Optical activity polarimeter

The experimental facts that led van t Hoff and Le Bel to propose that molecules having the same constitution could differ m the arrangement of their atoms m space concerned the physical property of optical activity Optical activity is the ability of a chiral sub stance to rotate the plane of plane polarized light and is measured using an instrument called a polarimeter (Figure 7 5)... 

Although the usual absorption and scattering spectroscopies caimot distinguish enantiomers, certain techniques are sensitive to optical activity in chiral molecules. These include optical rotatory dispersion (ORD), the rotation by the sample of the plane of linearly polari2ed light, used in simple polarimeters and circular dichroism (CD), the differential absorption of circularly polari2ed light. 

Polarimeter (Section 7.4) An instrument used to measure optical activity. 

Determination of Rotatory Power. —The 1 otatory p o < 1 of ethyl tartiate, which is an optically active substance, is determined by means of a polarimetcr. One of these insti U-ments known as Laurent s polarimeter is shown in l- igs. 71 and 72. 

Figure 9.5 Schematic representation of a polarimeter. Plane-polarized light passes through a solution of optically active molecules, which rotate the plane of polarization.

A polarimeter. The sample lube contains an optically active compound. The analyzing filter has been tuned clockwise to restore the light field and measure the rotation caused by the sample. 

A polarimeter is used to determine the optical activity of a substance by measuring the angle through which plane-polarized light is rotated by a sample. 

Optical activity Experimentally observed rotation of the plane of monochromatic plane-polarized light to the observer s right or left. Optical activity can be observed with a polarimeter. 

The rate of decomposition of an optically active compound was followed by a polarimeter when the tabulated results were obtained. The equilibrium constant is 3.B9. Find the specific rate. 

Optical isomerism is shown by compounds which are capable of rotating the plane of polarized light. The phenomenon is called optical activity and the substances possessing this property are called optically active substances. The optical activity is observed and measured by an instrument called polarimeter. ... 

Define plane-polarized light, optical rotation, optical activity, asymmetric carbon atom, enantiomers, racemic mixture, polarimeter, and specific rotation. 

The rotation of the plane of polarized light and hence the optical activity may be detected and measured accurately by an instrument known as the polarimeter. 

The two characteristic parameters related to optical activity of the pharmaceutical substances, namely (a) optical rotation, and (b) specific optical rotation, can be measured satisfactorily by the help of a Polarimeter as stated below ... 

A polarimeter can be used for the identification of optically active compounds,5 as shown in Figure 2.10. 

A soln of Boc-Phe-NCA (0.58g, 2.0mmol) in THF (6.0mL) was stirred at 20°C and TEA (0.42mL, 3.0 mmol) was added. An aliquot of the mixture was immediately removed and placed in a 1.00-dm polarimeter cell. The total optical activity of the mixture at the sodium D line (589 nm) was monitored over time. After 90 min, the solvent was removed under reduced pressure from the remainder of the mixture to give a white solid. H NMR analysis of this material showed it to be pure Boc-Phe-NCA, unchanged from starting material except for its optical activity. The material exhibited [a]D20 +17 (c 1.1, THF), a loss of 86% of the original value for Boc-L-Phe-NCA of [a]D20 +119 (c 1.1, TFIF). 

Substances that can rotate the orientation of plane-polarized light are said to have optical activity. Measurement of this change in polarization orientation is called polarimetry, and the measuring instrument is called a polarimeter. 

An unknown substance, X, was isolated from rabbit muscle. Its structure was determined from the following observations and experiments. Qualitative analysis showed that X was composed entirely of C, H, and 0. A weighed sample of X was completely oxidized, and the H20 and C02 produced were measured this quantitative analysis revealed that X contained 40.00% C, 6.71% H, and 53.29% O by weight. The molecular mass of X, determined by mass spectrometry, was 90.00 u (atomic mass units see Box 1-1). Infrared spectroscopy showed that X contained one double bond. X dissolved readily in water to give an acidic solution the solution demonstrated optical activity when tested in a polarimeter. 

Polarimeter. An instrument for determining the concentration of optically active compounds in solution by determining the angle of rotation of plane-polarized light passing through the sample. See also Polarimetry,... 

Optical rotation measures the degree that light is rotated (see Table Gl.5.7 in Anticipated Results). In citrus oils, d-limonene is the major enantiomer in the sample. Since other optically active compounds are often present in racemic mixtures, there is no net rotation and thus they are ignored. If a compound is a racemic mixture, the polarimeter will not give a reading. Readings can be verified with known standards. 

The polarimeter is used for determining the specific rotations of optically active substances and also for determining concentrations of solutions of optically active substances of known specific rotation. 

Polarimeter. An instrument for determining the rotation of polarization of light as the light passes through a solution containing an optically active substance. 

The labeled compounds are each checked with a polarimeter for the rotation of plane-polarized light. Only Compounds A. B. arui C are optically active. 

If reactants and products have different optical rotation properties, it is possible to study the transformation by monitoring the optical rotation using a polarimeter. When more than one optically active substance is present in the reaction, their combined optical rotation effect upon the plane of polarised light is observed. The angle of rotation (a) caused by a solution of a single pure compound is given by ... 

Chiral molecules are optically active. They rotate a beam of plane-polarized light. They are dextrorotatory (+) or levorotatory (-), depending on whether they rotate the beam to the right or left, respectively. The rotations are measured with a polarimeter and are expressed as specific rotations, defined as... 

Know the meaning of plane-polarized light, polarimeter, optically active or optically inactive, observed rotation, specific rotation, dextrorotatory, levorotatory. 

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. 

Each stereoisomer in a pair of enantiomers has the property of being able to rotate monochromatic plane-polarized light. The instrument chemists use to demonstrate this property is called a polarimeter (see your text for a further description of the instrument). A pure solution of a single one of the enantiomers (referred to as an optical isomer) can rotate the light in either a clockwise (dextrorotatory, +) or a counterclockwise (levorotatory, -) direction. Thus those molecules that are optically active possess a handedness or chirality. Achiral molecules are optically inactive and do not rotate the light. 

The existence of these different practices was not sufficient to create a discipline or subdiscipline of physical chemistry, but it showed the way. One definition of physical chemistry is that it is the application of the techniques and theories of physics to the study of chemical reactions, and the study of the interrelations of chemical and physical properties. That would mean that Faraday was a physical chemist when engaged in electrolytic researches. Other chemists devised other essentially physical instruments and applied them to chemical subjects. Robert Bunsen (1811—99) is best known today for the gas burner that bears his name, the Bunsen burner, a standard laboratory instrument. He also devised improved electrical batteries that enabled him to isolate new metals and to add to the list of elements. Bunsen and the physicist Gustav Kirchhoff (1824—87) invented a spectroscope to examine the colors of flames (see Chapter 13). They used it in chemical analysis, to detect minute quantities of elements. With it they discovered the metal cesium by the characteristic two blue lines in its spectrum and rubidium by its two red lines. We have seen how Van t Hoff and Le Bel used optical activity, the rotation of the plane of polarized light (detected by using a polarimeter) to identify optical or stereoisomers. Clearly there was a connection between physical and chemical properties. 

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