Lactide

Lactide

Names
Other names Dilactid, (R,R)-3,6-Dimethyl-1,4-dioxan-2,5-dion, (S,S)-3,6-Dimethyl-1,4-dioxan-2,5-dion, (meso)-3,6-Dimethyl-1,4-dioxan-2,5-dion, (R,R)-2,5-Dimethyl-3,6-dioxo-1,4-dioxan, (S,S)-2,5-Dimethyl-3,6-dioxo-1,4-dioxan, (meso)-2,5-Dimethyl-3,6-dioxo-1,4-dioxan
Identifiers
CAS Number	4511-42-6 [(S,S)-Lactide] 25038-75-9 [(R,R)-Lactide] 13076-19-2 [(R,S)-Lactide = meso-Lactide] 26680-10-4 [mixture of three isomers] 95-96-5 [mixture of three isomers] [1]
Properties
Chemical formula	C6H8O4
Molar mass	144.13 g·mol^−1
Melting point	95 to 97 °C (203 to 207 °F; 368 to 370 K) [(S,S)-Lactide and (R,R)-Lactide][2]
Solubility in water	Hydrolyses to lactic acid[2]
Solubility	soluble in chloroform, methanol slightly soluble in benzene [2]
Hazards
R-phrases	R36/37/38
S-phrases	S26 S37/39
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Lactide is the cyclic di-ester of lactic acid, i.e., 2-hydroxypropionic acid. Lactic acid cannot form a lactone as other hydroxy acids do because the hydroxy group is too close to the carboxylic group. Instead, lactic acid first forms a dimer, which is similar to a 5-hydroxyacid. The dimer contains a hydroxy group at a convenient distance from the carboxylic group for the formation of a lactone. Indeed, the dimer readily forms a six-membered cyclic diester known as lactide. Lactides may be prepared by heating lactic acid in the presence of an acid catalyst.

In general, a lactide is the cyclic diester, i.e., the di-lactone of two molecules of any 2-hydroxycarboxylic acid.

Stereoisomers

Lactic acid is chiral; two enantiomeric forms, (R)-lactic acid and (S)-lactic acid, may exist. Thus, lactide formed from two equivalents of lactic acid consists of two stereocenters. Three different stereoisomers of lactide are known:

(R,R)-Lactide (left above), (S,S)-lactide (right above) and meso-lactide (below)

Ball-and-stick models of the above isomers

Polymerization

Lactide can be polymerized to polylactic acid (polylactide) using suitable catalysts, with either syndiotactic or a heterotactic stereocontrol, to give materials with many useful properties:^[3]

References

1. Sigma Aldrich product page for lactide Retrieved 8th of July 2015
2. Römpp Online Chemielexikon Version 3.3 aufgerufen am 25. März 2009
3. R. Auras; L.-T. Lim; S. E. M. Selke; H. Tsuji (2010). Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications. Wiley. ISBN 978-0-470-29366-9.