Rotation of polarised light

A meso compound has a specific rotation of polarised light of 0°. A racemate is an equimolar mixture of two enantiomers and its specific rotation is also 0°. 

These simply refer to the direction of rotation of polarised light. Unlike R and S which are determined by configuration, their determination is entirely empirical. In other words, although we know that hypothetical compound 40 has the S configuation, we have no idea whether it is (+) or (-) and will not know until we put it into a polarimeter. 

Similarly, oxidative degradation of (+ )-a-phellandrene gives ( + )-isopropylsuccinic acid. This is identical to that obtained from (—)-glyceraldehyde except for the direction of rotation of polarised light. So the absolute stereochemistry of C-4 in ( + )-a-phellandrene is opposite to that of (-)-glyceraldehyde. 

Rotation of Polarised Light.— By use of the polarimeter, all carbohydrates can be divided into a dextro-rotatory class and a Isevo-rotatoiy class. The sugar D-fructose is the commonest Isevo-rotatoiy saccharide, and for this reason is termed Icevtdose. Each piu% saccharide has a specific rotatory constant, which is determined under equilibrium conditions so as to avoid confusion due to mutarotation. 

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. 

Enantiomer A single form of an optically active compound. Optically active compounds usually (but not exclusively) contain one or more chiral centres. Enantiomers are defined by their ability to rotate the plane of beam of polarised light one way or the other and these are referred to as either D or L , or alternatively + or, depending on whether the polarised light is rotated to the right (Dextro) or the left (Levo). 

A further complication is that for a carbon, which has four different groups attached to it in a tetrahedral arrangement, there are two optical isomers called laevo (l-) and dextro (d-) from their different abilities to rotate the plane of polarised light. They are structural mirror images which cannot be superimposed ... 

The reaction is pseudo-first order and rate is proportional to [Sucrose], The progress of the reaction can be studied by measuring the change in specific rotation of a plane of polarised light by sucrose. Let r0, r, and r are the rotation at initially (when t = 0), at any time t and final rotation, respectively. The initial concentration a is proportional to (r0 - r, ) and concentration at any time t, (a - x) is proportional to (r0 - rt). Thus, the rate constant may be obtained as... 

Optical isomerism of drug molecules is widespread. Many drug molecules only contain one or two chiral centres. A simple example is the naturally occurring neurotransmitter adrenaline. When a compound has no symmetry about a particular carbon atom the carbon atom is said to be a chiral centre. When a compound contains one or more chiral centres it is able to rotate plane-polarised light to the right (+) or the left (-). A chiral centre arises when a carbon atom has four structurally different groups attached to it. 

To relate the (+) (dextrorotatory) and (-) (laevorotatory) forms of a molecule to an absolute (R or S) configuration is complex and requires preparation of a crystal of the compound suitable for analysis by X-ray crystallography. In contrast the direction in which a molecule rotates plane-polarised light is easily determined using a polarimeter. 

It should be noted that in older literature the terms d and I are used to denote (+) and (-) respectively and D and L are used to denote R and 5 respectively. A mixture containing equal amounts of (+) and (-) adrenaline or indeed enantiomers of any drug is known as a racemic mixture and of course will not rotate plane-polarised light. The physical separation of enantiomers in a racemic mixture into their pure (-t) and (-) forms is often technically difficult. 

Chirality is an important aspect of aroma chemicals since enantiomers of the same compound may possess different organoleptic characters. Chirality means the occurrence of one or more asymmetric carbon atoms in an organic molecule. Such molecules exhibit optical activity and therefore have the ability to rotate plane-polarised light by equal amounts but in opposite directions. In other words, two stereoisomers which are mirror images of each other are said to be enantiomers. If two enantiomers exist in equal proportions, then the compound is called racemic. Enantiomers can be laevorotatory (, I, -, S), meaning rotating the plane of the polarised light to the left or dextrorotatory (°, d, -f, R), that is. 

Optical isomers are compounds differing only in their ability to rotate the plane of polarised light. The (+), or dextrorotatory (D), isomer rotates light to the right (clockwise), and the (-),... 

To express the rotations produced by different substances under comparable conditions, the specific rotation is calculated. This represents the rotation which would be produced by a liquid containing 1 gram of active substance per c.c. when a ray of polarised light traverses a layer of it 1 decimetre in length. The specific rotation is denoted by the symbol [a] or, when it refers to yellow sodium light and to the temperature 20°C., by... 

Compounds which rotate the plane of polarised light around its axis, whether they are in the gaseous, liquid or molten state, or in solution, are said to be optically active. This property arises from the lack of certain elements of symmetry in... 

If reactants and products have different optical rotation properties, it is possible to study the transformation by monitoring the optical rotation using a polarimeter. When more than one optically active substance is present in the reaction, their combined optical rotation effect upon the plane of polarised light is observed. The angle of rotation (a) caused by a solution of a single pure compound is given by ... 

Enantiomers have identical physical and chemical properties to one another except the direction in which they rotate plane polarised light (clockwise or anticlockwise). They may be separated by interaction with a second chiral species. This gives two diastereoisomers (if the two chiral centres are the same we can describe the diastereoisomers as optically pure meso AA and the racemic or rac form which itself occurs as two pairs of enantiomers, AA and AA) which do differ in their physical properties e.g. have different NMR spectra, can be separated by achiral chromatography etc). For example, Scheme 3.1 shows the experimental resolution of [Co(en)3]3+ using tartrate. 

Figure 13.22 Operation of a twisted nematic cell. In the absence of an applied electric field, the twisted nematic rotates the plane of polarised light from one side of the cell to the other allowing light to pass through. An applied potential renders it opaque.

Optical isomerism is also a type of configurational isomerism and is so named because of the ability of optical isomers to rotate plane-polarised light clockwise or counterclockwise. The existence of optical isomers has very important consequences for life, because optical isomers generally have significant differences in their biological activity. Apart from their biological activity and their effects on plane-polarised light, optical isomers have identical chemical and physical properties. 

A solution of each enantiomer or optical isomer can rotate plane-polarised light. One antiomer will rotate plane-polarised light clockwise while the other (the mirror image) will rotate it counterclockwise by the same amount. A mixture of the two isomers (a racemate) will not rotate plane-polarised light at all. In all other respects, the two isomers are identical in physical and chemical properties and are therefore indistinguishable. The asymmetric centres in the molecules shown (Following fig. B) have been identified with as asterisk. The structure lacking the asymmetric centre is symmetric or achiral and does not have optical isomers. A structure can also have more than one asymmetric centre. 

The assignment of an asymmetric centre as (R) or (S) has nothing to do with whichever direction the molecule rotates plane-polarised light. Optical rotation can only be determined experimentally. By convention, molecules which rotate plane-polarised light clockwise are written as (+) or d. Molecules which rotate plane-polarised light counterclockwise are written as (-) or 1. The (R) enantiomer of lactic acid is found to rotate plane-polarised light counterclockwise and so this molecule is defined as (R)-(-)-lactic acid. 

A substance is anisotropic, if it has different properties in different directions. If the refraction index n differs for perpendicular and parallel-polarised light (n and Hy) the result is birefringence or double refraction (when transparent). Accordingly, birefringence is evidenced by the ability of a material to rotate the plane of polarised light. It is defined as the difference in refractive indices in the directions parallel and perpendicular to the direction of orientation ... 

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