Optical inactivity, definition

The same kind of spontaneous racemization oc curs for any as 1 2 disubstituted cyclohexane in which both substituents are the same Because such compounds are chiral it is incorrect to speak of them as meso compounds which are achiral by definition Rapid chair-chair interconversion however converts them to a 1 1 mixture of enantiomers and this mix ture IS optically inactive... 

Chemical degradations have allowed the identification of both extremities of peripentadenine, hexanoic acid (present as an amide) and 2-hydroxy-6-meth-ylacetophenone. 13C NMR showed that the rest of the molecule included two nitrogen atoms, six methylenes, and one methine arranged in a (3-propylami-no)-l-pyrrolidine unit substituted at C-2. This formula was definitively proved by chemical degradations as well as by two total syntheses. As noted before in the cases of hygrine and of the ruspolinone alkaloids, peripentadenine is optically inactive. 

Note 2 A polymerization in which, starting from the racemate of a chiral monomer, two types of polymer molecules, each containing monomeric units derived from one of the enantiomers, form in equal amounts is termed racemate-forming enantiomer-differentiating polymerization . The resulting polymer is optically inactive (see Note 4 of Definition 2). 

The term meso (Greek, middle ) was used to describe an achiral member of a set of diastereomers, some of which are chiral. The optically inactive isomer seemed to be in the middle between the dextrorotatory and levorotatory isomers. The definition just given ( an achiral compound with chirality centers ) is nearly as complete, and more easily applied, especially when you remember that chirality centers are usually asymmetric carbon atoms. 

This is true because the operation consists of two parts a rotation C and a reflection . Since a reflection creates the mirror image, the operation is equivalent to rotating in space the mirror image. By definition, a molecule containing a axis is brought into coincidence with itself by the operation S and hence its mirror image, after rotation, is superimposable. The reader is reminded that 8i = a and >=, so that a molecule with either a plane or a centre of sym-metry is also optically inactive. However, the most general rule is a molecmle with a axis is optically inactive. Ck>aversely, it can be shown that a molecule without a axis is, in principle, optically active. 

The first really definitive work on glutose was carried out by Benedict, Dakin and West. They showed that glutose in vitro resembles D-fructose in that it is converted by sodium hydroxide into hydroxy acids (principally optically inactive lactic acid) to about the same extent as D-glucose. In slightly alkaline solution phenylhydrazine reacts to form the phenylosazone of methylglyoxal, and zinc ammonium hydroxide converts glutose into methylimidazole in yield comparable to that obtained from fermentable hexoses. But in vivo glutose ... 

Streptidine was best isolated by allowing a solution of streptomycin hydrochloride in N sulfuric acid to stand at 37° for forty-five hours. The sulfate monohydrate was precipitated in nicely crystalline form by adding three to five volumes of acetone to the reaction mixture. The substance is also isolable from the alkaline hydrolyzate of streptomycin. Streptidine and its salts have no true melting points and are optically inactive throughout the visible spectrum. The sulfate monohydrate has been definitely characterized by its x-ray diffraction pattern. 

One definition of chirality is that the molecule be nonsuperimposablc on its mirror image. An equivalent criterion is that it not possess an improper axis of rotation (page 52). The absence of a mirror plane does not insure optical activity because a molecule may have no mirror plane, yet may possess an improper rotational axis. We can, hpv/ever, be sure that the molecule with a mirror plane will be optically inactive. 

A last attempt at synthesis was made by the preparation of 2-(hydroxy-methyl)glyceronic acid from 1,3-dihydroxy-2-propanone, but this product has not to date been transformed into the desired saccharinic acid (VI). Thus, this optically inactive acid has not been obtained in definitive form, nor have any of its derivatives been reported in the literature. 

By definition, its mirror images are identical and so unresolvable, and the compound is optically inactive. 

Ault recommended using meso to define an optical activity relationship and not a structural feature (i.e., a macroscopic and not a molecular distinction). By this definition, meso refers to "an optically inactive member of a set of stereoisomers, at least two of which are optically active." Ault, A. /. Chem. Educ. 2008, 85, 441. 

TLCs are optically active mixtures of organic molecules. The correct name for TLCs used for temperature measurements is chiral nematic or cholesteric liquid crystals. TLCs are characterized hy well-analyzed reflections of visible light (color play) within a definite bandwidth of temperature. A certain temperature leads to reflections of an explicit spectrum of wavelengths, with a local maximum and a narrow bandwidth. Below the start temperature of the color play, called red-start temperature, the TLCs are transparent, when applied in thin layers the bulk looks milky and white. In this state the molecules, which are elongated like a cigar, have a typical size of about 2-5 nm [ 1 ], are well ordered and close to each other like in a solid crystal (see Fig. 2a). When the temperature increases the arrangement passes through the mesomorphic state, which characterizes the TLCs. The crystals are stiU optically inactive or transparent, hut the molecules are separated in layers. 

ABA is a C, organic acid with one asymmetric carbon atom at C-1 (Fig. 1). The naturally occurring form is 5-( + )-ABA the side chain of ABA is by definition 2-cis,4-trans [2]. The optical rotatory dispersion (ORD) spectrum of ABA exhibits an intense Cotton effect. This property was initially exploited to identify and measure ABA in purified extracts of a number of plants [11]. ABA in organic solvents can be photo-isomerized by ultraviolet radiation to give a mixture of approximately 50% ABA and 50% 2-trans,4 trans-ABA (f-ABA) (Fig. 1). The latter compound is biologically inactive. 

F. Beilstein/ by the action of silver oxide on j8-iodopropionic acid obtained what he called hydracrylic acid, since it decomposed on heating into water and acrylic acid, and he formulated it C12H22O11. Its correct formula was found by W. Moldenhauer.2 Heintz believed that it was the same as Wislicenus s ethylene lactic acid, giving a zinc-calcium double salt, but Wislicenus thought they were different. The position was first clarified by Erlenmeyer, who showed that Wislicenus s ethylene lactic acid is hydracrylic acid. Wisli-cenus had found that paralactic acid is optically active (dextro-rotatory), whilst ordinary fermentation lactic acid is inactive, and he supposed that they should be represented by the same formula, CH3 CH(0H)-C02H. He says (1869) Thus is given the first certainly proved case in which the number of isomers exceeds the number of possible structures. Facts like these compel us to explain diflFerent isomeric molecules with the same structural formula by different positions of their atoms in space, and to seek for definite representations of these. ... 

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