The polarimeter

When the axes of the analyzer and polarizer prisms are parallel (0°) and no optically active substance is present, the maximum amount of light is transmitteci. If the axes of the analyzer and polarizer are at right angles to 

This tilts the plane of polarization. The analyzer prism must be rotated to the left to maximize the transmission of Hght. If rotation is counterclockwise, the angle of rotation is defined as (—) and the enantiomer that caused the effect is called levorota-tory (Z). Conversely, clockwise rotation is defined as (-F), and the enantiomer is dextrorotatory (d). Tilting the plane of polarization is called optical activity. Note that if a solution of equal amounts of a d and an / enantiomeric pair is placed in the beam of the polarimeter, no rotation is observed. Such a solution is racemic it is an equimolar mixture of enantiomers. 

The magnitude of optical rotation depends on several factors (1) the nature of the substance, (2) the path length through which the light passes, (3) the wavelength of light used as a source, (4) the temperature, (5) the concentration of the solution used to make the measurement of optical activity, and (6) the solvent used in making the measurement. 

The results obtained from the measurement of the observed angle of rotation, ttobs/ are generally expressed in terms of specific rotation [a].The sign and magnitude of [a] are dependent on the specific molecule and are determined by complex features of molecular structure and conformation they cannot be easily explained or predicted. The specific rotation is a physical constant characteristic of a substance. The relationship of [a] to aobs is as follows  

These units are traditional, though most are esoteric by contemporary standards. The specific rotation for a given compound depends on both the concentration and the solvent, and thus both the solvent and concentration used must be specified. For example, [a]n (c = 0.4, CHCI3) = 12.3° implies that the measurement was recorded in a CHCI3 solution of 0.4 g/mL at 25 °C using the sodium D line (589 nm) as the light source. 

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. 

Furthermore, this excess of one enantiomer must persist long enough for the optical activity to be measured. If the enantiomers are rapidly interconverted, then before we could measure the optical activity due to one enantiomer, it would be converted into an equilibrium mixture, which—since enantiomers are of exactly the same stability—must be a 50 50 mixture and optically inactive. 

Even if all these conditions are met, the magnitude— and hence the detectability— of the optical rotation depends on the structure of the particular molecule concerned. In compound I, for example, the four groups attached to the chiral center differ only in chain length. 

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. 

At our present level of study, the matter of speed of interconversion will give us no particular trouble. Nearly all the chiral molecules we encounter in this book lie at either of two extremes, which we shall easily recognizef (a) molecules—like those described in this chapter—which owe their chirality to chiral centers here interconversion o enantiomers configurational enantiomers) is so slow—because bonds have to be broken—that we need not concern ourselves at all about inter-conversion (b) molecules whose enantiomeric forms conformational tn2LTi oTtiQTi) are interconvertible dimply by rotations about single bonds here—for the compounds we shall enrounter—interconversion is so fast that ordinarily we need not concern ourselves at all about the existence of the enantiomers. 

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When the solution is just cold the crystals, previously le-moved, are sown evenly over the bottom of the dish at distances of I—2 cms. apart and left for two days. The crystals will have now grown to a size which will enable the facets to be readily recognised. Each crystal is dried and carefully examined with a pocket lens in order to determine the position of the hemi-hedral facets, and placed in separate heaps. These facets lie to the right or left hand of the central prism face, as shown in Fig. 74. The crystals should be weighed, dissolved, and the solution diluted and examined in the polarimeter. The specific rotation may then be calculated. See Appe7idix., p. 264. 

The polarimeter is an instrument with which the essential oil chemist cannot possibly dispense. The hypothesis, first seriously enunciated by Le Bel and van t Hoff, that substances which contained an asymmetric carbon atom i.e. a carbon atom directly united to four different atoms or radicles) were capable of rotating the plane of polarisation of a beam of polarised light, has now become a fundamental theory of organic chemistry-. The majority of essential oils contain one or more components containing such a carbon atom, and so possess the power of effecting this rotation. In general, the extent to which a given oil can produce this effect is fairly constant, so that it can be used, within limits, as a criterion of the purity or otherwise of the oil. 

The analytical phase generally involves the use of very dilute solutions and a relatively high ratio of oxidant to substrate. Solutions of a concentration of 0.01 M to 0.001 M (in periodate ion) should be employed in an excess of two to three hundred percent (of oxidant) over the expected consumption, in order to elicit a valid value for the selective oxidation. This value can best be determined by timed measurements of the oxidant consumption, followed by the construction of a rate curve as previously described. If extensive overoxidation occurs, measures should be taken to minimize it, in order that the break in the curve may be recognized, and, thence, the true consumption of oxidant. After the reaction has, as far as possible, been brought under control, the analytical determination of certain simple reaction-products (such as total acid, formaldehyde, carbon dioxide, and ammonia) often aids in revealing what the reacting structures actually were. When possible, these values should be determined at timed intervals and be plotted as a rate curve. A very useful tool in this type of investigation, particularly when applied to carbohydrates, has been the polarimeter. With such preliminary information at hand, a structure can often be proposed, or the best conditions for a synthetic operation can be outlined. 

The polarimeter reading, a, varies linearly with concentration. 

The polarimeter essentially consists of two Nicol prisms, one the polarizer and the other the analyser with a sample tube kept in between them which contains the substance (a liquid or a solution) to be examined. 

Analytical Methods. The samples were analyzed by standard AACC (21) procedures for moisture (air-oven method), protein (Method 46-13), crude fat (Method 30-25), crude fiber (Method 32-10), insoluble dietary fiber (Method 32-20) and ash (600°C, 3 hr). Starch content was determined by the polarimeter method (Method 76-20) and total sugars by Method 80-60. Color characteristics of the dried products were evaluated with the Hunter Color Difference Meter. 

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 sample tube of the polarimeter is rinsed with the drug solution (2.5% w/v) and filled up with the same solution. The end glass-windows are closed properly. The angle of rotation of ibuprofen is now measured at 19.5° to 20.5°, using the D-line of polarized sodium light. Take at least five measurements and determine the mean value. 

The sensitivity of the polarimeter is low. Therefore, large sample sizes of high chemical purity are required for polarimetric measurements. In addition, the chiral sample must be isolated and purified without accidental enantiomeric enrichment due to fractionation. 

Routine polarimetric determinations are simple enough. First the polarimeter is balanced to zero degrees with the solvent. Then the solution is placed into the instrument, the instrument is rebalanced, and the angle a read off the scale. Nevertheless, when many measurements are taken, this becomes somewhat tedious. For the assessment of the half-shade field, the operator s eyes must be dark-adapted. Extended work in a darkened room peering through the eyepiece at an almost black field is tiring, The precision of visual polarimetric measurements will tend to increase rapidly at first, as die observer s eyes become adapted, but then it will decrease gradually... 

The polarimeter reading, a, varies linearly with concentration. 1-f 115.6-a... 

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. 

The polarimeter consists of two Nicol prisms N and Nx (Fig. 43) set at a distance from one another and on a common axis. Nx is the polariser and N the analyser. The polarimeter tube T, containing a definite... 

Light vibrating in only one plane passes through the polarimeter tube to the analyser N, which can be rotated about the main axis. The eyepiece at E consists of a system of lenses for focussing. 

The inverted liquid, read in the polarimeter according to 1, b, gave ... 

Electric lamps are also made which are fixed firmly to the polarimeter itself. 

The flask below the extractor is also charged with 90% alcohol, the total amount of the latter used being about 75 c.c. the apparatus is fitted together and the flask heated on a water-bath so that the alcohol boils. The extraction should be complete in about 2 hours. The residue may be extracted with fresh alcohol and the latter tested in the polarimeter, or one or two drops of the alcohol from the extractor may be mixed in a test-tube with 2 c.c. of distilled water and 5 drops of fresh 20% alcoholic a-naph-thol and 10 c.c. of concentrated sulphuric acid (free from any trace of nitric acid) added so that the two liquids do not mix in presence of sugar a violet ring forms at the surface between the acid and the alcoholic solution. 

The concentration c is expressed in grams per 100 mL and the length / of the polarimeter tube in decimeters. Since the problem specifies the concentration as 0.3 g/15 mL and the path length as... 

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. 

The presence of an asymmetric center in the sample can be derived from its behavior in solution to plane-polarized light. This can be accomplished through use of an automatic polarimeter (Perkin-Elmer Model 241, sodium lamp). It is strongly suggested that such equipment, rather than visual examination, be used to measure the optical activity of the phospholipids, which are notoriously low rotators. Phospholipids such as. vn-3-phosphatidyl-choline have a specific rotation, [a]g6, value of +6° to +7°. Comparison with the [ct]g6 value of glucose, +47.9, immediately defines the problem. Prior to the availability of an automatic polarimeter, the visual approach was really an act of faith. As expected, concentrated solutions were required and if they had a slight tan color, then differentiation between the two half-fields in the polarimeter was tenuous at best. So, if optical activity values are desired, it behooves one to use an automatic polarimeter. 

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