Rotation, specific

The amount of rotation observed in a polarimeter depends on the structure of the substance and on its concentration. The optical activity of a pure chiral substance is reported as its specific rotation, symbohzed by [a]jj. It is the number of degrees of rotation of a solution at a concentration measured in g mL in a tube 1 dm (10 cm) long. The standard conditions selected for polarimetry measurements are 25 °C, and a wavelength of 589 nm. This yellow light is the D line of a sodium vapor lamp. 

If a chiral substance rotates plane-polarized hght to the right—that is, in a positive (+) or clockwise direction—the substance is dextrorotatory (Latin dextra, right). If a chiral substance rotates plane-polarized light to the left—in a negative (—) or counterclockwise direction— the substance is levorotatory (Latin 

We often refer to an enantiomer by prefixing the sign of the optical rotation at 589 nm to the name of the compound. For example, one of the enantiomers of 2-iodobutane has [0 ] = -15.15. It is called (-)-2-iodobutane. The other enantiomer is (+)-2-iodobutane, [a]j = +15.15. 

The (+) isomer is sometimes called the d form because it is dextrorotatory the (—) isomer is sometimes called the / form because it is levorotatory. Earlier, we encountered levodopa, so named because it is levorotatory. It is also called L-dopa and (—)-dopa. The specific rotation of L-dopa is —13.1°. Table 8.1 lists the specific rotations of some common substances. 

and E to clarify that the resultant vector oscillates in the form of a sine wave. 

Optical rotation is the usual and most useful means of monitoring enantiomeric purity of chiral molecules. Therefore we need to know what variables influence the magnitude of optical rotation. 

The measured rotation, a, of a chiral substance varies with the concentration of the solution (or the density of a pure liquid) and on the distance through which the light travels. This is to be expected because the magnitude of a will depend on the number as well as the kind of molecules the light encounters. Another important variable is the wavelength of the incident light, which always must be specified even though the sodium D line (589.3 nm) commonly is used. To a lesser extent, a varies with the temperature and with the solvent (if used), which also should be specified. The optical rotation of a 

For example, quinine (Section 19-3A) is reported as having [a]D = —117° (c = 1.5, CHC13) (t = 17°), which means that it has a levorotation of 117 degrees for sodium D light (589.3 nm) at a concentration of 1.5 grams per 100 ml of chloroform solution at 17° when contained in a tube 1 decimeter long. 

Frequently, molecular rotation, [Af], is used in preference to specific rotation and is related to specific rotation by Equation 19-3  

Schematic representation of a polarimeter. Plane-polarized light passes through a sotufion of optically active molecules, which rotate the plane of polarization. 

The amount of rotation observed in a polarimetry experiment depends on the number of optically active molecules that the light beam encounters. The more molecules the light encounters, the greater the observed rotation. Thus, the amount of rotation depends on both sample concentration and sample pathlength. If we double the concentration of sample, the observed rotation doubles. Similarly, if we keep the concentration constant but double the length of the sample tube, the observed rotation doubles. It also turns out that the amount of rotation depends on the wavelength of the light used. 

When optical rotation data are expressed in this standard way, the specific rotation, lain, is a physical constant characteristic of a given optically active compound. For example, the (+ )-lactic acid that we saw in Section 9.1 has lain = +3.82°, and (-)-lactic acid has [aln = -3.82°. Some additional examples are listed in Table 9.1. 

TABLE 9.1 Specific Rotation of Some Organic Moiecuies  

Problem 9.S A 1.50 g sample of coniine, the toxic extract of poison hemlock, was dissolved in 10.0 mL of ethanol and placed in a sample cell with a 5.00 cm pathlength. The observed rotation at the sodium D line was +1.21°. Calculate lalp for coniine. 

Polarizer Sample tube containing Analyzer Observer 

To express optical rotation data in a meaningful way so that comparisons can be made, we have to choose standard conditions. The specific rotation, r Jo, of a compound is defined as the observed rotation when the sample pathlength l is 1 decimeter (1 dm = 10 cm), the sample concentration C is 1 g/mL, and light of 589 nanometer (nm) wavelength is used. (Light of 589 nm, the so-called sodium D line, is the yellow light emitted from common sodium street lamps 1 nm = 10 H m.) 

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The properties of a number of sugars are coUected in Table 111,139 the specific rotations in water are included for reference purposes. 

The observed rotation a of an optically pure substance depends on how many mol ecules the light beam encounters A filled polarimeter tube twice the length of another produces twice the observed rotation as does a solution twice as concentrated To account for the effects of path length and concentration chemists have defined the term specific rotation, given the symbol [a] Specific rotation is calculated from the observed rotation according to the expression... 

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... 

Cholesterol when isolated from natural sources is obtained as a single enantiomer The observed rotation a of a 0 3 g sample of cholesterol in 15 ml of chloroform solution contained in a 10 cm polarimeter tube is -0 78° Cal culate the specific rotation of cholesterol... 

It IS convenient to distinguish between enantiomeis by prefixing the sign of iota tion to the name of the substance Foi example we lefei to one of the enantiomeis of 2 butanol as (+) 2 butanol and the othei as (—) 2 butanol Optically pure (+) 2 butanol has a specific rotation ol of +13 5° optically pure (—) 2 butanol has an exactly oppo site specific rotation ol of —13 5°... 

The hydrolysis of sulfonate esters of 2 octanol is stereospecific and proceeds with complete inversion of configuration Write a structural formula that shows the stereochemistry of the 2 octanol formed by hydrolysis of an opti cally pure sample of (S) (+) 1 methylheptyl p toluenesulfonate identify the prod uct as / or S and deduce its specific rotation... 

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. 

Specific rotation is the number of degrees of rotation observed if a 1-dm tube is used and the compound being examined is present to the extent of 1 g per 100 mL. The density for a pure liquid replaces the solution concentration. 

Specific resistance Specific rotation Specific speed Specific viscosit Specimen contrast Speckle interferometry Spectazole... 

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 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. 

Capillary gc/ms, hplc, nmr, ir, and uv are all analytical methods used by the terpene chemist with a good Hbrary of reference spectra, capillary gc/ms is probably the most important method used in dealing with the more volatile terpenes used in the davor and fragrance industry (see Flavors and spices). The physical properties of density, refractive index, boiling point, melting point of derivatives, and specific rotation are used less frequendy but are important in defining product specifications. 

Fohc acid (1) is found as yellow, thin platelets which char above 250°C. The uv spectmm of L-foUc acid at pH 13 shows absorptions at A = 256 nm (e = 30, 000), 282 nm (e = 26,000), and 365 nm (e = 9800). FoHc acid has a specific rotation of [a] = +19.9 (c = 1, 0.1 NNaOH). Solutions of fohc acid are stable at room temperature and in the absence of light. It is slightly soluble in aqueous alkaU hydroxides and carbonates but is insoluble in cold water, acetone, and chloroform. Table 3 Hsts some physical properties of selected fohc acid derivatives. 

AH formulations of vitamin E must show low acidity, and contain not more than 0.004% heavy metals (reported as Pb) and not more than 10 ppm Pb. Eormulations that contain RRR-a-tocopherol must have a specific rotation of +24 ° for the oxidation product with alkaline potassium ferricyanide. 

The purity of (/-a-phenylethylamine-/-malate is not readily determined by its melting point or specific rotation, but rather by its massive crystalline form and solubility. The acid and neutral /-base-/-acid salts are much more soluble, and usually do not crystallize at all. 

The molecular ellipticity is analogous to specific rotation in that two enantiomers have exactly opposite values of 0 at every wavelength. Two enantiomers will thus show CD spectra having opposite signs. A compound with several absorption bands may show both... 

As Z-lelobanidine II also yields Z-lelobanine on oxidation the difference between the I and II forms must be stereochemical and lie in one of the side-chains, in spite of the quantitative identity of their specific rotations. The four asymmetric centres might have the following individual directional effects I. I. d. d. and d. 1. d. 1. in the two forms, but the total effect might be identical. 

Accordiug to Norkina c( the specific rotation varies with ti e solvent and racemisation is apt to occur during extraction. Values much lower than — 81° have been given by Smith.The salts are dextrorotatory. 

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