Problem 46 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reagents converts \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCHO}\) to \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH} ?\) (a) aq. \(\mathrm{NaOH}\) (b) Acidic \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{CrO}_{4} / \mathrm{H}_{2} \mathrm{SO}_{4}\) (d) \(\mathrm{NaNO}_{2} / \mathrm{HCl}\)

Step-by-Step Solution

Answer

(b) Acidic \( \mathrm{Na}_{2} \mathrm{SO}_{3} \) converts \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCHO} \) to \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH} \).

Step 1: Identify the Functional Groups in the Compound

The compound \( \text{C}_6 \text{H}_5 \text{COCHO} \) consists of a benzene ring \( \text{C}_6 \text{H}_5 \) attached to a keto group \( \text{CO} \) and an aldehyde group \( \text{CHO} \). The structure is essentially a benzaldehyde group attached to a ketone.

Step 2: Determine Desired Product's Structure Change

The desired product \( \text{C}_6\text{H}_5\text{CH}(\text{OH})\text{COOH} \) indicates a transformation of the aldehyde group to an alcohol group \( \text{CH}(\text{OH}) \) and the addition of a carboxylic acid group \( \text{COOH} \). The conversion of aldehydes to carboxylic acids can occur through the formation of intermediate alcohols (aldehyde to alcohol, then to acid).

Step 3: Analyze Possible Reagents

Evaluate the given reagents to determine which could make the desired transformation:- (a) Aqueous NaOH is typically used in basic conditions and transformations, like saponification or enolization, not likely to convert an aldehyde to an alcohol or acid.- (b) Acidic \( \text{Na}_2 \text{SO}_3 \): Sulfite ion is a known reagent for the conversion of keto to their alcohol form, especially effective under acidic conditions for aldehydes.- (c) \( \text{Na}_2 \text{CrO}_4 / \text{H}_2 \text{SO}_4 \) is a strong oxidizing agent, unlikely to convert aldehydes to alcohols directly without over-oxidizing.- (d) \( \text{NaNO}_2 / \text{HCl} \) is not relevant for this type of conversion as it would be used for diazotization reactions.

Step 4: Identify the Correct Reagent

Considering the reactions, acidic \( \text{Na}_2 \text{SO}_3 \) can convert the aldehyde group in the compound to an alcohol, assisting in the overall transformation in the presence of an appropriate acid (for complete oxidation to alcohol/acid).

Key Concepts

Aldehyde to Carboxylic Acid Conversion

In organic chemistry, converting an aldehyde into a carboxylic acid is a common reaction involving oxidation. The process typically requires an oxidizing agent to facilitate the conversion. Aldehydes contain a carbonyl group bonded to at least one hydrogen atom, making them susceptible to oxidation. This is why when an oxidizing agent is introduced, aldehydes can be incrementally oxidized first to alcohols and further to carboxylic acids.

Understanding this conversion involves recognizing the intermediate step, where aldehydes may first transform into alcohols before reaching the final carboxylic acid form. This pathway is relevant when mild conditions are employed to avoid over-oxidizing directly to acids. Specifically, reagents such as acidic sodium sulfite (Na₂SO₃) can play a critical role in moderating the reaction due to their ability to add to carbonyl groups, thus encouraging the sequential conversion of aldehydes.

Organic Functional Groups

Functional groups are specific clusters of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In organic chemistry, understanding functional groups like aldehydes, ketones, alcohols, and carboxylic acids is essential to predicting how molecules will react under certain conditions.

For example, the molecule in our exercise, phenylglyoxal (C₆ B₅COCHO), contains a carbonyl group (CO) and an aldehyde functional group (CHO). These groups are highly reactive and define the molecule's behavior in chemical reactions. A carboxylic acid group (COOH) is different, with a distinctive COOH structure, leading to different properties like acidity.

Recognizing these groups allows chemists to predict and manipulate pathways to convert one functional group to another, as performed in reactions like our exercise, where an aldehyde is transformed due to its reactive nature.

Sulfite Ion Reactions

Sulfite ions (SO₃²⁻) are known in chemistry for their ability to participate in reducing and oxidation reactions. Specifically, when present in acidic solutions, they have the capacity to interact with carbonyl compounds, such as aldehydes and ketones.

In the context of converting aldehydes to alcohols, and subsequently carboxylic acids, the sulfite ion acts by reducing the aldehyde to an alcohol under acidic conditions. This enables a practical transformation pathway without resorting to harsher oxidants that might over-oxidize the compounds.

Because of its bifunctional nature, the sulfite ion is an attractive reagent for selective transformations, aiding in the controlled conversion processes necessary for complex organic syntheses. This allows chemists to achieve particular reaction outcomes like selective group conversions.

Oxidation in Chemistry

Oxidation is a fundamental process in chemistry, involving the increase in oxidation state of a chemical species. This often involves the loss of electrons, and in organic contexts, is usually seen as the addition of oxygen or the removal of hydrogen.

In our targeted reaction, where an aldehyde is being converted into a carboxylic acid, oxidation is the key step. Initially, the aldehyde is changed into an alcohol, which requires the addition of a hydroxyl group. Following this, further oxidation converts the intermediate into the acid.

Oxidation requires careful choice of reagents and conditions to ensure complete and desired transformation without unintended overreaction. Reagents like chromates or permanganates are traditional oxidizing agents known for their efficacy, though their strength requires caution to avoid over-oxidizing beyond desired products. In less drastic transformations, using alternative agents like sodium sulfite ensures a controlled oxidation pathway.