Lactide

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Lactide
Lactide.png
R,R-Lactide xtal.png
Crystal structure of (R,R)-lactide.[1]
Meso-Lactide xtal.png
Crystal structure of meso-lactide.[2] Positions of the hydrogen atoms were not reported.
Identifiers
CAS number 4511-42-6 [(S,S)-Lactide] YesY, 25038-75-9 [(R,R)-Lactide], 13076-19-2 [(R,S)-Lactide = meso-Lactide], 26680-10-4 [mixture of three isomers]
Properties
Molecular 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][3]
Solubility in water Hydrolyses to lactic acid[3]
Solubility soluble in chloroform, methanol
slightly soluble in benzene

[3]

Hazards
R-phrases R36/37/38
S-phrases S26 S37/39
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
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[edit]

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)

Polymerization[edit]

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:[4]

Polylactide synthesis v.1.png

References[edit]

  1. ^ Van Hummel, G. J.; Harkema, S.; Kohn, F. E.; Feijen, J. (1982). "Structure of 3,6-dimethyl-1,4-dioxane-2,5-dione [D-,D-(L-,L-)lactide]". Acta Crys. B38 (5): 1679. doi:10.1107/S0567740882006840. 
  2. ^ Chisholm, Malcolm H.; Eilerts, Nancy W.; Huffman, John C.; Iyer, Suri S.; Pacold, Martha; Phomphrai, Khamphee (2000). "Molecular Design of Single-Site Metal Alkoxide Catalyst Precursors for Ring-Opening Polymerization Reactions Leading to Polyoxygenates. 1. Polylactide Formation by Achiral and Chiral Magnesium and Zinc Alkoxides, (η3-L)MOR, Where L = Trispyrazolyl- and Trisindazolylborate Ligands". J. Am. Chem. Soc. 122 (48): 11845. doi:10.1021/ja002160g. 
  3. ^ a b c Römpp Online Chemielexikon Version 3.3 aufgerufen am 25. März 2009
  4. ^ 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.