Problem 135 - A Complete Resource Book in Chemistry for JEE Main

Question

Keto-enol tautomerism is observed in (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CO}-\mathrm{CH}_{2}-\mathrm{CO}-\mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

Compounds (a) and (c) show keto-enol tautomerism.

Step 1: Identify Functional Groups for Tautomerism

Keto-enol tautomerism occurs between a ketone or aldehyde form and its corresponding enol form. We need to find the carbonyl groups (C=O) adjacent to a CH or CH2 group, which can undergo this shift.

Step 2: Analyze Compound (a): C6H5COCH3

In compound (a), there is an acetophenone group where the carbonyl (C=O) is adjacent to a methyl group. This means it can form an enol by shifting a hydrogen atom from the methyl group to the oxygen, creating a double bond (C=C) with the former carbonyl carbon.

Step 3: Analyze Compound (b): C6H5CHO

Compound (b) is benzaldehyde, with no adjacent hydrogen on a methyl or methylene group. It lacks the necessary structure for keto-enol tautomerism since the CH group is part of an aldehyde that does not participate in this tautomerism.

Step 4: Analyze Compound (c): C6H5COCH2COCH3

In compound (c), diketone is present. The central methylene (CH2) group is flanked by two carbonyls, making it conducive to keto-enol tautomerism. A hydrogen shift can occur here, creating an enol form.

Step 5: Determine Which Compounds Undergo Tautomerism

Based on the analysis, compounds (a) and (c) have structures that allow for keto-enol tautomerism.

Key Concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and determine how a molecule reacts in chemical reactions. In organic chemistry, these groups are crucial because they define the behavior of the molecule and its potential interactions with other substances.
For keto-enol tautomerism, the relevant functional groups are the carbonyl group (C=O) usually found in ketones and aldehydes, alongside adjacent acidic hydrogens on a CH or CH2 group.
Here's why they matter:

• A carbonyl group is a double-bonded carbon and oxygen, providing a site for chemical reactivity.
• When positioned next to hydrogen atoms, it becomes possible for these groups to undergo changes like shifting hydrogen atoms, fundamental for the keto-enol process.

Understanding functional groups helps us identify molecules such as ketones or aldehydes that can participate in tautomerism, as seen in compounds like \((\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3}\) or \((\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CO}-\mathrm{CH}_{2}-\mathrm{CO}-\mathrm{CH}_{3}\).

Carbonyl Group

The carbonyl group is a defining feature of both aldehydes and ketones. It's a carbon atom double-bonded to an oxygen atom, noted as C=O. The carbonyl group is highly polar, meaning it has an uneven distribution of electrons between the carbon and oxygen. This characteristic makes the group very reactive, particularly suited for various types of chemical transformations.
In keto-enol tautomerism, the carbonyl group's polarization is critical. It sets up conditions for the migration of an alpha-hydrogen atom (the hydrogen atom next to the carbonyl group) to the oxygen atom. The shift transforms the carbonyl compound into an enol form, characterized by an alcohol group attached to a double-bonded carbon.
Here are some important points about carbonyl groups related to this process:

• The electronegativity of oxygen makes the carbon atom an electrophile, drawing electrons towards itself.
• This character is key for the hydrogen shift that turns a ketone into its enol form, enabling keto-enol tautomerism.

Not all carbonyl-containing compounds participate in tautomerism, as shown by benzaldehyde \((\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\)), which lacks an adjacent methyl or methylene group for hydrogen migration.

Enol Form

The enol form of a molecule is an alternative structural arrangement to the ketone or aldehyde configuration. In chemistry, the term "enol" comes from the presence of an "alkene" (a carbon-carbon double bond) and an "alcohol" (a hydroxyl group).
When tautomerism happens, a hydrogen atom adjacent to a carbonyl group shifts to the oxygen atom of the carbonyl, forming an enol. The carbon atom which was initially double-bonded to oxygen now forms a double bond with a neighboring carbon atom instead.
Key points about the enol form include:

• The double bond in the enol form typically increases the molecule's nucleophilicity, making it more reactive in some cases.
• The stability of enol forms can be less than their keto counterparts but in some molecules like diketones, the enol form may be stabilized by hydrogen bonding.
• The ability to form an enol is a pivotal aspect of keto-enol tautomerism, as observed in structures such as \((\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3}\) and \((\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CO}-\mathrm{CH}_{2}-\mathrm{CO}-\mathrm{CH}_{3}\).

Conversion to enol forms plays a significant role in biochemical pathways and synthetic chemistry where the respect to such tautomerism is often utilized.