Rotation plane-polarized light

Addition of hydroxide occurs as a rapid follow-up reaction. Even if the alkyl halide was chiral before the carbocation formed, racemization occurs about the central carbon atom because the hydroxide can bond to the planar central carbon from either side (see Figure 8.17(b)). Statistically, equal numbers of each racemate are formed, so the angle through which the plane polarized light rotated during reaction will, therefore, decrease toward 0°, when reaction is complete. 

Plane-polarized light Rotated plane-polarized light... 

Why is Plane Polarized Light Rotated by a Chiral Medium ... 

A FIGURE 20.4 Rotation of Plane-Polarized Light Plane-polarized light rotates as it passes through a sample containing only one of two optical isomers. 

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)... 

A chiral molecule is one which exists in two forms, known as enantiomers. Each of the enantiomers is optically active, which means that they can rotate the plane of plane-polarized light. The enantiomer that rotates the plane to the right (clockwise) has been called the d (or dextro) form and the one that rotates it to the left (anticlockwise) the I (or laevo) form. Nowadays, it is more usual to refer to the d and I forms as the ( + ) and (—) forms, respectively. 

Very often, a sample of a chiral molecule exists as an equimolar mixture of (+) and (—) enantiomers. Such a mixture will not rotate the plane of plane-polarized light and is called a... 

The all trans" stmcture in Figure 4.9(a) is not chiral as it has an inversion centre i. This is called a meso stmcture. Although each CFIFCFI3 group rotates the plane of plane-polarized light in one direction, the other group rotates it an equal amount in the opposite direction. The result is that there is no rotation of the plane of polarization and, as the presence of an inversion centre tells us, the molecule is achiral. 

The analyzer is removed and the color of the sample is observed in plane-polarized light. If the sample is colored, the stage is rotated. Colored, anisotropic materials may show pleochroism—a change in color or hue when the orientation with respect to the vibration direction of the polarizer is changed. Any pleochroism should be noted and recorded. 

Normal measurements of optical activity are concerned with the ability of the optically active substance to rotate the plane of polarization of plane polarized light, its specific optical rotatory power ( ) being given by... 

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.

Today, we would describe Pasteur s work by saying that he had discovered enantiomers. Enantiomers, also called optical isomers, have identical physical properties, such as melting point and boiling point, but differ in the direction in which their solutions rotate plane-polarized light. 

Some physical properties of the three stereoisomers are listed in Table 9.3. The (+)- and (-j-tartaric acids have identical melting points, solubilities, and densities but differ in the sign of their rotation of plane-polarized light. The meso isomer, by contrast, is diastereomeric with the (+) and (-) forms. As such, it has no mirror-image relationship to (+)- and (-)-tartaric acids, is a different compound altogether, and has different physical properties. 

Problem 31.5 Polymers such as polypropylene contain a large number of chirality centers. Would you therefore expect samples of isotactic, syndiotactic, or atactic polypropylene to rotate plane-polarized light Explain. 

Dextrorotatory (Section 9.3) A word used to describe an optically active substance that rotates the plane of polarization of plane-polarized light in a right-handed (clockwise) direction. 

Plane-polarized light (Section 9.3) Ordinary light that has its electromagnetic waves oscillating in a single plane rather than in random planes. The plane of polarization is rotated when the light is passed through a solution of a chiral substance. 

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. 

---