Optical activity specific rotations

It is known that amines undergo fluxional inversion that leads to loss of optical activity. Phosphines (PR3) are analogs of amines in that phosphorus is immediately beneath nitrogen in the periodic table. Yet the phosphine shown has a specific rotation that can be measured if the temperature is kept below 135°C. Above 135°C, the phosphine loses optical activity (specific rotation goes to zero). What does this indicate about fluxional inversion in this phosphine ... 

Polarimetric detectors have demonstrated significant advantages for identification of optically active drug residues because optical activity is an extremely rare characteristic usually associated with biological activity. Specific rotation measurements on drug residues as they elute from the LC column have the potential to identify closely related structural analogues even when present in a compli-... 

Chiral molecules are optically active. They rotate a beam of plane-polarized light. They are dextrorotatory (+) or levorotatory (-), depending on whether they rotate the beam to the right or left, respectively. The rotations are measured with a polarimeter and are expressed as specific rotations, defined as... 

An optically active Grignard reagent has the ability to differentiate between the two enantiofaces of a carbonyl compound such as 45 [64], In the example shown, the (S)-enantiomer of the product alcohol, 46, is obtained with a high degree of optical purity (= specific rotation of mixture- - specific rotation of one pure enantiomer X 100 for definitions of other terms used in this work, see [65]). 

As noted in Section 17.3 of the text, some optically active compounds rotate plane-polarized light clockwise. These are said to be dextrorotatory and are designated by a plus sign (-t) before the specific rotation value. Substances that rotate plane-polarized light counterclockwise are called levorotatory and are designated by a minus sign (-) before the specific rotation value. 

Chiral compounds are optically active—they rotate the plane of polarized light achiral compounds are optically inactive. If one enantiomer rotates the plane of polarization clockwise (+), its mirror image will rotate the plane of polarization the same amount counterclockwise (—). Each optically active compound has a characteristic specific rotation. A racemic mixture is optically inactive. A meso compound has two or more asymmetric carbons and a plane of symmetry it is an achiral molecule. A compound with the same four groups bonded to two different asymmetric carbons will have three stereoisomers, a meso compound and a pair of enantiomers. If a reaction does not break any bonds to the asymmetric carbon, the reactant and product will have the same relative configuration—their substituents will have the same relative positions. The absolute configuration is the actual configuration. If a reaction does break a bond to the asymmetric carbon, the configuration of the product will depend on the mechaitism of the reaction. 

The specific rotation as a function of wavelength is known as optical rotatory dispersion [39]. The angle of rotation usually increases as wavelength decreases. This normal rotatory dispersion is found in broad spectral regions that are sufficiently remote from absorption bands of the optically active substance. Rotation can be represented by Drude s equation... 

Specific rotation (Section 7 4) Optical activity of a substance per unit concentration per unit path length... 

Specific Rotation. Optical rotation is caused by individual molecules of the optically active compound. The amount of rotation depends upon how many molecules the light beam encounters in passing through the tube. When allowances are made for the length of the tube that contains the sample and the sample concentration, it is found that the amount of rotation, as well as its direction, is a characteristic of each individual optically active compound. 

Optically Active PO. The synthesis of optically pure PO has been accompHshed by microbial asymmetric reduction of chloroacetone [78-95-5] (90). (3)-2-Meth5loxirane [16088-62-3] (PO) can be prepared in 90% optical purity from ethyl (3)-lactate in 44% overall yield (91). This method gives good optical purity from inexpensive reagents without the need for chromatography or a fermentation step. (3)-PO is available from Aldrich Chemical Company, having a specific rotation [0 ] ° 7.2 (c = 1, CHCl ). 

The optical activity of quartz and certain other materials was first discovered by Jean-Baptiste Biot in 1815 in France, and in 1848 a young chemist in Paris named Louis Pasteur made a related and remarkable discovery. Pasteur noticed that preparations of optically inactive sodium ammonium tartrate contained two visibly different kinds of crystals that were mirror images of each other. Pasteur carefully separated the two types of crystals, dissolved them each in water, and found that each solution was optically active. Even more intriguing, the specific rotations of these two solutions were equal in magnitude and of opposite sign. Because these differences in optical rotation were apparent properties of the dissolved molecules, Pasteur eventually proposed that the molecules themselves were mirror images of each other, just like their respective crystals. Based on this and other related evidence, in 1847 van t Hoff and LeBel proposed the tetrahedral arrangement of valence bonds to carbon. 

Quantitative measurements of optical activity are usually expressed in terms of the specific rotation, defined as... 

Normal measurements of optical activity are concerned with the ability of the optically active substance to rotate the plane of polarization of plane polarized light, its specific optical rotatory power ( ) being given by... 

By oxidation of d- and Z-pinene of high rotatory power, Barbier and Grignard obtained the optically active forms of pinonic acid. Z-pinene from French turpentine oil (boiling-point 155 to 157 , od - 37 2 157 to 160 , tto - 32 3°) was oxidised with permanganate. From the product of oxidation, which (after elimination of the volatile acids and of nopinic acid) boiled at 189 to 195 under 18 mm. pressure, Z-pinonic acid separated out in long crystalline needles, which, after recrystallisation from a mixture of ether and petroleum ether, melted at 67° to 69 . The acid was easily soluble in water and ether, fairly soluble in chloroform, and almost insoluble in petroleum ether. Its specific rotation is [a]o - 90-5 in chloroform solution. Oximation produced two oximes one, laevo-rotatory, melting-point 128 and the other, dextro-rotatory, melting-point 189° to 191°. 

For the identification of limonene, one of the most useful compounds is the crystalline tetrabromide, Cj(,HjgBr. This body is best prepared as follows the fraction of the oil containing much limonene is mixed with four times its volume of glacial acetic acid, and the mixture cooled in ice. Bromine is then added, drop by drop, so long as it becomes decolorised at once. The mixture is then allowed to stand until crystals separate. These are filtered off, pressed between porous paper, and recrystallised from acetic ether. Limonene tetrabromide melts at 104 5° and is optically active, its specific rotation being + 73 3°. The inactive, or dipeutene, tetrabromide melts at 124° to 125°. In the preparation of the tetrabromide traces of moisture are advisable, as the use of absolutely anhydrous material renders the compound very diflftcult to crystallise. 

When optical rotation data are expressed in this standard way, the specific rotation, (o-Jq, is a physical constant characteristic of a given optically active... 

Working carefully with tweezers, Pasteur was able to separate the crystal into two piles, one of "right-handed" crystals and one of "left-handed" crys tals like those shown in Figure 9.6. Although the original sample, a 50 50 mix lure of right and left, was optically inactive, solutions of the crystals from eacl of the sorted piles were optically active, and their specific rotations were equa in amount but opposite in sign. 

Comparison of the optical activity showed that the dendrimer 63 with branches of (S)-configuration has a specific rotation and a molecular ellipticity which clearly deviate from the expected values [88,90]. All other 2nd-generation dendrimers (even those with additional spacers between the branches and the core) have specific rotations that are comparable to those expected by simple addition of appropriate values for their building blocks. The deviation may therefore signal the presence of chiral conformational substructures in the 2nd-generation dendrimer 63. 

Marvel, Dec, and Cooke [J. Am. Chem. Soc., 62 (3499), 1940] have used optical rotation measurements to study the kinetics of the polymerization of certain optically active vinyl esters. The change in rotation during the polymerization may be used to determine the reaction order and reaction rate constant. The specific rotation angle in dioxane solution is a linear combination of the contributions of the monomer and of the polymerized mer units. The optical rotation due to each mer unit in the polymer chain is independent of the chain length. The following values of the optical rotation were recorded as a function of time for the polymerization of d-s-butyl a-chloroacrylate... 

Optically active telluronium ylides were not obtained for a long time. Optically active diastereomeric telluronium ylides 7 were obtained for the first time in 1995 by fractional recrystallization of the diastereomeric mixture.19 The absolute configurations of the chiral telluronium ylides were determined by comparing their specific rotations and circular dichroism spectra with those of the corresponding selenonium ylide with known absolute configuration. The telluronium ylides were found to be much more stable toward racemization than the sulfonium and selenonium ylides (Scheme 4). 

On the other hand, telluronium imides 13 were isolated for the first time in 2002 by optical resolution of their racemic samples on an optically active column by medium-pressure column chromatography.27 The relationship between the absolute configurations and the chiroptical properties was clarified on the basis of their specific rotations and circular dichroism spectra. The racemization mechanism of the optically active telluronium imides, which involved the formation of corresponding telluroxides by hydrolysis of the telluronium imides, was proposed (Scheme 6). 

Optical activity comes from the different refractions of right and left circularly polarized light by chiral molecules. The difference in refractive indices in a dissymmetric medium corresponds to the slowing down of one beam in relation to the other. This can cause a rotation of the plane of polarization or optical rotation. The value of specific rotation varies with wavelength of the incident polarized light. This is called optical rotatory dispersion (ORD). 

Let us now apply the technique in some specific cases. The existence of optical activity of several compounds in solution has been explained due to the presence of several active forms of the compound in equilibrium with each other and various assumptions about the forms were also put forward. The equilibrium between the different forms depended on external conditions. But a definite explanation was put forward in 1930 about tartaric acid and it was said that the molecule exists in the following three conformations and each of which makes a certain contribution to the rotation observed. 

Define plane-polarized light, optical rotation, optical activity, asymmetric carbon atom, enantiomers, racemic mixture, polarimeter, and specific rotation. 

The specific rotation of an optically active substance [a] at a given temperature is given by... 

As pharmacological activity is intimately related to molecular configuration, hence determination of specific rotation of pharmaceutical substances offer a vital means of ensuring their optical purity. 

Therefore, an optically active substance is one that rotates the plane of polarized light. In other words, certain specific substances by virtue of their internal structure may be able to transmit only such vibrations that are oriented along certain directions and entirely block vibrations in other directions. 

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