Which of the following pairs are keto–enol tautomers?
d.
e.

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Step 1: Understand the concept of keto–enol tautomerism. Keto–enol tautomerism is a chemical equilibrium between a ketone (or aldehyde) and its corresponding enol form. The keto form contains a carbonyl group (C=O), while the enol form contains a hydroxyl group (-OH) attached to a carbon-carbon double bond (C=C).

Step 2: Analyze the chemical structures provided in the image. In pair (a), the first structure is a ketone with a carbonyl group (C=O), and the second structure is an enol with a hydroxyl group (-OH) attached to a double bond. Similarly, in pair (b), the first structure is a ketone, and the second structure is an enol.

Step 3: Compare the pairs to determine if they are keto–enol tautomers. Keto–enol tautomers are interconvertible forms of the same compound, differing only in the position of a hydrogen atom and the double bond. In both pairs (a) and (b), the ketone and enol forms are interconvertible, satisfying the definition of keto–enol tautomerism.

Step 4: Evaluate the given options (d and e). Option (d) describes CH3CH2CH2CH=CHOH, which is an enol form, and its corresponding keto form would be CH3CH2CH2CHO (an aldehyde). Option (e) is not visible in the image, so it cannot be analyzed.

Step 5: Conclude that pairs (a) and (b) in the image are examples of keto–enol tautomers, as they meet the criteria for tautomerism. For option (d), the described structures also represent keto–enol tautomers. Option (e) cannot be evaluated due to lack of information.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Keto-Enol Tautomerism

Keto-enol tautomerism is a chemical equilibrium between a keto form (a carbonyl compound) and an enol form (an alcohol with a double bond). This process involves the transfer of a hydrogen atom and a shift of a double bond, allowing the interconversion between the two forms. Understanding this concept is crucial for identifying pairs of compounds that can exist in these two tautomeric states.

Structural Representation of Tautomers

Tautomers are often represented structurally to highlight the differences in bonding and functional groups. In keto-enol pairs, the keto form typically features a carbonyl (C=O) group, while the enol form has a hydroxyl (–OH) group adjacent to a double bond. Recognizing these structural differences is essential for determining whether two given compounds are tautomers.

Stability of Tautomers

The stability of keto and enol forms can vary significantly, often influenced by factors such as sterics and electronic effects. Generally, the keto form is more stable due to the stronger C=O bond compared to the C=C bond in the enol form. This stability can affect the equilibrium position between the two forms, which is important when analyzing reactions involving these tautomers.

Which of the following pairs are keto–enol tautomers?d. e.