Problem 142 - A Complete Resource Book in Chemistry for JEE Main

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 \(\mathrm{HCl}\)

Step-by-Step Solution

Answer

Aqueous HCl will maximize the enol content.

Step 1: Understand the question

The exercise asks us to identify which solvent will maximize the enol content of the given compound, \( \mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{2}-\mathrm{CO}-\mathrm{CH}_{3} \), which is a diketone. Enol content refers to the acidic hydrogen being abstracted to form a double bond (alkene) and an alcohol group simultaneously.

Step 2: Consider solvent polarity

Enol content of a compound depends significantly on the solvent in which it is dissolved. Polar solvents can stabilize charged transition states and intermediates. Therefore, solvents with higher polarity can typically promote enolization better than non-polar ones. Among the choices, water and aqueous \( \mathrm{HCl} \) are polar solvents, whereas \( \mathrm{n} \)-hexane is non-polar.

Step 3: Examine solvent acidity

The stability of the enol form in acidic environments is due to facilitation of the hydrogen transfer from carbonyl to keto-enol tautomer by acidic protons. \( \mathrm{CH}_{3}-\mathrm{COOH} \) and aqueous \( \mathrm{HCl} \) are the acidic solvents in this list. The acidic nature of these solvents can increase the enol content by facilitating the enolization process.

Step 4: Determine the effect of specific acids

Among the acidic solvents, \( \mathrm{CH}_{3}-\mathrm{COOH} \) is a weak acid, whereas \( \mathrm{HCl} \) is a strong acid. Stronger acids promote a higher degree of enolization because they better stabilize the transition states and intermediates by proton donation, helping in shifting the equilibrium towards the enol form. Therefore, aqueous \( \mathrm{HCl} \) will maximize the enol content.

Key Concepts

Diketone Structure

At the core of this problem is the compound CH extsubscript{3}-CO-CH extsubscript{2}-CO-CH extsubscript{3}, which is characterized as a diketone. Understanding a diketone structure is vital to grasping its chemical behavior in different environments. A diketone contains two carbonyl groups (C=O) within its molecular skeleton. In this case, the carbonyl groups are located on either side of a central methylene group (CH extsubscript{2}), forming an interesting setup for chemical reactivity.

The presence of two carbonyl groups increases the molecule's susceptibility to chemical transformations, such as enolization, due to their inherent electronegativity. - **Carbonyl groups:** Capable of stabilizing intermediates through resonance and inductive effects. - **Methylene group:** Acts as the site for hydrogen abstraction, facilitating enol formation. The conformation and interaction of these groups define the chemical behavior and reactivity of diketones, especially under different solvent conditions. Recognizing these aspects is key to solving problems involving enol content.

Solvent Polarity

The polarity of a solvent plays an important role in the enolization process of diketones. Enolization involves a transition state where charges are partially developed. Polar solvents can stabilize these charged or polar transition states by interacting with them via dipole interactions.

When evaluating how solvent polarity affects enol content: - **Polar solvents** like water and aqueous HCl are known to assist in stabilizing the intermediate states formed during enolization. - **Non-polar solvents** such as n-hexane lack the ability to stabilize charged species, making them less favorable for promoting enol content. Thus, using a polar solvent can significantly increase enol content as it helps in stabilizing the transition states involved in shifting from the keto to the enol form.

Solvent Acidity

Solvent acidity is a crucial factor that can affect the enol content in a diketone. Acidic solvents provide protons, facilitating the keto-enol tautomerization process. This process is where the keto form of a compound is converted to its enol form. - **Weak acids** such as acetic acid (CH extsubscript{3}-COOH) provide an environment for moderate enolization. - **Strong acids** like HCl, in aqueous solution, offer a high concentration of protons that can significantly enhance the enolization process.

The stronger the acid, the better it can protonate the carbonyl oxygen or stabilize the enolate anion, thus shifting the equilibrium towards the enol form. Therefore, when attempting to maximize enol content, using a strong acid like aqueous HCl is more effective than weaker acids.