Lactic acid, configuration structure

Some times the sign and extent of rotation help in determining which isomer has which configuration. This happens because rotation of structurally related compounds of identical configuration undergoes analogous changes under the influence of temperature, solvent or other factors. Let us study the molecular rotations of L(+) lactic acid and alanine. 

Exercise 19-7 (-F)-Lactic acid has the l configuration. On the basis of the following transformations, deduce the absolute configurations of (—)-1-phenylethanol and (+)-1-phenylethyldimethylsulfonium fluoroborate. Write equations to show the structure and configuration of the products in each step. Reactions 5 and 7 both give E with the same sign of rotation. 

You can tiy working the other way, from the configurational label to the structure. Take lactic acid as an example. Lactic acid is produced by bacterial action on milk it s also produced in your muscles when they have to work with an insufficient supply of oxygen, such as during bursts of vigorous exercise. Lactic acid produced by fermentation is often racemic, though certain species of bacteria produce solely (R)-lactic acid. On the other hand, lactic acid produced by anaerobic respiration in muscles has the S configuration. 

Pasteur was unable to interpret his ideas in precise terms of molecular structure, because Kekul s theory of molecular structure, leading to structural formulae, had not been put forward at the time of Pasteur s experiments. With the advent of Kckul s theory it became possible, during the next few years, to examine the structural formulae of the tartaric and lactic acids, and of various other substances which were known to exhibit optical activity. As a result, it was found that the molecules of all these substances assumed an asymmetric form—or configuration, as it is usually called—when their flat Kekulean representations were converted into three-dimensional ones, accoiding to a very simple principle. This principle was embodied in the Theory of Molecular Configuration, which was advanced independently and almost simultaneously in 1874 by the French chemist Lc Bel, and the Dutch chemist, van t Hoff. 

We may also determine the relative configuration of chiral structures even though bonds are broken to the chiral center if we are able to determine whether the reaction proceeds with retention or with inversion of configuration. The Sn2 reaction of (S)-(—)-2-chloropropanoic acid (87) with hydroxide ion produces (—)-lactic acid, which must have the R configuration (88, Figure 2.34). Note that in this case the direction of rotation of plane-polarized fight stays the same, even though there has been an inversion of the substituents at the chiral center. 

Chabot F., Vert M., Chapelle S., Granger R, Configurational structures of lactic acid stereocopolymers as determined by 13C-1H N.M.R, Polymer, 24,1983, 53-59. 

L-lactic acid is a metabolic intermediate. It can be obtained at low cost from the fermentation of agricultural and food by-products containing carbohydrates such as corn and sugar [30-31]. Racemic lactic acid is produced by petrochemistry or racemisation of the L-enantiomer [32]. As mentioned before, the main advantage of this family is an outstanding versatility due to the chirality, and polymerisation and copolymerisation of various lactides. Figure 9.2 shows the formula of various lactides with links to derived homo- and stereocopolymers whose configurational structures (distribution of L- and D-lactyl units) are represented schematically. 

Draw skeletal structures of both configurational stereoisomers of lactic acid (shown in Model 2). 

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