion (I-) which can be used to remove a hydrogen atom from the alpha carbon (the carbon next to the carbonyl group) of a ketone, thus promoting a reaction called halogenation. (D) I2 in NaOH is a combination of iodine and sodium hydroxide. This mixture can also generate iodine ion (I-), but additionally, it can produce hypoiodite ion (OI-). This ion is a strong oxidizing agent that can perform oxidative cleavage of a ketone to produce a carboxylic acid and a carboxylate ion. After acidification, both products will become carboxylic acids.

Determine the correct reagent to produce propanoic acid

As we have analyzed all the given reagents, we can now determine the correct reagent. Among all of the given reagents, I2 in NaOH (option D) is the correct reagent for transforming butan-2-one into propanoic acid. The hypoiodite ion (OI-) present in the reagent mixture can perform oxidative cleavage, and after acidification, it will produce the desired product, propanoic acid.

Key concepts

Conversion from Butan-2-one to Propanoic Acid

Understanding the conversion from butan-2-one to propanoic acid is crucial for students studying organic chemistry, especially in the context of JEE Advanced exam preparation. Butan-2-one, a four-carbon ketone, must undergo a reaction that removes one carbon atom and adds an oxygen to form propanoic acid, a three-carbon carboxylic acid.

The correct process involves the use of an oxidizing agent that can break the carbon chain at the appropriate place to form the acid. The iodine in sodium hydroxide solution (\textbf{I}\(_{2}\) in \textbf{NaOH}) is the reagent capable of achieving this transformation. This solution generates hypoiodite ions (OI-), which are strong enough to cleave the carbon chain next to the carbonyl group of the ketone, resulting in propanoic acid upon subsequent acidification.

Fehling's Solution and Ketones

Coming to the interaction of Fehling's solution and ketones, it's essential to clarify that Fehling's solution is a chemical reagent used to differentiate aldehydes from ketones. It consists of copper(II) sulfate, sodium potassium tartrate (Rochelle salt), and strong alkali like sodium hydroxide.

The copper(II) ions in Fehling's solution are reduced to copper(I) oxide, which precipitates as a red to brick-red color if an aldehyde is present. However, ketones do not usually react with Fehling's solution due to their lower reactivity towards oxidation. So, a negative result (no color change) would be observed if you test a ketone like butan-2-one with Fehling’s solution.

Tollen's Reagent and Aldehydes

Similarly, Tollen's reagent has a specific role in organic chemistry, primarily to test for the presence of aldehydes. Tollen's reagent is a solution of silver nitrate (\textbf{AgNO}\(_3\)) in aqueous ammonia. When an aldehyde is present, it reduces the silver ions in the reagent to elemental silver, leaving a shiny silver mirror on the walls of the test container.

Aldehydes are easily oxidized compared to ketones, hence their positive reaction with Tollen's reagent. Ketones, including butan-2-one, typically do not undergo oxidation with Tollen's reagent and thus do not give a positive test. This forms the basis of distinction between aldehydes and ketones in qualitative organic analysis.

Oxidative Cleavage in Organic Synthesis

Lastly, the concept of oxidative cleavage is a fundamental one in organic synthesis. It refers to the splitting of a carbon-carbon bond in an organic molecule, using an oxidizing agent. This reaction is valuable for reducing the size of carbon chains and introducing oxygen atoms into the molecule.

Reagents capable of oxidative cleavage, such as hypoiodite ions generated from \textbf{I}\(_{2}\) in \textbf{NaOH}, are potent enough to split ketones like butan-2-one to form smaller molecules, which upon acidification yield carboxylic acids. This type of reaction not only illustrates the reactivity of different functional groups but also showcases the strategic methods used in organic synthesis to achieve specific transformations, a key topic for students aiming for high scores on exams like the JEE Advanced.