Glyceraldehyde optical rotation

The small capital letter prefix refers to configuration, related to n glyceraldehyde, and not to the direction of optical rotation. The sign of optical rotation is expressed as (+) and (—) or as d and I or by the words dextro and Uuvo. Th is we have n.( —)-fructose and ,-(+).arabinose. 

No relationship exists between the R and 5 prefixes that designate configuration and the (+) and (-) designations indicating optical rotation. For example, the S enantiomer of lactic acid is dex-trorotatoiy (+), whereas the S enantiomer of glyceraldehyde is levorotatory (-). 

Further examples are provided by naturally occurrii (-)-glyceraldehyde and (+ )-alanine, which both have the S configuration, as shown in Figure 9.9 (p. 318). Note that the sign of optical rotation, (+) or (- i is not related to the R,S designation. (S)-GlycerEiIdehyde happens to be levorotatory (-) and (S)-alanine happens to be d trorotatory (+). There is no simple correlation between R.S conjuration and direction or magnitude of optical rotation. 

Whether a sample is dextrorotatory (abbreviated "(+) ) or levorotatory (abbreviated (-)") is determined experimentally by a polarimeter. Except for the molecule glyceraldehyde, there is no direct, universal correlation between direction of optical rotation ((-i-) and (-)) and designation of configuration (R and 5). In other words, one dextrorotatory compound might have R configuration while a different dextrorotatory compound might have S configuration. 

Like R and S, d and l indicate the configuration of an asymmetric carbon, but they do not indicate whether the compound rotates polarized light to the right (-h) or to the left (-) (Section 5.7). For example, o-glyceraldehyde is dextrorotatory, whereas D-lactic acid is levorotatory. In other words, optical rotation, like melting or boiling points, is a physical property of a compound, whereas R, S, d, and l are conventions humans use to indicate the configuration of a molecule. 

The two enantiomeric forms of lactic acid. The + and signs indicate optical rotation to the right and to the left. The letters D and l indicate the relationship to D- and L-glyceraldehyde see p. 11 2). 

Until relatively recently, interest in chiral chemistry has been largely academic and, as a consequence, has occupied a relatively minor position in the analytical chemistry syllabuses of most universities. Despite the emphasis that has been placed on the recent advances in chiral chemistry, optical isomers have been know for many years and were first identified by Biot [1] in the early 1800s, and their existence was established by the work of Pasteur [2] in 1848. Both van t Hoff [3] and Le Bel [4] proposed the existence of the asymmetric carbon atom and used it to explain the cause of optical rotation. However, it was Emil Fisher [5], who made the first serious attempts to relate the absolute stereochemistry of optical isomers and determined the configuration of (+)-glucose for which he received the Nobel prize. Fisher predicted that the (+)-isomer of glyceraldehyde was the D-isomer and arbitrarily assigned the stereochemistry as ... 

The absolute structures of such molecules can be related to the structure of glyceraldehyde, and they are accordingly referred to by the symbols (see Fig. 1-24). There is no constant correlation between the direction of the optical rotation and the absolute structure of the molecule. Thus, a compound may be dextrorotatory and have an absolute formula related to that of the glyceraldehyde. With these concepts in mind, the specificity of crotonase can be easily understood. 

FIGURE 22.4 Other than for glyceraldehyde, there is no relation between D and L in a carbohydrate name and the sign of optical rotation, (4-) and (-). 

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