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Introduction - Isomerism |
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What is isomerism? Using building blocks various structures can be built. Similarly using the same atoms, different organic compounds can be formed. Therefore, organic compounds that have the same molecular formula but different structural formulae and different arrangement in space are called isomers or isomerides. The property of organic compounds to have same molecular formula but different structural formulae and spatial arrangement is called Isomerism. |
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Types of Isomerism |
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The relationship may be derived by combining the mass-energy relationship proposed by Max Planck and Einstein. According to Planck, photon of light having energy E is associated with a wave of frequency n
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Structural Isomerism |
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When two or more organic compounds have the same molecular formula but different structural formulae, they exhibit structural isomerism. |
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Stereoisomerism |
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When organic compounds having same molecular and structural formulae have different arrangements in the three dimensional space, they are said to exhibit stereoisomerism. |
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Geometrical Isomerism |
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Geometrical isomerism is a kind of stereoisomerism. The isomerism due to the difference in spatial arrangements of groups about the doubly bonded carbon atoms is known as Geometrical Isomerism. |
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Properties of Geometrical Isomers |
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Melting point and stability of the cis isomers are lower than those of the trans isomer. |
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Nomenclature of Geometrical Isomers |
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Geometrical isomers are usually named using the naming convention cis and trans or syn and anti. In some cases, these names are inadequate. A new system of nomenclature has been introduced namely, the E and Z isomers. E stands of Entgegen (meaning "opposite" in German) and Z stands for Zusammen (meaning "follow together" in German). |
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Conformations or Conformational Isomers (Rotational Isomers) |
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Consider an ethane molecule (H3C-CH3). Infinite number of arrangements are possible if one methyl group is allowed to remain stationery and the other rotated through the C-C axis depending on the angle of rotation. This angle is called the dihedral angle or the angle of torsion and the different arrangements are called rotational isomers or conformations or conformers. |
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Two-dimensional Conventions |
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In these drawings for ethane, the molecule is viewed slightly from above and from the right and the lower left hand carbon is always considered towards the front. |
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Conformations of Cyclohexane |
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In 1890, Sachse said that two puckered (non planar) models of cyclohexane are possible where the valence bonds of all the carbons were at tetrahedral angle to each other. They were found to be free from strain. |
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Optical Isomerism |
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The earliest optically active substances were all inorganic like quartz, rock crystals and so on. Later crystals of potassium chlorate, potassium bromate and sodium periodate were also found to exhibit optical activity. However, when these crystalline inorganic and organic compounds like benzil are fused or dissolved in a solvent, they lose their crystalline structure and their optical activity as well. This shows that these compounds are optically active because of their crystalline nature. |
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Determination of Optical Activity using a Polarimeter |
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Two Nicol prisms (got by combining two prisms of Iceland spar suitably cut) are placed between the source of monochromatic light, S, and the eye. Light passes through the first Nicol prism, P, the polarizer and gets plane polarized (vibrations are restricted to one plane only). |
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Specific Rotation |
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Specific rotation refers to the rotatory power of a given solution. It is defined as the angle of rotation (a-alpha) produced by one decimetre length of solution that has one gram of the substance per cubic centimetre. The measurement of rotation is carried out at temperature T using sodium light. |
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Chirality |
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A compound must be chiral for it to exhibit optical activity. Chiral is derived from the Greek word cheir meaning hand. A chiral molecule is one that is not super imposable on its reflection. |
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Symmetric Elements and Chirality |
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The presence of a single chiral center indicates chirality of a molecule. Another way to recognize chirality is to detect the presence of some symmetry elements. |
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Asymmetric and Chiral Molecules |
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An object or molecule is not symmetrical or is asymmetrical when it doesn't have any element of symmetry. Chiral molecules are asymmetrical in nature. However some symmetrical molecules are also chiral when their symmetry is limited to their rotation axes. Chiral molecules are also called dissymmetric. Compounds that are not chiral are called achiral. |
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R and S Notation of Optical Isomers |
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The scheme of representing optical isomers using the prefix R (derived from Greek word Rectus) and prefix S (derived from Greek Sinister) was proposed by R.S.Cahn, C.K.Ingold and V.Prelog. |
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Sequence Rules |
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When four different atoms are attached to the chiral carbon, priority depends on atomic number. The atom with the highest atomic number gets top priority. If the two atoms are isotopes of the same element, the atom of higher mass number has higher priority. |
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Fischer Projection |
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The intersection of the horizontal and vertical lines represents the chiral center (the chiral carbon). |
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Specification of Configuration of Compounds with more than one Chiral Centre |
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Specification of configuration of compounds with more than on chiral carbon such as tartaric acid can be done by
a) Specifying the configuration about each of the chiral carbons and
b) Numbering the chain of carbon atoms present to easily denote which configuration refers to which carbon. |
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Enantiomers and Diastereomers |
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Optical isomers like that of lactic acid and tartaric acid are mirror images of each other and are called enantiomers. They have identical physical and chemical properties except the direction in which they rotate the plane of polarized light. |
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Mechanism of Racemisation |
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A compound where the chiral carbon atom is attached to a hydrogen atom and an electron-attracting group can undergo racemisation readily. The mechanism is enolisation. |
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Resolution |
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Equal amount of dextro and laevo varieties are got when an optically active compound is synthesized in the lab. The result is a racemic mixture. The two forms called enantiomorphs can be separated from the racemic mixture. This process is called resolution. |
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Importance of Steoreochemistry |
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Stereochemistry, an important aspect of carbon compounds, is a feature that is widely prevalent in nature. The human body is structurally chiral with the heart lying to the left and the liver to the right. Many climbing plants show chirality in the way that they wind around supporting structures. Most of the molecules found in plants and animals are chiral and usually only one form of chiral molecules occurs in a species. |
