Optical Rotation

Only chiral materials, which lack tiny mirror symmetry, have optical activity. Optical rotation can be observed in fluids, unlike other forms of birefringence that change the state of the polarization of a beam. This might include chiral gasses or liquids, like sugars, helical secondary structure molecules like specific proteins, and chiral liquid crystals. 

OPTICAL ISOMERS

Optical isomers, also known as enantiomers, have the same atomic and bond sequence but differ in their 3D structure. The most typically stated example of two enantiomers is human hands, which are nonsuperimposable mirror images of one another (i.e., chiral). Even though our left hand is a mirror image of our right, our left thumb cannot be placed over our right thumb if our palms face the same direction and are positioned over one another. Optical isomers also lack an axis of symmetry, which means no line divides the compound into two halves, with the left half mirroring the right.

There are certain exceptions to the general features of optical isomers (melting points, boiling points, etc.). (uses in biological mechanisms and optical activity). Enantiopure medications are those that have varied effects depending on whether they are a racemic mixture or entirely one enantiomer. D-ethambutol, for example, is used to cure tuberculosis, whereas l-ethambutol causes blindness. The interaction of these enantiomers with plane polarized light is referred to as optical rotation.

ROTATION OF LIGHT

Dextrorotatory [(+), or d-] is the enantiomer that rotates plane-polarized light in the positive direction, or clockwise, whereas levorotatory [(-), or l-] is the enantiomer that rotates the light in the negative direction or counterclockwise, this phenomenon is possible in an optically active substance. A racemic mixture is one in which both the d- and l- isomers are present in equal proportions. Optical rotation is a term used to describe compounds with these features, which are made up of chiral molecules. An enantiomer of a chiral molecule (geometric mirror image) shall be dextrorotary if it is laevorotatory and vice versa. Enantiomers tend to rotate under the effect of plane-polarized light in entirely opposite directions, retaining the same amount of degrees.

A polarized source and polari-meter are used to detect optical rotation. This is a tool used in the sugar business to determine the sugar content of syrup and chemistry to determine concentration or enantiomeric ratio of chiral compounds in solution.

MEASURING OPTICAL ACTIVITY

A polarimeter is used to detect optical rotation, which is affected by various parameters, including sample concentration, temperature, sample tube or cell length, and wavelength of light passing through the sample. The rotation is provided in degrees depending on whether the sample contains d- (positive) or l- (negative) enantiomers.

The specific rotation is defined as an angle measured at a route length of 1 decimeter and a concentration of 1g/ml; this is the standard measurement for rotation for a given chemical compound. A pure substance’s unique rotation is an inherent characteristic. The formula for specific rotation in solution is:

[α]Tλ=α / (c x l)

where,

The specific rotation [α] is measured in degrees cm3 dm-1 g-1.

λ is the wavelength in nanometers, T is the temperature in degrees, l is the path length in decimeters, and c is the concentration in g/ml.

STEREOISOMERISM

Stereoisomerism, also referred to as spatial isomerism, may be a sort of isomerism during which molecules have an equivalent formula and bound atom sequence, but their atoms are oriented uniquely in three dimensions in space, optically active substances are a major part of discussion over here. This is different from structural isomers, which have an identical formula but differ in bond connections or order. Molecules that are stereoisomers of 1 other have an equivalent structural isomer by definition.

• ENANTIOMERS

Enantiomers, also known as optical isomers, are two stereoisomers connected through reflection: they are non-superimposable mirror images of one another. A macroscopic analog of this is human hands. On the other hand, every stereogenic center has the opposite layout, except for the direction in which they twist polarized light and how they interact with various optical isomers of other compounds.

Two compounds that are enantiomers of one another exhibit the same physical properties as one another. As a result, the biological effects of different enantiomers of a chemical may differ significantly.Pure enantiomers exhibit optical rotation as well and can only be differentiated with the aid of a chiral agent.

• DIASTEREOMERS

Diastereomers are stereoisomers of material that do not mirror images of each other. Melting temperatures, boiling points, densities, solubilities, refraction etc and specific rotations are different physical properties of diastereomers. Except for the opposite sign of a particular optical rotation, enantiomers have similar physical characteristics.

Other diastereomers may or may not remain optically active depending on geometrical isomers.

Due to differences in their properties, diastereomers vary from each other and can thus be separated using the common chromatographic mode. This system of separation has been named the indirect mode of separation. The formation of diastereomeric compounds precedes the proper chromatographic process. 

CIS-TRANS AND E-Z ISOMERISM

• CIS-TRANS

The atoms in cis-trans isomers contain numerous spatial placements in 3-D space, it’s a kind of stereoisomerism. In the organic chemistry, cis-isomers have functional-groups on same side of carbon chain, whereas in trans-isomers have active groups on the completely opposing sides of the compound.

Both organic and inorganic compounds have the ability to exhibit this exquisite type of isomerism. The Latin roots of the prefixes “cis” and “trans” can be rendered as “this side of” and “other side of” respectively. There exist also certain cis-trans isomers in some co-ordination complexes and the physical properties of cis-trans isomers of a molecule differ in many circumstances. 

Changes in the dipole moment of the molecule or variances in the spatial arrangement of atoms might cause these differences. The cis isomer of pent-2-ene reaches a boiling point of 37°C, while the trans isomer has a boiling temperature of 36°C. Because bond polarity is low, the change is minor.

The cis isomer’s boiling point is 60.3°C, while the trans isomers are 47.5°C, due to the polar nature of bonds in 1,2-dichloroethylene (the C-Cl dipole moments in trans isomer cancel each other out).

Because of the differences in their characteristics, the cis and trans isomers of the butenedioic acid have extremely distinct reactivities. The cis isomer of maleic acid is maleic acid, while the trans isomer is fumaric acid.

The cis-trans isomers of elaidic and oleic acid are elaidic acid and oleic acid. At room temperature, the former is solid (melting point = 43°C), whereas the latter is liquid (melting point = 13.4°C).

• E-Z

Geometric-isomers of alkene are the ones in which same groups are organized differently and are traditionally referred to as cis or if it is trans. However, there are many instances where the cis-trans approach is challenging to apply. The International Union of Pure and Applied Chemistry (IUPAC) has a more comprehensive system for designating alkene isomers. The R-S approach is built on a set of “priority rules” that can rank any group. The E-Z system used by IUPAC to name alkene isomers is based on the same priority rules. The priority criteria are commonly referred to as Cahn-Ingold-Prelog (CIP) rules, named after chemists who created the system. The E-Z system’s general technique is basically to look at two groups at each end of a double bond. This process is used to check whether the higher-priority group at the one end of the double bond is on the same side (Z, from the German zusammen = together) or on the opposite side (E, from the German language entgegen = opposing) of the double bond.

The two priority groups are zusammen = together because they are on the same sides of a double-bond (“down” in the example). As a result, it’s (Z)-2-butene. In the second case, the priority group is “down” on  double bond’s left end and “up” on double bond’s right end.Two of the priority groups are entgegen = opposing because they are on different sides of double bond. As a result, it’s (E)-2-butene. 

CONCLUSION

Isomerism refers to molecules with the same number of atoms of the same type (and thus the same formula) but different chemical and physical properties. This intricate concept is the stepping stone in understanding the properties of substances and their different reactions to the activities in nature.