Question

Esterification takes place when \((-)\)-lactic acid is treated with methanol. (+) methyl lactate is obtained product. The correct statement regarding reaction is (A) Sign of rotation toward PPL changes (B) Configuration of chiral centre is not changed (C) Configuration of chiral centre is changed (D) Both (A) and (B).

Short answer

The correct statement regarding the esterification reaction of \((-)\)-lactic acid with methanol to form \((+)\) methyl lactate is (D), both (A) and (B). The sign of rotation towards PPL changes and the configuration of the chiral center is not changed during the reaction.

Step-by-step solution

Write the esterification reaction

The esterification reaction between \((-)\)-lactic acid and methanol is represented as follows: \(-\text{lactic acid} + \text{methanol} \rightarrow (+)\text{methyl lactate}\) Step 2: Analyze the stereochemistry of reactants and products

Compare the stereochemistry of reactants and products

In this case, the reactant (-)-lactic acid has a chiral center at the alpha-carbon (the carbon next to the carboxyl group). This means it exists in two enantiomeric forms with opposite optical rotations. Similarly, the product (+)-methyl lactate also has a chiral center at the same carbon. Thus, the product (+)-methyl lactate also exists in two enantiomeric forms with opposite optical rotations. Step 3: Analyze the configuration of the chiral center

Determine the configuration change at the chiral center

Since the reaction involves esterification, only the carboxyl group of lactic acid is involved in the reaction. The chiral center at the alpha-carbon remains unchanged during the reaction. This implies that the configuration of the chiral center in the reactant and product remains the same. Step 4: Determine the correct statement(s)

Identify the correct statement(s) regarding the reaction

Based on our analysis, we can now evaluate the statements: (A) Sign of the rotation towards PPL changes. This statement is true as the reactant has a negative optical rotation while the product has a positive optical rotation. (B) Configuration of the chiral center is not changed. This statement is also true as we established that the configuration of the chiral center remains the same during the reaction. (C) Configuration of the chiral center is changed. This statement is false for the reasons explained above. (D) Both (A) and (B). This statement is true because both statements (A) and (B) are true. Therefore, the correct statement regarding the reaction is both (A) and (B), So, the answer is (D).

Key concepts

Stereochemistry

Stereochemistry is the branch of chemistry that deals with the three-dimensional arrangement of atoms in molecules and the impact of this arrangement on chemical reactions and properties. In esterification reactions, like the one between lactic acid and methanol, understanding stereochemistry is crucial because it determines the physical and chemical characteristics of the product formed.

For example, the lactic acid used in the reaction is a chiral molecule, meaning it has a carbon atom connected to four different groups, also known as a chiral center. The presence of this chiral center means the molecule exists in two forms, known as enantiomers, which are non-superimposable mirror images of each other. These enantiomers can have dramatically different effects in biological systems and can also exhibit distinct optical activity, which is critical for applications in pharmaceuticals, where the efficacy and safety of the drugs depend on the correct stereochemistry.

Optical Rotation

Optical rotation is a physical property of chiral substances which measures the degree to which they rotate the plane of polarized light. A substance that rotates the plane of polarized light to the right, or clockwise, is said to have dextrorotatory (usually denoted by the prefix (+)) optical activity, whereas a substance that rotates it to the left, or counterclockwise, is referred to as levorotatory (denoted by (−)).

In the esterification problem given, we encounter (-) lactic acid and (+) methyl lactate, which indicate the direction of the optical rotation caused by each compound. The change from negative to positive rotation in the product formed signifies that the plane of polarized light is rotated in the opposite direction due to the change in the molecular environment around the chiral center upon esterification. However, it's important to note that while the direction of optical rotation changes, this does not necessarily imply a change in the chiral center's configuration.

Chiral Center Configuration

The configuration of a chiral center determines the spatial arrangement of the substituent groups attached to the chiral atom. In chiral molecules, the configuration is often denoted by R (from the Latin rectus, meaning right) or S (from the Latin sinister, meaning left) which are derived from the Cahn-Ingold-Prelog priority rules. The configuration is crucial for the molecule's interaction with other chiral entities, such as enzymes and receptors.

In the case of the esterification of lactic acid with methanol, the configuration at the chiral center of lactic acid remains unchanged after the reaction. This is because the reaction does not involve or alter the groups attached to the chiral carbon. Instead, it involves the reaction of the carboxyl group of lactic acid with methanol to form methyl lactate, while the chiral center retains its original configuration. This subtle yet essential concept demonstrates that while the optical activity of a compound can change through chemical reactions, the underlying spatial configuration of a chiral center can remain intact, an important consideration in the synthesis and analysis of chiral compounds.