Observed rotation compounds

To express optical rotations in a meaningful way so that comparisons can be made, we have to choose standard conditions. The specific rotation, [a ]D, of a compound is defined as the observed rotation when light of 589.6 nanometer (nm 1 nm = 10-9 m) wavelength is used with a sample pathlength / of 1 decimeter (dm 1 dm = 10 cm) and a sample concentration C of 1 g/mL. (Light of 589.6 nm, the so-called sodium d line, is the yellow light emitted from common sodium lamps.)... 

Besides the effect of solvent polarity, the C=C rotation in many push-pull ethylenes is sensitive to acid catalysis (143). This is probably explained by protonation of the acceptor groups, for example, the oxygen atoms in C=0 groups (16), which increases their acceptor capacity. Small amounts of acids in halogenated solvents, or acidic impurities, may have drastic effects on the barriers, and it is advisable to add a small quantity of a base such as 2,4-lutidine to obtain reliable rate constants (81). Basic catalysis is also possible, but it has only been observed in compounds containing secondary amino groups (38). 

Optical activity is the ability of a compound to rotate the plane of polarized light. This property arises from an interaction of the electromagnetic radiation of polarized light with the unsymmetric electric fields generated by the electrons in a chiral molecule. The rotation observed will clearly depend on the number of molecules exerting their effect, i.e. it depends upon the concentration. Observed rotations are thus converted into specific rotations that are a characteristic of the compound according to the formula below. 

Recently Isbell and coworkers have published the results of an extensive study of the behavior of solutions of sorbitol and D-mannitol in the presence of tetraborates. They found that sorbitol appears to form three complex borate compounds, whereas D-mannitol forms only two. Since the specific rotation in the tetraborate-D-mannitol system is a function of the ratio of the components and is independent of concentration at constant tetraborate-D-mannitol ratios, D-mannitol can be determined quantitatively by this method. However, sorbitol cannot be determined this way because the change in observed rotation at constant tetraborate concentration shows a reversal with increasing amounts of sorbitol. 

The specific rotation of a compound, designated as [aj, is defined as the observed rotation, a, when the sample path length Z is 1 dm, the sample concentration C is Ig/mL and light of 599.6 nm wavelength (the D line of a sodium lamp, which is the yellow light emitted from common sodium lamps) is used. 

As Louis Pasteur first observed (Box 1-2), enantiomers have nearly identical chemical properties but differ in a characteristic physical property, their interaction with plane-polarized light. In separate solutions, two enantiomers rotate the plane of plane-polarized light in opposite directions, but an equimolar solution of the two enantiomers (a racemic mixture) shows no optical rotation. Compounds without chiral centers do not rotate the plane of plane-polarized light. 

The direction and the degree to which a compound rotates plane-polarized light is given by its observed rotation. 

Given the concentration of an optically active compound, length of the polarimeter tube, and observed rotation, calculate the specific rotation. Given any three of the four quantities mentioned, calculate the fourth. 

For a particular compound the observed rotation depends on the concentration of the compound, the path length of the sample tube, and the wavelength of the light that is used. Often the yellow light produced by a sodium lamp, called the sodium D line (wavelength = 589 run), is used. The specific rotation, a constant characteristic of each... 

To use the rotation of polarized light as a characteristic property of a compound, we must standardize the conditions for measurement. We define a compound s specific rotation [a] as the rotation found using a 10-cm (1-dm) sample cell and a concentration of 1 g/mL. Other cell lengths and concentrations may be used, as long as the observed rotation is divided by the path length of the cell (Z) and the concentration (c). 

With chiral compounds, the plane of the polarized light is rotated through an angle a. The angle a, measured in degrees (°), is called the observed rotation. A compound that rotates the plane of polarized light is said to be optically active. 

A natural product was isolated in the laboratory, and its observed rotation was +10° when measured in a 1 dm sample tube containing 1.0 g of compound in 10 mL of water. What is the specific rotation of this compound ... 

Observed rotation (Section 5.12A) The angle that a sample of an optically active compound rotates plane-polarized light. The angle is denoted by the symbol a and is measured in degrees (°). 

Only the methoxy- and chloro-compounds (38, 3 ) exhibit large deviations between calculated and observed rotations. In general, the 1,1-diphenylcyclopropanes XVIII have rather large optical rotations ( [ ]d > 90°). Inspection of Table 3 reveals that the molar rotations of XVIII are almost entirely due to helix optical activity ( 0 q > 0 d ). This is in contrast to an earlier assumption which has attributed optical rotations of XVIII to atomic asymmetry. Further comparisons between calculated and observed optical rotations of complex cyclopropanes I are presented in Table 4. 

Specific rotation The amount by which a chiral molecule rotates the plane of plane polarised light, [a], under standard conditions it is defined for solutions as [a]=a//c and for pure compounds as being equal to did, where a is the observed rotation, / is the cell length in decimetres, c is the concentration in grams per millilitre and d is the density in the same units. 

Irradiation performed with racemic substrate at room temperature, unless noted otherwise. Anisotropy (g) factor at or around irradiation wavelength, if reported or estimated. Extent of destruction. Maximum observed rotation a of irradiated solution, or specific rotation [a] of isolated sample or of residue obtained upon evaporation. Maximum observed ellipticity of irradiated solution or molar ellipticity of isolated sample. Enantiomeric excess of isolated sample. Not reported. Compound (mp 113 C) of unknown structure, obtained in a reaction of humulene with sodium nitrite, according to the reported procedure Chapman, AC. J. Chem. Soc. 1895 67 780. A mixed case of asymmetric destruction and photoderacemization irradiation performed at 0 C. Enantiomerically enriched sample used. Estimated g factor enhanced by two-quantum excitation with high intensity picosecond laser pulse. High-inten-sity laser of indicated pulse duration used. "Irradiation performed at 77 K in a hydrocarbon glass matrix. Optically pure sample photolyzed only to evaluate the enhanced g factor. Estimated g factor enhanced by two—quantum excitation with high-intensity femtosecond laser pulse. 

Specific rotation is a physical property of an optically active compound. The specific rotation of plane polarized light by an optically active compound is the observed rotation to the left, Ievorotatory (I, -) or to the right, dextrorotatory (d,+) divided by the length of the sample tube in decimeters and the concentration of the sample in g/cm3. 

Naya and Kotake, in an examination of Japanese hop oil, have isolated three humulane-type compounds, viz., humuladienone (161, R = Me), humulenone II (161,R = =CH2), and humulol (162), in addition to the tricyclic diol (163, R = OH), m.p. 207 °C. This diol has already been prepared in two different ways (a) Sutherland et treated humulene (164) with AT-bromosuccini-mide in aqueous acetone and converted the resultant bromohydrin (163, R = Br) to the diol (163, R = OH), m.p. 205—206 °C, by hydrolysis, (b) McKervey and Wright obtained the same diol, m.p. 201—203 °C, by acid-catalysed (20% sulphuric acid) rearrangement of humulene 1,2-epoxide (165), a known natural product. On the basis of these findings and the fact that both caryophyllene (166) and humulene can be derived from the above bromohydrin by two in vitro steps, McKervey and Wright postulated that humulene 1,2-epoxide may be involved in the biosynthesis of the tricyclic diol and caryophyllene. This postulate does not, however, readily accommodate the observed rotations of the relevant... 

Physical Properties.—S-Deoxy-D-erj/i ro-hexosulose has been ob-tained " " as a colorless, hygroscopic powder of empirical formula CeHioOs. The compound is soluble in water, methanol, and ethanol, but insoluble in dry acetone, ethyl acetate, and ether. In water, a small mutarotation is observed [ajn — 2.5 — + 1.5° — 2.6 —> + 1.2° 0° 1°. However, the observed rotations could be due to impurities. The ultraviolet spectra in the range 400-210 m revealed no peak or strong absorption. 

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