Enantiomers polarimeter

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... 

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. 

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

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. 

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. 

When one of the enantiomers of butan-2-ol is placed in a polarimeter, the observed rotation is 4.05° counterclockwise. The solution was made by diluting 6.00 g of butan-2-ol to a total of 40.0 mL, and the solution was placed into a 200-mm polarimeter tube for the measurement. Determine the specific rotation for this enantiomer of butan-2-ol. 

If you had the two enantiomers of carvone in unmarked bottles, could you use just your nose and a polarimeter to determine ... 

The polarimeter is commonly used in organic and analytical chemistry as an aid in identification of optically active compounds (especially natural products) and in estimation of their purity and freedom from contamination by their optical enantiomers. The polarimeter has occasional application to chemical kinetics as a means of foUowing the course of a chemical reaction in which opticaUy active species are involved. Since the rotation a is a linear function of concentration, the polarimeter can be used in studying the acid-catalyzed hydrolysis of an optically active ester, acetal, glycocide, etc. 

Is the reverse true Whenever we deal with chiral molecules— with compounds that exist as enantiomers—must we always observe optical activity No. We have just seen that a 50 50 mixture of enantiomers is optically inactive. Clearly, if we are to observe optical activity, the material we are dealing with must contain an excess of one enantiomer enough of an excess that the net optical rotation can be detected by the particular polarimeter at hand. 

It has been calculated that this compound should have the tiny specific rotation of0.00001 —far below the limits of detection by any existing polarimeter. In 1965, enantiomerically pure samples of both enantiomers of I were prepared (see Problem 19, p. 1026), and each was found to be optically inactive. 

Each enantiomer should, of course, be optically active. Now, if we were to put the jec-butyl chloride actually prepared by the chlorination of -bulane into a polarimeter, would it rotate the plane of polarized light The answer is no, because prepared as described it would consist of the racemic modification. The next question is why is the racemic modificcUion formed ... 

Glyceraldehyde, the simplest aldose, has only one chirality center and thus ha.s two enantiomeric (mirror-image) forms. Only the dextrorotatory enantiomer occurs naturally, however. That is, a sample of naturally occurring glyceraldehyde placed in a polarimeter rotates plane-polarized light in a clockwise direction, denoted (+). 

The simplest class of optically active molecules has one carbon bonded to four different substituents, and of course it is necessary for one enantiomer to be in excess over the other. Measurements of optical activity are carried out with a polarimeter, which is shown schematically in Figure 2.3. The direction and extent of the rotation of the plane of polarized light is the basis of specific rotation, which is dealt with in this section. 

The terms in the numerator arise because what the polarimeter registers is a measure of the difference in population of the enantiomers in a sample. An enantiomeric excess of 50% signifies that the sample contains 75% of the enantiomer with, say, R configuration and 25% of S, that is 50% of a 1 1 mixture and 50% of the R enantiomer. 

Breakthrough curves (lines in Fig. 6.20) are calculated from the detector signals (symbols). The rotation angle detected by the polarimeter is zero if both enantiomers are present (t > 800 s) while the UV signal is additive and has the highest value there. 

Figure 17.18 Internal Concentration Profiles of the Troger s base enantiomers measured with (a) a polarimeter and (b) a UV detector. Concentrations of each enantiomer. Solid line experimental data at port 6. Dotted line experimental data at port 5, ) Calculated profiles using the 3-layer isotherm model. K. Mihlbachler, A. Seidel-Morgenstem, G. Guiochon, AIChE J., 50 (2004) 611 (Figs. 8 and 9). Reproduced by permission of the American Institute of Chemical Engineers. 1997 AIChE. All rights reserved.

Enantiomers rotate plane-polarised light in the opposite directions. The (-l-)-enantiomer (or dextrorotatory enantiomer) rotates the light to the right, while the (—)-enantiomer (or laevorotatory enantiomer) rotates the light to the left. The amount of rotation is called the specific rotation, [a]D, and this is measured using a polarimeter. (By convention, the units of [a]D are often not quoted.)... 

A molecule that is asymmetric or dissymmetric (and therefore not superimposable on its mirror image) is called a chiral compound this means that all enantiomers are chiral. Such a compound will display optical activity as an individual enantiomer, which is the ability to rotate the plane of plane-polarized light (measured using a polarimeter), which is one way that we can detect the presence of an enantiomer and define its optical purity. Whereas diastereomers usually differ appreciably in their chemical and physical properties, enantiomers differ only in their ability to rotate polarized light and related optical properties. Normally, when a diastereomer that has an enantiomer is synthesized, a 50 50 mixture of the two enantiomeric forms of the compound is produced, and thus no optical activity is observed. However, if the compound is separated into its two enantiomers (or resolved), each enantiomer will show optical activity in a polarimeter the responses of the two enantiomers will differ only in the sign of rotation of plane-polarized light. 

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