Optical Isomerism– Optical Activity, Enantiomerism, Diastereomerism and Meso Compounds : Pharmaceutical Guidelines -->

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Optical Isomerism– Optical Activity, Enantiomerism, Diastereomerism and Meso Compounds

Stereoisometry, in its simplest form, is optical isomerism. Isomers and stereoisomers are two different kinds of molecules.

Optical Isomerism

Stereoisometry, in its simplest form, is optical isomerism. Isomers and stereoisomers are two different kinds of molecules. Before we discover what, optical isomerism is, let us quickly review what they are. The same molecule can have more than one isomer, but each isomer has a distinct bonding pattern among atoms. Despite the presence of different atomic bonding arrangements, however, the stereoisomer displays the same molecular formula. They differ in their arrangement of atoms (in three dimensions). Because the whole molecule rotates in its entirety, the different arrangements are done away with by a molecule revolving around unique bonds.

A case of optical isomerism occurs when two isomers possess identical molecular weights, chemical compositions, and physical properties. The rotation of polarized light is affected differently by these two processes. Optically equivalent substances do not superimpose each other, and therefore exhibit optical isomerism. In many ways, they resemble one another. The asymmetric carbon atom may also be found in other substances. In optical isomerism, the polarized light plane is rotated by stereoisomers. Enantiomers exist in both the (+) and (-) forms when the polarized light passes through enantiomer solutions in clockwise and anti-clockwise directions.
If the polarized plane of light is rotated clockwise and anti-clockwise, alanine (an amino acid) looks like (+) alanine and (-) alanine.

The plane-polarized light will rotate in the same direction in both enantiomeric forms, but in opposite directions. A racemic mixture occurs when both enantiomers appear in equal amounts. The mixture is composed of 50% (+) and 50% (-) components. The rotation of the polarized light plane is zero with racemic mixture because it rotates equally in opposite directions. Therefore, racemic mixture has no optical effect at all.

Optical Activity

The French physicist Francious Arega surmised in 1811 that when polarized plane light passes through certain materials, most notably through some crystals such as quartz, the emerging plane is not always the same as the incident plane. In these crystals, light is polarized in a different plane. Such a phenomenon is called an optical rotation or activity.
  • Let's look at how optical activity works. Consider a standard light source that provides unpolarized light rectilinearly. Light is further polarized by passing it through a polarizer (a Nicol prism and a polarizer).
  • Nicol prisms are capable of double refraction, so that when light enters, the ordinary and extraordinary rays are split. Planar polarization is a feature of both E-rays and O-rays. A polarizer is used to focus the plane-polarized light.
  • In the ordinary ray, the vibrations are parallel to the direction of the plane of paper across which we are looking, but in the extraordinary ray, the vibrations are parallel to the plain paper.
  • In other words, after the ordinary gets completely reflected inside the Nicol prism, the extraordinary ray enters the prism and so it vibrates in one direction.
  • An analyzer cannot detect any emerging light after passing plane-polarized light through it. When polarizer and analyzer are placed together, an optically active substance can be used in the same experimental setup to reveal emerging light. The light is rotated due to plane-polarization.
A typical light wave oscillates randomly because ordinary light is unpolarized. Polarizing filters or a Nicol prism polarizer are used to convert the unpolarized light into polarized light in the polarimeter experiment. In polarimeters, samples (optically active) are kept in tubes passing polarized light through them. Nicol prism analyzers reflect the polarized light after rotating it.
Substances that are optically active can be categorized into two categories:

Dextrorotatory substances - Also known as right substances, dextrorotatory substances are among the substances that contribute to rotation. The direction of polarization of light can be rotated by right-handed or dextrorotatory substances, which are optically active. This group of substances makes plane-polarized light rotate clockwise or in the right direction.

Laevororatory substances - The term "left-handed" describes substances that rotate the polarization plane of the light in a levorotatory way. Left-handed materials are those that rotate the plane of polarized light in a left-handed direction. Dextrorotatory substances are those which rotate plane-polarized light in an anticlockwise or left-hand direction.

Enantiomers

The enantiomers have all the same other chemical properties. If dissolved in solution a pair of enantiomers rotates polarized light in either a positive or a negative way, thus they are referred to as optical isomers; hence, the dextro and laevo rotatory directions. The same enantiomers when present in equal proportions are referred to as racemic mixtures, which do not rotate polarized light because each enantiomer's optical activity cancels the others.

Properties of Enantiomers

  • It is generally accepted that enantiomers have identical properties, including melting point, boiling point, and infrared absorption spectra.
  • While both enantiomers will have the same melting point, it's important to realize that a mixture of the two enantiomers may have a different melting point.
  • Due to the difference in the stereochemistry of opposite enantiomers between R and S enantiomers, the intermolecular interactions between these enantiomers may be different from those between like enantiomers between two molecules of the same R or S stereochemistry.
  • Chiroptical techniques, of which optical rotation is the most common, are the only physical techniques capable of distinguishing enantiomers of compounds.
  • Molecular chiroptical properties are determined by more than bond lengths and angles; they are affected by the sign and magnitude of torsional angles, with the difference between enantiomers determining the sign.

Diastereomers

Molecularly identical compounds are called diastereomers, because they share the same formula and sequence of bonded elements but are not superimposable or mirror images.

Stereoisomers and enantiomers are both considered isomerism, and they always involve the comparison of at least two species. Stereoisomers are the same, but distinct. There are differences between diastereomers in terms of distances of certain characteristic atom pairs, that is, scalar terms.

Characteristics of Diastereomers

  • Among the physical differences between diastereomers are their melting points, boiling points, densities, solubilities, indexes of refraction, dielectric constants and specific rotations. Their rotational specificities are opposite, but they have virtually identical physical properties.
  • The optical activity of diastereomers other than geometric isomers may or may not be determined.
  • Their chemical properties converge but do not match. Provided that the reagent is not rapidly active, the rates of reactions between the two diastereomers.
  • Through methods such as fractional crystallization, fractional distillation, chromatography, etc., diastereomers are separated from one another due to differences in their physical properties. These techniques are not able to separate enantiomers.

Meso-compounds

Structures and objects around us are commonly symmetrical, such as houses, buildings, and even people's faces. It is symmetrical, which means that a straight line can be drawn dividing the smiley face in half. In that case, we have an internal mirror plane or plane of symmetry, which gives our smiley face its left and right side mirror images.

The left and right sides of some chemical compounds are symmetrical, just like this smiley face. Compounds such as these are called mesocompounds. The left and right sides of a meso compound are mirror images of one another, since they have an internal mirror plane.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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