Question

Discuss possible procedures for resolution of ethyl \(D, L\) -lactate (ethyl 2-hydroxypropanoate bp \(155^{\circ}\) ) into ethyl \(D\) -lactate and ethyl \(L\) -lactate.

Short answer

Separate ethyl \(D\)-lactate and ethyl \(L\)-lactate using a chiral reagent to form diastereomers, then resolve them via crystallization or chromatography.

Step-by-step solution

Understanding Racemic Mixtures

Ethyl \(D, L\)-lactate is a racemic mixture, meaning it contains equal amounts of two enantiomers: ethyl \(D\)-lactate and ethyl \(L\)-lactate. The goal of resolution is to separate these two enantiomers.

Choosing Resolution Method

One common method for resolving racemic mixtures is by using a chiral reagent. This can involve converting the racemic mixture into diastereomers, which have different chemical properties and can be separated using techniques like crystallization or chromatography.

Formation of Diastereomers

React ethyl \(D, L\)-lactate with a chiral reagent, such as a chiral acid or base, to form diastereomeric esters or salts. These diastereomers can be separated by exploiting their differences in solubility or stability.

Separation of Diastereomers

Use a separation method, such as fractional crystallization or column chromatography, to separate the diastereomers. The separation is based on the different physical properties of the diastereomers, such as solubility or polarity.

Recovering the Enantiomers

Once separated, convert the diastereomers back to the original enantiomers, ethyl \(D\)-lactate and ethyl \(L\)-lactate. This might involve reversing the reaction used to create the diastereomers, such as by hydrolysis, and removing the chiral reagent.

Key concepts

Chiral Reagents

A chiral reagent plays a crucial role in the resolution of racemic mixtures, such as ethyl \(D, L\)-lactate. Chiral reagents are substances that possess chirality themselves, meaning they have a non-superimposable mirror image. This property allows them to interact distinctly with each enantiomer of a racemic mixture. Since the enantiomers have the same physical properties but differ in spatial configurations, using a chiral reagent introduces asymmetry, transforming the enantiomers into diastereomers. Chiral reagents usually include:

• Chiral acids or bases
• Chiral auxiliary agents

These reagents are chosen based on their ability to form stable diastereomers with the enantiomers in question. In practice, the chiral reagent is added to the racemic mixture, and their interaction converts the enantiomers into diastereomers, which have different physical and chemical properties.

Diastereomers

Diastereomers are stereoisomers that are not mirror images of each other. Unlike enantiomers, which have identical physical properties (like melting point and boiling point), diastereomers exhibit different properties. This distinction is key in separating the enantiomers within a racemic mixture.When a racemic mixture is reacted with a chiral reagent, each enantiomer forms a different diastereomer. The resulting diastereomers vary in solubility and boiling points, making physical separation possible using traditional laboratory techniques. This principle is particularly important because it provides the basis by which the resolution of enantiomers is achieved. After separation, these diastereomers can be converted back into enantiomers, culminating in the isolation of pure ethyl \(D\)-lactate and ethyl \(L\)-lactate.

Fractional Crystallization

Fractional crystallization is a technique used to separate substances based on differences in their solubility. It is one of the most traditional and common methods to separate diastereomers formed during the resolution of racemic mixtures. In the context of racemic resolution, diastereomers exhibit distinct solubility properties in a given solvent. By carefully controlling the temperature and solvent conditions, one diastereomer can be made to crystallize while the other remains dissolved. The process involves:

• Dissolving the racemic mixture in a solvent
• Slowly cooling the solution to promote crystallization
• Filtering out the formed crystals, which contains the less soluble diastereomer

This separation process efficiently isolates one diastereomer, which can later be converted back into its corresponding enantiomer.

Column Chromatography

Column chromatography is a widely used technique in chemistry for separating mixtures. It leverages the differential interaction of compounds with a stationary phase (solid) and a mobile phase (liquid). For the separation of diastereomers in racemic mixtures, column chromatography takes advantage of the differing polarities and sizes of the diastereomers. As the mixture passes through a column packed with the stationary phase, individual diastereomers interact differently with it, resulting in varied speeds of travel and, ultimately, separation. Key steps include:

• Packing the column with a suitable stationary phase
• Loading the diastereomer mixture onto the column
• Eluting with an appropriate mobile phase to separate the diastereomers

Column chromatography is efficient and versatile, providing excellent separation that aids in the resolution process and the subsequent recovery of individual enantiomers.