Question

(S)-Butan-2-ol slowly racemizes on standing in dilute sulfuric acid. Explain.

Short answer

(S)-Butan-2-ol racemizes in dilute sulfuric acid due to acidic conditions forming a carbocation intermediate, leading to equal amounts of (S) and (R) enantiomers.

Step-by-step solution

Understanding Racemization

Racemization is a process by which an optically active compound is converted into a racemic mixture, i.e., a mixture with equal amounts of left- and right-handed enantiomers, resulting in a net optical rotation of zero. In the case of (S)-Butan-2-ol, it is likely converting to its enantiomer (R)-Butan-2-ol slowly.

Role of Dilute Sulfuric Acid

Dilute sulfuric acid acts as a catalyst in this process. It provides acidic conditions that can protonate the hydroxyl group of butan-2-ol, making it a better leaving group. This step is crucial for enabling the subsequent formation of a carbocation intermediate.

Formation of Carbocation Intermediate

Once protonated, the hydroxide group is released as water, leaving behind a carbocation. This carbocation is planar and can be attacked from either side by a nucleophile due to its sp2 hybridization. This lack of chirality at the carbocation stage is a key step in racemization.

Re-formation of Butan-2-ol

The planar carbocation can be attacked by water, leading to the re-formation of butan-2-ol. The attack of the nucleophile (water) on the carbocation is random, resulting in the formation of both (S) and (R) enantiomers in equal proportions.

Key concepts

Carbocation Intermediate

In many organic reactions, a carbocation intermediate plays a pivotal role. It's a species formed during the conversion of one compound to another, acting as a temporary stepping stone in the reaction pathway.

In the racemization of (S)-Butan-2-ol, the carbocation intermediate is formed after the hydroxyl group is protonated by dilute sulfuric acid and leaves as water. What remains is a positively charged carbon atom, or a carbocation.

• The carbocation is unstable and has a full positive charge on the central carbon.
• It adopts a planar structure due to its sp2 hybridization, which makes it susceptible to attacks from either side.
• This intermediate is crucial in reactions because it lacks chirality, paving the way for racemization, where both enantiomers are formed eventually.

Optical Activity

Optical activity refers to a compound's ability to rotate plane-polarized light. This is an important concept in stereochemistry, which studies the spatial arrangement of atoms in molecules.

Chiral molecules, such as (S)-Butan-2-ol, are optically active due to their asymmetric carbon atoms. This asymmetry allows them to rotate plane-polarized light, a property measured using a polarimeter.

• Enantiomers are optical isomers, meaning they rotate light in opposite directions.
• When a racemic mixture is formed, the rotations of enantiomers cancel each other out, resulting in no net optical activity.
• Optical activity is a key concept in understanding how racemization leads to an optically inactive mixture over time.

Enantiomers

Enantiomers are stereoisomers that are non-superimposable mirror images of each other. They are like left and right hands, structurally identical but distinct in spatial arrangement.

In the context of (S)-Butan-2-ol, it's important to understand that enantiomers have unique properties.

• They have identical physical properties except for their ability to rotate plane-polarized light and interact with other chiral substances.
• Each enantiomer will rotate light in different directions: one clockwise (dextrorotatory) and the other counterclockwise (levorotatory).
• The formation of equal amounts of two enantiomers results in a racemic mixture, which is a key outcome of the racemization process.

Acid Catalysis

Acid catalysis is a process where an acid accelerates a chemical reaction. The acid is not consumed in the reaction but helps speed up the transformation of reactants to products.

In our example with (S)-Butan-2-ol, dilute sulfuric acid acts as an acid catalyst.

• It donates a proton to the hydroxyl group of alcohol, converting it into better leaving water.
• This protonation step facilitates the departure of the hydroxyl group, forming a carbocation.
• The acid catalysis is crucial for the reaction to proceed, enabling the interconversion of enantiomers and ultimately leading to a racemic mixture.