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

Question

Tautomerism and resonance are two different phenomena. In tautomerism, two structural isomers differing in the relative positions of their atoms are spontaneously interconvertible and can exist in dynamic equilibrium. The interconvertibility of tautomers involves making and breaking of bonds. In resonance, two or more structures are written for the same compound as each structure could not explain all the properties of the compound. The resonating or canonical structures do not differ in the position of atoms but only bonds and unpaired electrons have different positions in the different structures. The actual structure of the compound is the hybrid of the canonical structures. 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

The enol content will be maximum in acetic acid (c).

Step 1: Understand the Question

We need to determine in which solvent the enol content of pentanedione (\( \mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{2}-\mathrm{CO}-\mathrm{CH}_{3} \)) will be maximum. Tautomerism and resonance concepts will help us understand where the dynamic equilibrium between keto and enol forms lies.

Step 2: Identify Tautomerization Process

Pentanedione exhibits keto-enol tautomerism, where the keto form can change to the enol form. The equilibrium between these forms is influenced by the solvent's ability to stabilize the enol form.

Step 3: Analyze Solvent Effects

The stability of the enol form is enhanced in solvents that can form strong hydrogen bonds or solvents that are slightly acidic, promoting enolization. Analyze the options: (a) Water can stabilize enol through hydrogen bonding, (b) \(\mathrm{n}\)-hexane is non-polar and does not favor hydrogen bonding, (c) Acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\)) provides a hydrogen-bonding environment and is slightly acidic, (d) Aqueous \(\mathrm{HCl}\) is strongly acidic, which might lead to protonation affecting enol formation.

Step 4: Choose the Optimal Solvent

Acetic acid (c) is both a hydrogen-bond donor and slightly acidic, making it the most favorable environment for maximizing the enol content by stabilizing the enol form effectively compared to the other solvents.

Key Concepts

Resonance

Resonance is a concept in chemistry that explains how certain molecules or ions can be represented by two or more different structures. These structures, called "resonance structures" or "canonical forms," involve the same arrangement of atoms but differ in the positioning of electrons. For instance, the placement of

• double bonds
• single bonds
• lone pairs

can vary between different resonance structures.

The actual molecule or ion, however, doesn't flip between these canonical forms. Instead, it is best thought of as a resonance hybrid—a blend of all the canonical forms. This hybrid state provides a more accurate depiction of the compound’s structure. This resonance effect helps stabilize the molecule because the electron density is spread out over the structure. Understanding resonance enables chemists to predict molecular stability, reactivity, and properties, giving insight into the complex behaviors of molecules.

Keto-enol tautomerism

Keto-enol tautomerism is an important concept in organic chemistry, where two isomers, called tautomers, interconvert. These isomers have different connectivity of atoms: one form is a keto form, which typically has a carbonyl group \[ (\text{-CO-}) \]and the other is an enol form, characterized by a hydroxyl group (-OH) adjacent to a double bond. This equilibrium process involves the exchange of a hydrogen atom and the shift of a double bond.

The conversions can be affected by several factors such as solvent, temperature, and pH. Keto forms are generally more stable due to the strength of the carbonyl bond, but under certain conditions, enol forms can be favored, especially if the molecule can achieve additional stability via hydrogen bonding or resonance.

Understanding this concept is crucial in fields like biochemistry and pharmaceuticals since the properties and reactivities of molecules can vary significantly between their keto and enol forms.

Solvent effects on tautomerism

Solvent choice can significantly influence the tautomeric balance between the keto and enol forms of a compound. Depending on the solvent’s characteristics, it can either stabilize or destabilize one of the tautomers, thus tipping the equilibrium.

• **Polar solvents**, such as water, can stabilize tautomers through hydrogen bonding, potentially increasing the concentration of the enol form.
• **Non-polar solvents**, like n-hexane, generally do not favor the enol form since they lack hydrogen bonding capabilities, leaving the keto form more prevalent.
• **Acidic solvents**, such as acetic acid, not only provide hydrogen bonding but can also protonate or deprotonate the molecule, leading to a possible increase in enol content.
• **Strongly acidic environments**, like aqueous hydrochloric acid, may lead to excessive protonation, affecting equilibrium by possible side reactions.

By analyzing these solvent effects, one can manipulate the conditions to enhance the presence of a desired tautomer, crucial in synthesis and material science applications.