Concept Explanation: Tautomerism

Tautomerism is a phenomenon where a single compound exists in two or more readily interconvertible structures that differ in the relative position of a hydrogen atom and a double bond. These forms are called tautomers, and the most common type is keto-enol tautomerism.

Key Characteristics for Tautomerism:

Presence of an acidic α-hydrogen (hydrogen attached to the carbon adjacent to a carbonyl group or other electron-withdrawing group).
Compounds with carbonyl groups (like aldehydes, ketones) often exhibit keto-enol tautomerism.
Compounds must have a mobile hydrogen atom and a π-bond that can shift.

Step-by-Step Analysis of Options:

Step 1: Check for acidic α-hydrogen

2-Butane: This is an alkane (CH₃-CH₂-CH₂-CH₃). No carbonyl group, no acidic α-hydrogen. Does not exhibit tautomerism.
2-Pentanone: Ketone (CH₃-C(O)-CH₂-CH₂-CH₃). Has α-hydrogens (on carbons adjacent to carbonyl). Can exhibit keto-enol tautomerism.
Lactic acid: Carboxylic acid (CH₃-CH(OH)-COOH). Has α-hydrogen (on chiral carbon) and carbonyl group. Can exhibit tautomerism (keto-enol and also lactone forms).
Phenol: Aromatic alcohol (C₆H₅OH). No α-hydrogen relative to carbonyl (no carbonyl in phenol itself). Does not exhibit typical keto-enol tautomerism, but phenol has resonance, not tautomerism.

Step 2: Evaluate tautomeric capability

2-Pentanone and lactic acid both have carbonyl groups and acidic α-hydrogens, so they can exhibit tautomerism.
However, lactic acid (2-hydroxypropanoic acid) has an additional -OH group, which can participate in tautomerism, making it prominent.
Phenol does not have a carbonyl group; it shows resonance stabilization, not tautomerism.

Step 3: Compare stability and occurrence

Between 2-pentanone and lactic acid, lactic acid is more known for tautomerism due to its structure which allows for keto-enol and also potential ring formation (lactide).
Lactic acid exists in equilibrium between its keto and enol forms:

Keto form: $C H 3 - C H (O H) - C O O H$

Enol form: $C H 3 - C (O) - C H O H (a l d e h y d e f o r m)$ , but actually it enolizes to: $C H 3 - C (O H) = C H - C O O H$

2-Pentanone also shows tautomerism: Keto $C H 3 - C (O) - C H 2 - C H 2 - C H 3$ and enol $C H 3 - C (O H) = C H - C H 2 - C H 3$

Step 4: Final answer

Both 2-Pentanone and Lactic acid exhibit tautomerism, but lactic acid is more commonly associated and prominent due to additional functional group. However, all options except 2-Butane and Phenol can show tautomerism, but lactic acid is a classic example.

Thus, Lactic acid is the compound that exhibits tautomerism.

Related Topics:

Keto-enol tautomerism
Acidic α-hydrogen
Resonance vs Tautomerism
Carbonyl chemistry

Formulae and Theory:

General keto-enol tautomerism equilibrium:

$R - C H 2 - C (O) - R' ⇌ R - C H = C (O H) - R'$

Condition: Compound must have at least one α-hydrogen.

For lactic acid (2-hydroxypropanoic acid), the tautomerism involves:

Keto: $C H 3 - C H (O H) - C O O H$

Enol: $C H 3 - C (O H) = C H - C O O H$