Enantiomers plane-polarized light

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... 

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. 

Except for their effect on plane-polarized light, two enantiomers of a chiral compound have identical physical properties. For example, the two isomers of lactic acid shown below have the same melting point, 52°C, and density, 1.25 g/mL. 

You will never be expected to look at a compound that you have never seen and then predict in which direction it will rotate plane-polarized light (nnless you know how the enantiomer rotates plane-polarized light, becanse enantiomers have opposite effects). Bnt yon will be expected to assign confignrations (R and S) for stereocenters in componnds that you have never seen. 

Most of the physical properties (e.g., boiling and melting point, density, refractive index, etc.) of two enantiomers are identical. Importantly, however, the two enantiomers interact differently with polarized light. When plane polarized light interacts with a sample of chiral molecules, there is a measurable net rotation of the plane of polarization. Such molecules are said to be optically active. If the chiral compound causes the plane of polarization to rotate in a clockwise (positive) direction as viewed by an observer facing the beam, the compound is said to be dextrorotatory. An anticlockwise (negative) rotation is caused by a levorotatory compound. Dextrorotatory chiral compounds are often given the label d or ( + ) while levorotatory compounds are denoted by l or (—). 

Enantiomers rotate the plane of plane-polarized light in equal amounts but in opposite directions. 

A young Louis Pasteur observed that many salts of tartaric acid formed chiral crystals (which he knew was related to their ability to rotate the plane of polarization of plane-polarized light). He succeeded in solving the mystery of racemic acid when he found that the sodium ammonium salt of racemic acid could be crystallized to produce a crystal conglomerate. After physical separation of the macroscopic enantiomers with a dissecting needle, Pasteur... 

Enantiomers have identical chemical and physical properties in the absence of an external chiral influence. This means that 2 and 3 have the same melting point, solubility, chromatographic retention time, infrared spectroscopy (IR), and nuclear magnetic resonance (NMR) spectra. However, there is one property in which chiral compounds differ from achiral compounds and in which enantiomers differ from each other. This property is the direction in which they rotate plane-polarized light, and this is called optical activity or optical rotation. Optical rotation can be interpreted as the outcome of interaction between an enantiomeric compound and polarized light. Thus, enantiomer 3, which rotates plane-polarized light in a clockwise direction, is described as (+)-lactic acid, while enantiomer 2, which has an equal and opposite rotation under the same conditions, is described as (—)-lactic acid. 

The first example of the deliberate separation of optically active molecules is appropriate as an example of physical separation in the clearest sense of the term. The molecules are referred to as optically active because polarized light interacts differently with right- and left-handed molecules. In the case of simple diastereomers the RR and SS forms are enantiomers while the RS and SR forms are not. The separation of the latter and former was first done under a microscope using crossed polarizers and the crystals which were seen were separated from those that caused little or no rotation of plane-polarized light by hand using tweezers. A truly physical separation of chemical species using a physical property of chemical origin ... 

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

Figure 9.1 Enantiomers of glyceraldehyde. Enantiomers are mirror images of each other and are chemically similar but will cause a beam of plane polarized light to rotate in opposite directions. Glyceraldehyde has only one asymmetric centre ( ) and the designation of D or L is determined by the orientation of the O and OH groups about this carbon atom. For carbohydrates with more than one asymmetric carbon atom, the prefix d or l refers only to the configuration about the highest numbered asymmetric carbon atom, although in such enantiomers the configuration about all the asymmetric centres will also be reversed.

A carbon atom with four different groups attached is chiral. A chiral carbon rotates plane-polarized light, light whose waves are all in the same plane, and has an enantiomer (non-superimposable mirror image). Rotation, which may be either to the right (dextrorotatory) or to the left (levorotatory), leads to one optical isomer being d and the other being 1. Specific rotation (represented... 

Enantiomers can be distinguished by their rotation of plane-polarized light at a specific wavelength, or over a range of wavelengths (optical rotatory dispersion, ORD), as well as by the difference in absorption of right and left circularly polarized light (circular dichroism. Cotton effect, CD). 

Properties of enantiomers Enantiomers share same physical properties, e.g. melting points, boiling points and solubilities. They also have same chemical properties. However, they differ in their activities with plane polarized light, which gives rise to optical isomerism, and also in their pharmacological actions. 

When we have a pair of enantiomers, each rotates the plane-polarized light hy the same amount, hut in the opposite direction. A mixture of enantiomers with the same amount of each is called a racemic mixture. Racemic mixtures are optically inactive (i.e. they cancel each other out) and are denoted hy ( ). 

However, the optical activity does not tell us the actual configuration of an enantiomer. It only gives us the information whether an enantiomer rotates the plane-polarized light clockwise or anti-clockwise. 

The spectrum of A -acetyl-L-phenylalanine also introduces us to another important consideration in H NMR spectra the effect of stereoisomerism on the H NMR spectrum. Each of a pair of enantiomers has the same physical properties (apart from their ability to rotate the plane of plane polarized light) and so will have identical H NMR spectra. The spectra for TV-acetyl-L-phenylalanine and its enantiomer, 7V-acetyl-D-phenylalanine, will therefore be identical in every respect. Diastereoisomers, on the other hand (no pun intended), have different physical properties and so give different H NMR spectra. 

As Louis Pasteur first observed (Box 1-2), enantiomers have nearly identical chemical properties but differ in a characteristic physical property, their interaction with plane-polarized light. In separate solutions, two enantiomers rotate the plane of plane-polarized light in opposite directions, but an equimolar solution of the two enantiomers (a racemic mixture) shows no optical rotation. Compounds without chiral centers do not rotate the plane of plane-polarized light. 

Many substances can rotate the plane of polarization of a ray of plane polarized light. These substances are said to be optically active. The first detailed analysis of this phenomenon was made by Biot, who found not only the rotation of the plane of polarization by various materials (rotatory polarization) but also the variation of the rotation with wavelength (rotatory dispersion). This work was followed up by Pasteur, Biot s student, who separated an optically inactive crystalline material (sodium ammonium tartrate) into two species which were of different crystalline form and were separately optically active. These two species rotated the plane of polarized light equally but in opposite directions and Pasteur recognized that the only difference between them was that the crystal form of one was the mirror image of the other. We know to-day, in molecular terms, that the one necessary and sufficient condition for a substance to exhibit optical activity is that its molecular structure be such that it cannot be superimposed on its image obtained by reflection in a mirror. When this condition is satisfied the molecule exists in two forms, showing equal but opposite optical properties and the two forms are called enantiomers. 

This particular example represents one class of stereoisomers known as enantiomers, which may be defined as two molecules that are mirror images but are nonetheless nonsuperimposable. Such molecules are said to possess opposite configuration. If these isomers are separated (resolved), the separate enantiomers have been found to rotate the plane of plane-polarized light. This phenomenon of optical activity has been known for well over a century. A 50-50 mixture of two enantiomers is optically inactive or racemic, since the rotation of light by one enantiomer is precisely compensated by the rotation of tight in the opposite direction by the other enantiomer. 

The single most important physical property that differentiates enantiomers is their ability to rotate the plane of plane polarized light. This property is called optical activity and is displayed only by chiral molecules. Thus, stereoisomers which are also chiral are known as optical isomers. Chiral molecules that rotate polarized light in a clockwise fashion are termed dextrorotatory (d) while those that rotate the beam counterclockwise are levorotatory (/). Enantiomers have optical rotations of die same magnitude but of different signs (d or /). 

Enantiomers are stereoisomers that are non-superimposable mirror images of each other. EnrirTm mers have identical physical and chemical (except towards optically aQtJvp reagents) properties except for the direction in which plane-polarized light is rotated. Enantiomers account for a compound s optical activity. 

The upper half of(the )nolecule is a non-superimposable mirror image of the lower half, making the top half an enantiomer of the lower half However, since the two halfs are in the same molecule the rotation of plane-polarized light by the upper half is cancelled by the lower half and the compound is optically inactive. 

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