Problem 53 - A Complete Resource Book in Chemistry for JEE Main

Question

Keto \(-\) enol tautomerism is observed in : (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OCH}_{3}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{COCH}_{2} \mathrm{COOH}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COC}_{6} \mathrm{H}_{5}\)

Step-by-Step Solution

Answer

The compound \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{COCH}_{2} \mathrm{COOH}\) exhibits keto-enol tautomerism.

Step 1: Understanding Keto-Enol Tautomerism

Keto-enol tautomerism occurs in compounds that have a hydrogen atom adjacent to a carbonyl group (known as alpha hydrogen). The compound must have this hydrogen and the ability to form a double bond between the carbonyl carbon and the adjacent carbon.

Step 2: Analyzing Each Compound for Tautomerism

Let's look at each compound to see if they contain an alpha hydrogen that can participate in tautomerism:(a) \(\text{C}_{6}\text{H}_{5}\text{CHO}\) is an aldehyde with no alpha hydrogen, so no tautomerism.(b) \(\text{C}_{6}\text{H}_{5}\text{OCH}_{3}\) is an ether and lacks a carbonyl group, so no tautomerism.(c) \(\text{C}_{6}\text{H}_{3}\text{COCH}_{2}\text{COOH}\) has a keto group, carboxyl group, and hydrogens adjacent to the keto group indicative of the potential for tautomerism.(d) \(\text{C}_{6}\text{H}_{5}\text{COC}_{6}\text{H}_{5}\) is a ketone, but has no alpha hydrogen, thus cannot tautomerize.

Step 3: Conclusion

The only compound that meets all the criteria for keto-enol tautomerism (presence of an alpha hydrogen next to a carbonyl group) is \(\text{C}_{6}\text{H}_{3}\text{COCH}_{2}\text{COOH}\). This compound can exhibit keto-enol tautomerism.

Key Concepts

Understanding Alpha Hydrogen

In organic chemistry, the term "alpha hydrogen" refers to a hydrogen atom that is situated next to a carbonyl group. This hydrogen plays a pivotal role in various chemical reactions, particularly in keto-enol tautomerism. The alpha hydrogen is bonded to the carbon atom adjacent to the carbonyl carbon. Here's why it's important:

• The presence of an alpha hydrogen is a prerequisite for the keto-enol tautomerism to occur. Without it, the transformation between the keto and enol forms cannot happen.
• In compounds where alpha hydrogens are present, these hydrogens can be acidic. This acidity allows them to be abstracted easily by a base, facilitating the conversion into an enolate intermediate.

This enolate is key for the reversible transformation in tautomerism. Not all compounds with carbonyl groups have alpha hydrogens, hence not all can undergo tautomerism. For instance, in the exercise, the compound \( ext{C}_{6}\text{H}_{3}\text{COCH}_{2}\text{COOH}\) has an alpha hydrogen next to a carbonyl group, which makes it capable of undergoing keto-enol tautomerism.

The Role of the Carbonyl Group

A carbonyl group, characterized by a carbon atom double-bonded to an oxygen atom (503;O), is a common feature in many organic molecules. It's a defining component of various functional groups like ketones and aldehydes. Here's why the carbonyl group is crucial in tautomerism:

• The carbon-oxygen double bond is polarized, with oxygen being more electronegative than carbon. This polarity makes the carbon atom electrophilic, meaning it will seek to attract electrons.
• In the context of keto-enol tautomerism, the carbonyl group's electron-deficient nature facilitates the shift and redistribution of electrons, allowing the conversion to the enol form.

In \( ext{C}_{6}\text{H}_{3}\text{COCH}_{2}\text{COOH}\), the carbonyl group is instrumental because, together with the alpha hydrogen, it drives the molecule toward achieving equilibrium between the keto and enol forms. This balance is essential as it affects the compound's properties and reactivity.

Tautomerism Analysis: Keto to Enol Transformation

Tautomerism analysis, particularly the transition from the keto form to the enol form, involves scrutinizing the structural changes in a molecule. Here's an overview of the process:

• Initiation Step: The process begins with the abstraction of an alpha hydrogen, leading to the formation of an enolate ion. This ion is a key intermediate in the mechanism.
• Electron Movement: The electrons from the enolate ion are redistributed to form a new double bond between the alpha carbon and the carbonyl carbon, while the carbon-oxygen double bond becomes a single bond.
• Enol Formation: The oxygen in the enol form gains a hydrogen, completing its transformation structure. Now, the former carbonyl oxygen is hydroxylated, resulting in the enol form where an alcohol group is adjacent to a double bond.

Through this dynamic equilibrium, a molecule like \( ext{C}_{6}\text{H}_{3}\text{COCH}_{2}\text{COOH}\) can interconvert between its keto and enol forms. This balance affects multiple properties such as acidity, reactivity, and its ability to participate in subsequent reactions.