Question

Enol content of \(\mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{2}-\mathrm{CO}-\mathrm{CH}_{3}\) will be maximum in which of the solvents? (a) Water (b) \(\mathrm{n}\) - hexane (c) \(\mathrm{CH}_{3}-\mathrm{COOH}\) (d) Aqueous \(\mathrm{HCl}\)

Short answer

The enol content is maximum in aqueous HCl.

Step-by-step solution

Understanding the Concept

The compound given is a beta-diketone known to be capable of tautomerizing between keto and enol forms. The enol form proportion increases in solvents that can stabilize it through hydrogen bonding or by providing an acidic environment that favors the enol form.

Analyzing Solvent Effects

Each solvent has different properties: Water (polar, protic), n-hexane (non-polar, aprotic), acetic acid ( CH₃COOH - polar, protic, slightly acidic), and aqueous HCl (polar, protic, and strongly acidic). The presence of an acidic medium tends to favor the enol form as it helps in the stabilization of enols through hydrogen bonding and extending the conjugation.

Identifying Maximum Enol Content Solvent

Among the given solvents, aqueous HCl provides the strongest acidic environment. Acidity increases the stability of the enol form by facilitating the transfer of hydrogen, which other solvents do not do as effectively.

Key concepts

Beta-Diketone

Beta-diketones are organic compounds characterized by having two ketone groups separated by a single methylene ( CH₂) group. One of the most well-known examples is the compound 2,4-pentanedione, also known as acetylacetone. This structure is significant because it allows for keto-enol tautomerism. In beta-diketones, the presence of two carbonyl groups with the methylene group in between creates an environment that is conducive to tautomerism. The hydrogen atom on the methylene group can easily migrate, turning the compound into its enol form. This migration results in the formation of a carbon-carbon double bond adjacent to an alcohol group, creating the enol form. This balance between the keto and enol forms is the essence of keto-enol tautomerism, and the dynamic nature of this process relies heavily on external factors such as the surrounding solvent and the intrinsic properties of the diketone itself.

Solvent Effects on Tautomerism

Solvents have a substantial impact on the equilibrium between the keto and enol forms of a beta-diketone. The properties of the solvent can either promote or hinder the enolization process.

• Polarity: Polar solvents are more likely to stabilize the enol form via hydrogen bonding.
• Protic vs. Aprotic: Protic solvents, which can donate hydrogen bonds, are better at supporting enol formation compared to aprotic solvents.
• Acidity: Solvents with acidic properties further enhance enol content by promoting hydrogen transfer, thus stabilizing the enol structure.

When analyzing the solvents from the exercise: - Water provides a polar environment, beneficial for hydrogen bonding. - n-Hexane, being non-polar and aprotic, offers little support to enol formation. - Acetic acid, though polar and protic, provides only a slightly acidic environment. - Aqueous HCl, being both polar and significantly acidic, maximizes enol stabilization through enhanced hydrogen bonding and acidity.

Acidity and Stability of Enol Form

The stability of the enol form in keto-enol tautomerism is greatly influenced by the acidity of the environment. When a solution is more acidic, it tends to stabilize the enol form more effectively. This happens for several reasons:

• Hydrogen Bonding: Acidic solutions facilitate stronger hydrogen bonding interactions, which can stabilize the enol form by anchoring the hydroxyl group.
• Proton Contribution: Acidity provides a conducive environment for the intramolecular or intermolecular proton transfers that are crucial in maintaining the enol form.
• Conjugation Enhancement: An acidic medium can extend conjugation and increase stability by helping to delocalize electrons in the enol form.

In the context of the given question, aqueous HCl represents the optimal environment for enol stabilization because its strong acidity not only supports hydrogen bonding but also promotes a more favorable energy landscape for the enol form, ultimately leading to maximum enol content.