Question

The decreasing order of enol content of following 1,3 -dicarbonyl compound is (A) I > II > III (B) II > I \(>\) III (C) \(\mathrm{III}>\mathrm{II}>\mathrm{I}\) (D) II > III >I

Short answer

The short answer is: The decreasing order of enol content for the given 1,3-dicarbonyl compounds is \( II > I > III \), which corresponds to answer choice (B).

Step-by-step solution

Identify the given compounds and their enol forms

First, let's identify the 1,3-dicarbonyl compounds and draw their enol forms: I: Acetylacetone CH_3COCH_2COCH_3 in enol form becomes CH_3C(OH)=C(O^-)CH_3 II: Acetoacetate CH_3COCH_2COO^- in enol form becomes CH_3C(OH)=C(O^-)O^- III: Diethyl malonate (CH_2COOEt)_2 in enol form becomes CH(COOEt)(OH)=C(O^-)(OEt) Now that we have identified the compounds and their enol forms, we will analyze the stability for each enol in the next steps.

Analyze the enol form of Compound I (Acetylacetone)

Compound I has an enol form CH_3C(OH)=C(O^-)CH_3 with an intramolecular hydrogen bonding between an oxygen atom and a hydrogen atom. The presence of this hydrogen bond increases the stability of the enol form.

Analyze the enol form of Compound II (Acetoacetate)

Compound II has an enol form CH_3C(OH)=C(O^-)O^- which has an intramolecular hydrogen bonding and a charge delocalization via resonance with the neighboring oxygen atom. This increased stabilization due to resonance makes the enol form of Compound II more stable than that of Compound I.

Analyze the enol form of Compound III (Diethyl Malonate)

Compound III has an enol form CH(COOEt)(OH)=C(O^-)(OEt) which has an intramolecular hydrogen bond similar to Compounds I and II. However, the presence of two ester groups hinders the conjugation of the enol, resulting in a less stabilized enol form than Compounds I and II.

Compare the stability of enol forms and determine the decreasing order of enol content

Comparing the stability of enol forms, we find the following order: Compound II > Compound I > Compound III Hence, the decreasing order of enol content of the 1,3-dicarbonyl compounds is II > I > III. Therefore, the correct answer is (B).

Key concepts

Enol Stability

The concept of enol stability is fundamental when discussing organic compounds, specifically those with carbonyl groups such as 1,3-dicarbonyl compounds. An enol is a form of a carbonyl compound where the double bond is between carbon and oxygen has shifted to be between two carbon atoms, with a hydroxyl group (-OH) attached to one of them. The stability of this form is key in determining the reactivity and properties of the molecule.

The enol content in a compound refers to the proportion of molecules that exist in this tautomeric form at any given time.

Resonance in Organic Chemistry

Resonance is a concept in organic chemistry that describes the delocalization of electrons within a molecule. It helps to explain the stability of molecular structures beyond single Lewis structures. Structures that can resonate or show different forms of electron arrangements are said to have resonance forms or contributors.

When it comes to enol forms of 1,3-dicarbonyl compounds, resonance allows the negative charge to be distributed over multiple atoms, which contributes to the overall stability of the enol form. The more resonance forms available to a molecule, generally, the more stable it is because the charge or the electron density is spread out over a larger volume of space. Compound II in our exercise exemplifies this with its negative charge being delocalized across two oxygen atoms, leading to a more stable enol form.

Resonance and Stability

Resonance effects can significantly heighten the stability of enols. A molecule that benefits from extensive charge delocalization through resonance will more readily adopt and maintain the enol form.

Intramolecular Hydrogen Bonding

In the world of organic molecules, intramolecular hydrogen bonding is another critical stablizing factor. It occurs when a hydrogen atom is shared between two electronegative atoms like oxygen or nitrogen within the same molecule. This type of bonding plays an essential role in dictating the molecule's structure and stability.

In the context of enol forms of 1,3-dicarbonyl compounds, intramolecular hydrogen bonding between the hydroxyl group and a carbonyl oxygen increases stability, just like in Compounds I and II from our exercise. This internal bond formation can make the enol form more favorable compared to structures without such bonding opportunities.