Racemic mixtures optical activity

Section 7 9 A chemical reaction can convert an achiral substance to a chiral one If the product contains a single chirality center it is formed as a racemic mixture Optically active products can be formed from optically inactive... 

What is a racemic mixture Is a racemic mixture optically active That is, will it rotate the plane of polarized light ... 

Many of the physical properties are not affected by the optical composition, with the important exception of the melting poiat of the crystalline acid, which is estimated to be 52.7—52.8°C for either optically pure isomer, whereas the reported melting poiat of the racemic mixture ranges from 17 to 33°C (6). The boiling poiat of anhydrous lactic acid has been reported by several authors it was primarily obtained duriag fractionation of lactic acid from its self-esterification product, the dimer lactoyUactic acid [26811-96-1]. The difference between the boiling poiats of racemic and optically active isomers of lactic acid is probably very small (6). The uv spectra of lactic acid and dilactide [95-96-5] which is the cycHc anhydride from two lactic acid molecules, as expected show no chromophores at wavelengths above 250 nm, and lactic acid and dilactide have extinction coefficients of 28 and 111 at 215 nm and 225 nm, respectively (9,10). The iafrared spectra of lactic acid and its derivatives have been extensively studied and a summary is available (6). 

What products are formed from acid-catalyzed hydration of racemic ( )-4-methyl-1-hexene What can you say about the relative amounts of the products Is the product mixture optically active ... 

A special case of isomerization is the racemization of optically active compounds catalyzed, for example, by sulfuric acid or promoted A1C13. Thus, treatment of (+)-(5)-3-methylhexane at 60°C with 96% sulfuric acid yields a mixture of racemic 2- and 3-methylhexane68 (Scheme 4.4). At lower temperature (0 or 30°C), racemization occurs, but shift of the methyl group does not take place. It can be concluded that at 60°C methyl migration is faster than hydride abstraction to yield isomeric alkanes. At 0 or 30°C, hydride transfer occurs before methyl... 

In the presence of a cationic initiator such as trifluoromethanesulfonic acid and boron trifluoride etherate, 58, gave a mixture of dimers and higher oligomers. [69] On the basis of structural analyses of the dimers prepared from the racemic and optically active monomers, it was concluded that the cationic oligomerization involved protonation onto the amide nitrogen of 58 followed by selective scission of the C(5)—N(6) bond, while the C(5)—0(8) bond remained intact. 

Symmetrically disubstituted allenes, which may be optically active, have been dimerized. Racemic or optically active 1,3-diphenylallene gives both a cis 37,124) and a racemic trans 38> isomer of l,2-bis(phenylmethyl-ene)-3,4-diphenylcyclobutane. Racemic 1,3-dimethylallene affords a complex mixture of dimeric products 38 51>, and partially resolved 1,3-dimethylallene gives a similar spectrum of dimers 51>. 

Optical activity Rotation of the plane of polarized light by an enantiomer racemate (racemic mixture) Optically inactive 1 1 mixture of two enantiomers (5-2)... 

Pfeiffer effect The change in rotation of a solution of an optically active substance on the addition of a racemic mixture of an asymmetric compound. 

In this as m other reactions m which achiral reactants yield chiral products the product IS formed as a racemic mixture and is optically inactive Remember for a substance to be optically active not only must it be chiral but one enantiomer must be present m excess of the other... 

It IS a general principle that optically active products cannot be formed when opti cally inactive substrates react with optically inactive reagents This principle holds irre spective of whether the addition is syn or anti concerted or stepwise No matter how many steps are involved m a reaction if the reactants are achiral formation of one enan tiomer is just as likely as the other and a racemic mixture results... 

Structures A and A are nonsuperimposable mirror images of each other Thus although as 1 2 dichloro cyclohexane is chiral it is optically inactive when chair-chair interconversion occurs Such interconver Sion IS rapid at room temperature and converts opti cally active A to a racemic mixture of A and A Because A and A are enantiomers interconvertible by a conformational change they are sometimes re ferred to as conformational enantiomers... 

Section 7 4 Optical activity, or the degree to which a substance rotates the plane of polarized light is a physical property used to characterize chiral sub stances Enantiomers have equal and opposite optical rotations To be optically active a substance must be chiral and one enantiomer must be present m excess of the other A racemic mixture is optically inactive and contains equal quantities of enantiomers... 

The product is chiral but is formed as a racemic mixture because it anses from an achiral intermediate (the enol) it is therefore not optically active... 

Optically Inactive Chiral Compounds. Although chirality is a necessary prerequisite for optical activity, chiral compounds are not necessarily optically active. With an equal mixture of two enantiomers, no net optical rotation is observed. Such a mixture of enantiomers is said to be racemic and is designated as ( ) and not as dl. Racemic mixtures usually have melting points higher than the melting point of either pure enantiomer. 

Lactic acid is also the simplest hydroxy acid that is optically active. L (+)-Lactic acid [79-33-4] (1) occurs naturally ia blood and ia many fermentation products (7). The chemically produced lactic acid is a racemic mixture and some fermentations also produce the racemic mixture or an enantiomeric excess of D (—)-lactic acid [10326-41-7] (2) (8). 

Chemical Properties. The notation used by Chemical Abstracts to reflect the configuration of tartaric acid is as follows (R-R, R )-tartaric acid [S7-69A-] (4) (S-R, R )-tartaric acid [147-71-7] (5) and y j O-tartaric acid [147-73-9] (6). Racemic acid is an equimolar mixture of the two optically active enantiomers and, hence, like the meso acid, is optically inactive. 

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

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