Rotation levorotatory

Dextrorotatory dek-(i)stro- ro-t o- tor-e (1878) adj. To turn clockwise (sym. is D) or toward the right esp rotating the plane of polarization of light toward the right, <-crystals> - compare levorotatory (rotates light to the left). Morrison RT, Boyd RN (1992) Organic chemistry, 6th edn. Prentice-Hall, Englewood Cliffs, NJ. 

Levorotatory Rotating the plane of polarized light in a polarimeter to the left. 

Optical isomers interact with polarized light in different wa)rs. Separate equimolar solutions of each enantiomer rotate plane-polarized light (Figures 25-5 and 25-6) by equal amounts but in opposite directions. One solution is dextrorotatory (rotates light to the right) and the other is levorotatory (rotates light to the left). Optical isomers are called... 

Occasionally an optically inactive sample of tartaric acid was obtained Pasteur noticed that the sodium ammonium salt of optically inactive tartaric acid was a mixture of two mirror image crystal forms With microscope and tweezers Pasteur carefully sep arated the two He found that one kind of crystal (m aqueous solution) was dextrorota tory whereas the mirror image crystals rotated the plane of polarized light an equal amount but were levorotatory... 

The optical activity of malic acid changes with dilution (8). The naturally occurring, levorotatory acid shows a most peculiar behavior in this respect a 34% solution at 20°C is optically inactive. Dilution results in increasing levo rotation, whereas more concentrated solutions show dextro rotation. The effects of dilution are explained by the postulation that an additional form, the epoxide (3), occurs in solution and that the direction of rotation of the normal (open-chain) and epoxide forms is reversed (8). Synthetic (racemic) R,.9-ma1ic acid can be resolved into the two enantiomers by crystallisation of its cinchonine salts. 

Natural Occurrence of ( — )-proto-Quercitol. Although the dextrorotatory form (12) of proto-quercitol was discovered in acorns more than a century ago by Braconnot (5), who at first thought that it was lactose, the levorotatory form (13) remained unknown until 1961. In that year, Plouvier isolated it from leaves of the tree Eucalyptus populnea the yield was 0.55% (36). The optical rotation of the new compound was equal and opposite to that of the dextro enantiomer, and it was identical to the latter in its crystal form, melting point, solubilities, molecular formula and infrared spectrum. 

The discoverer of levorotatory proto-quercitol unfortunately described it (36) as L-quercitol. The capital letter l should of course be understood to designate configuration, not rotation. And according to one widely accepted convention (18,19), the quercitol stereosiomer which has the configuration 13 would be designated V , not l . (See formulas 12 and 13.) The name quercitol is now used in a generic sense (cyclo-hexanepentol), so that there are actually six diastereomers to which the name L-quercitol might apply. 

The answer is that Pasteur started with a 50 50 mixture of the two chiral tartaric acid enantiomers. Such a mixture is called a racemic (ray-see-mi c) mixture, or racemate, and is denoted either by the symbol ( ) or the prefix cl,I to indicate an equal mixture of dextrorotatory and levorotatory forms. Racemic mixtures show no optical rotation because the (+) rotation from one enantiomer exactly cancels the (-) rotation from the other. Through luck, Pasteur was able to separate, or resolve, racemic tartaric acid into its (-f) and (-) enantiomers. Unfortunately, the fractional crystallization technique he used doesn t work for most racemic mixtures, so other methods are needed. 

Levorotatory (Section 9.3) An optically active substance that rotates the plane of polarization of plane-polarized light in a left-handed (counterclockwise) direction. 

The presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized hght is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+) or to the left, levorotatory (—). The direction of rotation is independent of the stereochemistry of the sugar, so it may be designated d(—), d(+), l(—), or l(+). For example, the naturally occurring form of fructose is the d(—) isomer. 

Most of the physical properties (e.g., boiling and melting point, density, refractive index, etc.) of two enantiomers are identical. Importantly, however, the two enantiomers interact differently with polarized light. When plane polarized light interacts with a sample of chiral molecules, there is a measurable net rotation of the plane of polarization. Such molecules are said to be optically active. If the chiral compound causes the plane of polarization to rotate in a clockwise (positive) direction as viewed by an observer facing the beam, the compound is said to be dextrorotatory. An anticlockwise (negative) rotation is caused by a levorotatory compound. Dextrorotatory chiral compounds are often given the label d or ( + ) while levorotatory compounds are denoted by l or (—). 

A substance that rotates plane-polarized light in the clockwise direction is said to be dextrorotatory, and one that rotates plane-polarized light in a counterclockwise direction is said to be levorotatory (Latin dexter, right and laevus, left). 

An equal molar mixture of the dextrorotatory and levorotatory enantiomers of a chiral compound is called a racemic mixture or a racemate. Racemates do not show overall optical rotation because the equal and opposite rotations of the two enantiomers cancel each other out. A racemic mixture is designated by adding the prefix (+) or rac- before the name of the molecule. 

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