Rotation dextrorotatory

Dextrorotatory Rotating the plane of polarized light in a polarimeter to the right. 

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... 

Accordiug to Norkina c( the specific rotation varies with ti e solvent and racemisation is apt to occur during extraction. Values much lower than — 81° have been given by Smith.The salts are dextrorotatory. 

The directions of rotation at C and C have been arrived at from the following considerations. The deoxy-bases (II p. 443 Q = quinoline residue) obtained from cinchonine and cinchonidine are structurally identical, i but optically different, and since they must be optically identical at C and C, and C is no longer asymmetric, the difference between them (see table, p. 446) must be due to difference in direction of rotation at C , which must therefore be dextrorotatory in cinchonine and laevorotatory in cinchonidine, and this must also be true of quinidine and quinine respectively and of the corresponding dihydro-bases. The keto-bases, cinchoninone and quininone, might be expected to exist each in two pairs, since carbon atom 8 is, according to the formula (p. 442), asymmetric, but it is better represented by the tautomeric grouping —... 

The data given in the table for corynanthine are due to Foumeau and Fiore. Raymond-Hamet later found that corynanthine on alkaline hydrolysis gave a dextrorotatory acid, which Scholz confirmed and identified its methyl ester as yohimbine and suggested that corynanthine is a stereoisomeride of yohimbine. Fourneau and Benoit have confirmed that corynanthine on acid hydrolysis gives 1-corynanthic acid (m.p. 284°, [a]i) — 85-9° pyridine), but on alkaline hydrolysis, even in the cold, some racemisation occurs and the resulting acid is of low lasvo-rotation. They also point out that both yohimbine and corynanthine yield yohimbone on dehydrogenation cf., p. 504) and are probably stereoiso-merides. 

Occasionally, an optically inactive sfflnple of tartaric acid was obtained. Pasteur-noticed that the sodium ammonium salt of optically inactive tartaric acid was a mixture of two minor-image crystal forms. With microscope and tweezers, Pasteur carefully separated the two. He found that one kind of crystal (in aqueous solution) was dextrorotatory, whereas the minor-image crystals rotated the plane of polarized light an equal fflnount but were levorotatory. 

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 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. 

Dextrorotatory (Section 9.3) A word used to describe an optically active substance that rotates the plane of polarization of plane-polarized light in a right-handed (clockwise) 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). 

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