Specific rotation measurement

As the specific rotation also depends on temperature, the temperature at which the rotation is measured is often shown in the equation. A specific rotation measured at 25 °C is denoted more precisely as... 

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-... 

Specific Rotation Measure the optical rotation of the effluent sample and of the starting Glucose Substrate at 25.0°, and calculate their specific rotations [see Optical (Specific) Rotation, Appendix IIB] by the equation... 

A major disadvantage of optical rotation as a measure of stereochemical purity is that interpretation of the single value lacks significance if any impurities are chiral. To insure the validity of an optical rotation specification, it should be supported by specific rotation measurements on any known chiral impurity or degradation product. This is, of course, in addition to the usual studies of the effects of solvent, concentration, and temperature on the specific rotation of the principal analyte. 

Figure 2, range A) Figure 3 represents the experimentally found dependence of the optical yield of dimerization (proved by n.m.r. and specific-rotation measurements) on the initial R) S) composition of the monomers in samples crystallized under conditions near the thermodynamic equilibrium as expected, the values approximate well to 100% in range A, while dropping quickly to 0% in the eutectic range. 

The product Is very moisture sensitive and should be handled under dry nitrogen or argon. Despite this precaution, the product 1s always contaminated by small amounts of (+)-c1trone11a1, which has an optical rotation opposite to that of the enamlne. To determine the optical purity of the product enamlne, the specific rotation measured therefore must be corrected for the (+)-c1tronellal Impurity. It is more reliable to base optical purity on the specific rotation of the citronellal obtained by hydrolysis of the enamlne (Part C). The absolute method using HPLC of the diastereomeric amide derivative also may be useful as a check of the optical purity. 

The rotation, a, may be measured in an instrument called a polarimeter (Figure 3.17). In practice, the amount of rotation depends upon the wavelength of the light, temperature and the concentration of compound present in solution. The specific rotation, [a], for a chiral compound in solution is given by equation 3.7. Light of a single frequency is used for specific rotation measurements and a common choice is the sodium D-line in the emission spectrum of atomic sodium the specific rotation at this wavelength is denoted as [q]d. 

Excitation of the outer ns electron of the M atom occurs easily and emission spectra are readily observed. We have aheady described the use of the sodium D-line in the emission spectrum of atomic Na for specific rotation measurements (see Section 3.8). When the salt of an alkali metal is treated with concentrated HCl (giving a volatile metal chloride) and is heated strongly in the non-luminous Bunsen flame, a characteristic flame colour is observed (Li, crimson Na, yellow K, lilac Rb, red-violet Cs, blue) and this flame test is used in qualitative analysis to identify the M ion. In quantitative analysis, use is made of the characteristic atomic spectrum in flame photometry or atomic absorption spectroscopy. 

Because the stereochemistry is fixed at all three chiral centers of the molecule, one would expect to observe optical rotation by the molecule. The optical rotation of pergolide mesylate was measured with the sodium d line (589 nm) as the light source at a concentration of 10 mg/ml in dimethylformamide (DMF) in a 100 mm cell using a Perkin-Elmer Polarimeter Model 241MC. The specific rotation measured at 20°C has been observed to be between -18.0 and -23.0°. 

Light of a single frequency is used for specific rotation measurements and a common choice is the sodium D-line in the emission spectrum of atomic sodium. The specific rotation at this wavelength is denoted as [a]o. 

Specific rotation is a physical property of a substance just as melting point boil mg point density and solubility are For example the lactic acid obtained from milk is exclusively a single enantiomer We cite its specific rotation m the form [a]o =+3 8° The temperature m degrees Celsius and the wavelength of light at which the measure ment was made are indicated as superscripts and subscripts respectively... 

The a-carbon of glutamic acid is chiral. A convenient and effective means to determine the chemical purity of MSG is measurement of its specific rotation. The specific optical rotation of a solution of 10 g MSG in 100 mL of 2 A/HQ is +25.16. Besides L-glutamic acid [56-86-0] D-glutamic acid [6893-26-1] and the racemic mixture, DL-glutamic acid [617-65-2] are known. Unique taste modifying characteristics are possessed only by the L-form. 

Polarimetry. Polarimetry, or polarization, is defined as the measure of the optical rotation of the plane of polarized light as it passes through a solution. Specific rotation [ a] is expressed as [cr] = OcjIc where (X is the direct or observed rotation, /is the length in dm of the tube containing the solution, and c is the concentration in g/mL. Specific rotation depends on temperature and wavelength of measurement, and is a characteristic of each sugar it may be used for identification (7). 

The specific rotation ia water is [0 ] ° — +66.529° (26 g pure sucrose made to 100 cm with water). This property is the basis for measurement of sucrose concentration ia aqueous solution by polarimetry. 100°Z iadicates 100% sucrose on soHds. 

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

Nuclear magnetic resonance spectroscopy of the solutes in clathrates and low temperature specific heat measurements are thought to be particularly promising methods for providing more detailed information on the rotational freedom of the solute molecules and their interaction with the host lattice. The absence of electron paramagnetic resonance of the oxygen molecule in a hydroquinone clathrate has already been explained on the basis of weak orientational effects by Meyer, O Brien, and van Vleck.18... 

By measuring the optical rotation as it changes with time, after a gelatin solution is rapidly cooled to the temperature of interest, and extrapolating back to zero time, one can determine the initial specific rotation. It is approximately constant with the concentration, but varies with temperature. This initial specific rotation probably represents that of the sol molecule at that temperature before it is converted into the gel form. 

A comparison of the thus calculated with the measured specific rotations of the 0th- to 4th-generation dendrimers of this kind gave a close resemblance, with a curve, approaching asymptotically a limiting value (Fig. 26). It was also shown that the shape of this curve was independent of solvent, concentration and temperature. This was not the case when CD spectra of these dendrimers were compared (Fig. 27) in solvents such as CH2C12 and f-butyl methyl ether a constant rise of the Cotton effect was observed, which correlates with the increasing amount of benzene chromophores in the dendrimers. However, in the... 

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... 

If di-D-fructose anhydride II has formula XIX, a mixture of 1,3,4-and 1,4,6-trimethyl-D-fructoses would be present in the hydrolytic product. Such a mixture would have a specific rotation of — 10° to — 20° (water) in contrast to the value of + 25 to + 30 found by McDonald and Jackson.76 The rotation of 1,4,6-trimethyl-D-fructose was measured by Montgomery76 in chloroform (+ 29.7°), but it has not been measured directly in water. However, the hydrolysis data of McDonald and Jackson76 for hexamethyl-di-D-fructose anhydride III show that 1,4,6-trimethyl-D-fructose has about the same rotation in water as in chloroform. The argument thus appears to exclude structure XIX. 

The following Tables constitute a list of most of the known, characterized derivatives of sucrose. The names of the solvents used for measuring the specific rotations are abbreviated as follows A, acetone C, chloroform Dm, dichloromethane E, ethanol M, methanol Mf, N,N-dimethylformamide P, pyridine and W, water. 

The reaction is pseudo-first order and rate is proportional to [Sucrose], The progress of the reaction can be studied by measuring the change in specific rotation of a plane of polarised light by sucrose. Let r0, r, and r are the rotation at initially (when t = 0), at any time t and final rotation, respectively. The initial concentration a is proportional to (r0 - r, ) and concentration at any time t, (a - x) is proportional to (r0 - rt). Thus, the rate constant may be obtained as... 

Problem 1.7 The specific rotation of sucrose in presence of hydrochloric acid at 35°C was measured and is given as follows ... 

Specific Rotation A polarized light when passed through an optically active substance, each molecule of it encountered by the light beam rotates the plane of polarization by a constant amount characteristic of the substance. Consequently, a measure of the rotary power of the individual molecule, irrespective of the two parameters, namely the path length and the concentration, is achieved by converting the measured rotation into a specific rotation by the help of the following expressions ... 

For a nonracemic mixture of enantiomers prepared by resolution or asymmetric synthesis, the composition of the mixture was given earlier as percent optical purity (equation 1), an operational term, which is determined by dividing the observed specific rotation (Mobs) of a particular sample of enantiomer with that of the pure enantiomer ( max), both of which were measured under identical conditions. Since at the present, the amount of enantiomers in a mixture is often measured by nonpolarimetric methods, use of the term percent optical purity is obsolete, and in general has been replaced by the term percent enantiomeric excess (ee) (equation 2) introduced in 197163, usually equal to the percent optical purity, [/ ] and [5] representing the relative amounts of the respective enantiomers in the sample. 

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