Levorotatory light

Occasionally an optically inactive sample of tartaric acid was obtained Pasteur noticed that the sodium ammonium salt of optically inactive tartaric acid was a mixture of two mirror image crystal forms With microscope and tweezers Pasteur carefully sep arated the two He found that one kind of crystal (m aqueous solution) was dextrorota tory whereas the mirror image crystals rotated the plane of polarized light an equal amount but were levorotatory... 

Levorotatory (Section 9.3) An optically active substance that rotates the plane of polarization of plane-polarized light in a left-handed (counterclockwise) direction. 

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 (—). 

A substance that rotates plane-polarized light in the clockwise direction is said to be dextrorotatory, and one that rotates plane-polarized light in a counterclockwise direction is said to be levorotatory (Latin dexter, right and laevus, left). 

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... 

Both types proved to have all the chemical properties of tartaric acid, but in solution one type rotated polarized light to the left (levorotatory), the other to the right (dextrorotatory). Pasteur later described the experi ment and its interpretation ... 

RGURE 1 Pasteur separated crystals of two stereoisomers of tartaric acid and showed that solutions of the separated forms rotated polarized light to the same extent but in opposite directions. These dextrorotatory and levorotatory forms were later shown to be the (R,R) and (S,S) isomers represented here. The RS system of nomenclature is explained in the text. 

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 /). 

We have seen that individual enantiomers have identical physical properties and only can be distinguished in a chiral environment. Plane-polarized light is such a chiral environment, and one enantiomer is dextrorotatory and one is levorotatory. Another way to distinguish enantiomers is to allow them to react (or interact) with other chiral molecules. The interaction of a mixture of enantiomers with a single enantiomer of a chiral molecule produces a mixture of diastereomers as illustrated. 

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. 

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