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Lactic acid is a chiral molecule with two enantiomers, L‐(+)‐lactic acid (also known as S‐lactic acid) and D‐(−)‐lactic acid (also known as (R)‐lactic acid). L‐lactic acid is dextrorotatory and D‐lactic is levorotatory. The racemic mixture is termed rac‐lactic acid. The L‐ and D‐forms of lactic acid have melting points near 53°C, while rac‐lactic acid has a melting point of 16.8°C (Table 2.1). The optically active forms will racemize slowly to rac‐lactic acid when held at 200°C for 500 h [1].

Lactide is the dilactone of lactic acid. Having two stereocenters, lactide has three unique configurations: SS (L‐lactide), RR (D‐lactide), and RS (meso‐lactide). Note that a blend of L‐lactide and D‐lactide is called rac‐ or DL‐lactide and is not the same as meso‐lactide. The lactides rotate polarized light in the opposite direction of the constituent lactic acids; SS (L‐lactide) is levorotatory (−) and RR (D‐lactide) is dextrorotatory. Meso‐lactide is optically inactive. Melting points are indicated in Table 2.2.

The optical rotation for D‐ and L‐forms of lactic acid in water is complicated by oligomer equilibration. Literature values for the optical rotation of L‐lactic acid in water range from −13° to 3.9°. Bancroft and Davis [2] show that specific rotation ([α]_D) for L‐lactic acid as a function of apparent concentration changed linearly from 5.26° at 76 g/100 mL to 0.85° at 5 g/100 mL. Sodium lactate, on the other hand, exhibits nonlinear optical rotation with concentration and changes from −7.8° at 47 g/100 mL to −12.2° at 0.7 g/100 mL. They also showed that specific rotation of a freshly prepared lactic acid (76 g/100 mL) decreased from 5.2° to 1.37° over 17 days. The lactic acid was prepared by acidifying zinc lactate, filtering of zinc sulfate and removing the water under vacuum, keeping the temperature < 40°C. It should be noted that their structural interpretations were wrong. We recommend not to use optical rotation as a measure of the enantiomeric purity of lactic acid but instead rely on HPLC [3], GC [4], or NMR [5] methods.