Measuring Specific Rotation

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... 

The solvents used for measuring specific rotation are abbreviated as follows A, acetone B, benzene Bu, butanone C, chloroform Cy, cyclohexane E, ethanol M, methanol Mf, NN-dimethylformamide Ms, methyl sulfoxide P, pyridine Pe, petroleum ether T, 1,1,2,2-tetrachloroethane and W, water. 

The specific rotation of (—)-2-bromobutane was reported to be - 23.13°. A sample prepared by nucleophilic attack of bromide ion on the tosylate of partially resolved 2-butanol had a measured specific rotation of —16.19°. What is the mole fraction of each enantiomer in this sample of 2-bromobutane ... 

Mannose has three species in equilibrium a-mannopyranose, the aldehyde form, and P-mannopyranose. The position of this equilibrium can be measured, so it is possible to determine the relative percentage of these three species. This discussion must begin with an assumption, however. Assume that the open-chain aldehyde form of mannose (21) accounts for less than 1% of the equilibrium (for most sugars in this chapter, it is about 0.1-0.5% or less) therefore, the pyranose forms are the major conformations. Because these are chiral compounds, one way to measure the equilibrium is to measure the specific rotation (see Chapter 9, Section 9.2). The experimentally measured specific rotation for a pure sample of a-d-mannopyranose (37) is -1-29.3°. The experimentally measured specific... 

Table IV shows the measured specific rotation of various poly-(Bu-l-acid) polymers from which the chiral amine has been removed by repetitive deamination in acetic acid-benzene mixtures. The non negligible specific rotations observed prove that asymmetry was introduced into the main chain of the polymer. of deaminated poly-(Bu-l-acid) was 210 nm, which is close to the absorption wavelength attributed to the n-n transition of carbonic acids.

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... 

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. 

Fig. 26. a Specific optical rotations for the model building blocks for core 78, interior building block 79 and peripheral unit 80 of dendrimers of type 77 b calculated average optical rotations for the doubly branching dendrimers of Oth to 4th generation c measured optical rotations in CHC13 at different temperatures and concentrations [94]... 

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 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 ... 

The beams of reactant molecules A and B intersect in a small scattering volume V. The product molecule C is collected in the detector. The detector can be rotated around the scattering centre. Various devices may be inserted in the beam path, i.e. between reactants and scattering volume and between scattering volume and product species to measure velocity or other properties. The angular distribution of the scattered product can be measured by rotating the detector in the plane defined by two molecular beams. The mass spectrophotometer can also be set to measure a specific molecular mass so that the individual product molecules are detected. 

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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