Question

Consider the following substances: (1) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{COCH}_{3}\) (2) \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCO}\right)_{2} \mathrm{CH}_{2}\) (3) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{COOC}_{2} \mathrm{H}_{5}\) (4) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{COCH}_{3}\) Place these dicarbonyl compounds in decreasing order of their enol content. (a) \(4>3>2>1\) (b) \(1>4>3>2\) (c) \(1>2>3>4\) (d) \(1>4>2>3\)

Short answer

The correct order is (1) > (4) > (3) > (2), corresponding to option (b).

Step-by-step solution

Identify Dicarbonyl Compounds

The given compounds have two carbonyl groups (-CO-). We need to identify these to determine the enol content transformation potential. For compound (1) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{COCH}_{3}\), there are two ketone groups. For compound (2) \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCO}\right)_{2} \mathrm{CH}_{2}\), these appear as ester derivatives. Compound (3) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{COOC}_{2} \mathrm{H}_{5}\) has a ketone and an ester. Compound (4) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{COCH}_{3}\) has two ketone groups.

Analyze the Stability of Enol Forms

The stability of enol forms is generally influenced by hydrogen bonding and conjugation. Compound \( (1) \) has the potential for aromatic conjugation (phenyl group), making its enol form quite stable. Both compounds \( (3) \) and \( (4) \) have isolated ketones, which can still form stable enol due to keto-enol tautomerism, with \( (4) \) having more steric factors that might lower stability compared to \( (3) \). Compound \( (2) \), with its ester groups, does not enolize easily. Thus, its enol content is minimal.

Arrange Compounds Based on Predicted Enol Content

Based on our analysis in Step 2, we predict the order of enol content. Compound \( (1) \) likely has the highest enol content due to aromatic stabilization. Compound \( (4) \) follows due to its straightforward ketone-ketone system. Compound \( (3) \) is next, being a mixed system but lacking significant resonance assistance. Lastly, compound \( (2) \), with ethyl ester groups, has minimal enol content compared to others.

Key concepts

Dicarbonyl compounds

Dicarbonyl compounds are chemical structures that contain two carbonyl groups represented by the functional group C=O. These groups can be found in various forms, such as aldehydes, ketones, and esters. Understanding these compounds is crucial in organic chemistry, especially when discussing keto-enol tautomerism. In the original exercise, we identify and focus on dicarbonyl compounds, which are pivotal when evaluating the potential for enolization.
Dicarbonyl compounds can bond with nearby hydrogen atoms, facilitating a chemical process called tautomerism. Here, compounds like ketones and esters are essential. Ketones, with two carbonyl groups, often form stable structures, influencing the enol content. Remember that in compound structures, dicarbonyl configurations can affect stability and reaction properties, paving the way for fascinating tautomeric transformations.

Enol content

Enol content refers to the concentration of the enol form present in a compound containing keto-enol tautomers. Enols are alcohols characterized by a C=C double bond and an OH group. They usually exist in equilibrium with ketones or aldehydes. High enol content is often a result of factors like conjugation, hydrogen bonding, and steric considerations.
In compounds, like those in the given exercise, enolization allows the molecule to stabilize through the formation of an additional resonance structure. For compound (1) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{COCH}_{3}\), the presence of a phenyl group results in significant conjugation, increasing its enol content. This is due to the aromatic nature of the phenyl group that allows overlap with the enol form.
For compound (2) \((\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCO})_{2} \mathrm{CH}_{2}\), the ethyl ester groups hinder enol content due to their structural nature, resulting in minimal enol formation. Therefore, understanding how these elements impact enol content helps in predicting the behavior of these compounds under various conditions.

Tautomeric stability

Tautomeric stability is a concept based on the equilibrium between two structural isomers, such as keto and enol forms. It is crucial in determining the predominant structure of a compound under specific conditions. Stability is often enhanced by strong factors like hydrogen bonding, conjugation, and steric effects.For compounds in keto-enol equilibrium, like compound (1) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{COCH}_{3}\), aromatic conjugation provides substantial stability to the enol form. This compound, with its phenyl group, showcases enhanced enol content due to this effect.
Compound (4) \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{COCH}_{3}\) contains ketone-ketone linkage, which generally allows for stable enol because of internal hydrogen bonding possibilities. However, steric hindrances might come into play, affecting overall stability compared to more favorable conditions seen in simpler systems like compound (1). Thus, predicting and understanding tautomeric stability helps chemists tailor reactions and synthesize desirable products by controlling the structural strain and resonance possibilities.