Crystal optical activity

The measured crystal optical activity, in general, can be either of molecular origin or due to the chiral helical arrangement of chiral or achiral molecules in the crystal, or both. The two factors are difficult to separate. Kobayashi defined a chirality factor r = (pc — ps)/pc = 1 — pslpc, where pc is the rotatory power per molecule of a randomly oriented crystal aggregate derived from the gyration tensors determined by HAUP, and ps that in solution [51]. It is a measure of the 4 crystal lattice structural contribution to the optical activity and represents the severity of the crystal lattice structural contribution to the optical activity, and represents the severity of the restriction of the freedom of molecular orientation by forming a crystal lattice. Quartz is a typical example of r = 1, as it does not contain chiral molecules or ions and its optical activity vanishes in random orientation (ps = 0). 

Properties Colorless crystals. Optically active D(-)-lysine, mp 224C with decomposition L(+)-ly-sine mp 224C with decomposition. Soluble in water slightly soluble in alcohol insoluble in ether. 

This reaction is more stereoselective than the corresponding synthesis of menthyl sulfinate diastereoisomers in the Andersen procedure, allowing for easier fractional crystallization. Optically active (/ )-(+)-methyl phenyl sulfoxide (13) is obtained on reaction of (lf ,2S)-(12) with methyllithium (Scheme 2.14). 

Elgavi, A., Green, B. S., and Schmidt, G. M. J., Reactions in chiral crystals. Optically active heterophotodimer formation from chiral single crystals,/. Am. Chem. Soc., 95, 2058-2059,1973. 

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

Schadt M and Flelfrich W 1971 Voltage-dependent optical activity of a twisted nematic liquid crystal Appl. Phys. Lett. 18 127-8... 

Nonlinear Optical Devices. A transparent, optically active, sol—gel-derived organic—inorganic glass has been synthesized (68). This hybrid consists of a 2,4-dinitroaminophenylpropyl-triethoxysilane covalently bound to a siUcon alkoxide-derived siUca network. This hybrid exhibits a strong electric field-induced second harmonic signal and showed no signs of crystallization. 

The racemic acid is not a primary product of plant processes but is formed readily from the dextrorotatory acid by heating alone or with strong alkaU or strong acid. The methods by which such racemic compounds can be separated into the optically active modifications were devised by Pasteur and were apphed first to the racemic acid. Racemic acid crystallizes as the dihydrate triclinic prisms. It becomes anhydrous on drying at 110°C... 

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). 

Optically active films used to polarize light or enhance brightness in a liquid crystal display. 

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. 

The diastereoselectivity is observed in the Henry reaction using optical active niti o compounds or a-heteroatom substituted aldehydes. Lor example, the reaction of O-benzyl-D-lactal-dehyde with methyl 3-niti opropionate in the presence of neubal alumina leads to a mixture of three niti o-aldol products from which D-ribo isomer is isolated by direct crystallization. D-Ribo... 

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. 

Bornyl Acetate.—The acetic acid ester is the most important of the series. It is a constituent of pine-needle and rosemary oils, and has a most fragrant and refreshing odour. It is prepared artificially by the action of acetic anhydride on borneol, in the presence of sodium acetate, or by the condensation of borneol with glacial acetic acid in the presence of a small amount of a mineral acid. It is absolutely necessary in the reproduction of any pine odour. It is a crystalline body, crystallising from peDroleum ether in rhombic hemihedric crystals melting at 29°. The optical activity depends on that of the borneol from which it has been prepared. It has the following characters —... 

V. M. L. Wood, Crystal science techniques in the manufacture of chiral compounds in Chirality and Industry II. Developments in the Manufacture and applications of optically active compounds, A. N. Collins, G. N. Sheldrake, J. Crosby (Eds.), John Wiley Sons, New York (1997) Chapter 7. 

Little was done after Biot s discovery of optical activity until 1848, when Louis Pasteur began work on a study of crystalline tartaric acid salts derived from wine. On crystallizing a concentrated solution of sodium ammonium tartrate below... 

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. 

Let s return for a last look at Pasteur s pioneering work. Pasteur took an optically inactive tartaric acid salt and found that he could crystallize from it two optically active forms having what we would now call the 2R,3R and 2S,3S configurations. But what was the optically inactive form he started with It couldn t have been meso-tartaric acid, because meso-tartaric acid is a different chemical compound and can t interconvert with the two chiral enantiomers without breaking and re-forming chemical bonds. 

The resultant mixture can be separated by fractional crystallization as the trans-isomer is more soluble the m-isomer can be resolved into its enantiomers using optically active anions like a-bromocamphor 7r-sulphonate. These chlorides can be converted into the bromide or iodide complex by refluxing with a solution of the appropriate potassium halide. 

These polymers could not be crystallized, despite their apparent stereoregularity, probably because of the sterically-hindered character of the chains. It was proposed by Farina and Bressan62 that the chain growth was stereoregulated by the optically active anion of the ion-paired chain carrier. Further studies63 showed that the first portion of the polymer produced in a given reaction always possessed a less regular structure than later portions, unless the reaction was started in the presence of previously prepared polymer. This observation was interpreted as evidence for the pre-... 

The crude product is dissolved in five times its weight of water, and after clearing with a little Norite the solution is diluted with one and one-half volumes of 95 per cent alcohol. The product separates in well-formed, snow-white crystals, and after standing for several days in an ice chest is collected with suction on a Buchner funnel. The yield of purified histidine monohydrochloride is 75-80 g. (Note 5). The compound melts at 251-2520, with decomposition. The amino acid is not race-mized by the procedure employed, and shows the characteristic optical activity, [a]n6° = +8.00, in the presence of three moles of... 

Sulfoxides were first prepared in optically active form in 1926 by the classical technique of diastereomeric salt formation followed by separation of the diastereomers by recrystallization16 17. Sulfoxides 1 and 2 were treated with d-camphorsulfonic acid and brucine, respectively, to form the diastereomeric salts. These salts were separated by crystallization after which the sulfoxides were regenerated from the diastereomers by treatment with acid or base, as appropriate. Since then numerous sulfoxides, especially those bearing carboxyl groups, have been resolved using this general technique. 

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