Racemic mixtures and meso compounds are key concepts in stereochemistry. They both appear optically inactive, but for different reasons. Racemic mixtures contain equal amounts of enantiomers, while meso compounds have internal symmetry.

Understanding these compounds is crucial for grasping chirality and optical activity. It's important to know how to separate racemic mixtures into individual enantiomers, as this process is vital in many chemical and pharmaceutical applications.

Racemic Mixtures and Meso Compounds

Racemic mixtures vs meso compounds

• Racemic mixtures contain equal amounts of two enantiomers (mirror-image molecules) resulting in optical inactivity due to cancellation of opposite rotations, have a specific rotation $[\alpha]$ of zero, can be separated into individual enantiomers through resolution techniques (crystallization, chromatography)
• Meso compounds possess a plane of symmetry making them achiral despite having stereogenic centers, optically inactive due to internal mirror symmetry, cannot be separated into enantiomers
• Racemic mixtures and meso compounds have identical chemical properties in achiral environments (solvents, reagents) but may exhibit different reactivities in chiral environments (enzymes, chiral catalysts)

Optical Activity and Chirality

• Optical activity refers to the ability of a substance to rotate plane-polarized light
• Chiral molecules lack a plane of symmetry and exist as non-superimposable mirror images (enantiomers)
• Enantiomers rotate plane-polarized light in equal but opposite directions
• Racemic mixtures (racemates) contain equal amounts of enantiomers, resulting in no net rotation of polarized light
• Enantiomeric excess (ee) measures the optical purity of a mixture, indicating the predominance of one enantiomer over the other

Resolution of racemic mixtures

• React racemic acid with a single enantiomer of a chiral amine forming a mixture of diastereomeric salts, each containing one acid enantiomer paired with the chiral amine
• Diastereomeric salts have different physical properties (solubility, melting point, crystallization rates) allowing separation by fractional crystallization or chromatography
• Treat separated diastereomeric salts with strong acid to displace the chiral amine and yield individual enantiomers of the original acid

Reactions of racemic acids

• Racemic acid + achiral amine forms a single achiral salt as both acid enantiomers react equally, resulting in a racemic mixture that cannot be used for resolution
• Racemic acid + chiral amine forms diastereomeric salts with distinct stereochemistries ($(R)$-acid-$(R)$-amine and $(S)$-acid-$(R)$-amine), separable due to different physical properties, enabling resolution of the racemic acid into constituent enantiomers