Isomer and Isomerism
Isomers are compounds having the same chemical formula but having different arrangements of atoms. There are many types of isomers that are defined based on the way in which the atomic arrangements differ.
Structural isomerism is a form of isomerism in which the atoms are arranged in a different order. For example, the ions SNC- and SCN- have the same atoms but they are connected to one another in a different order. Stereoisomers have the same bonds but in different spatial arrangements. For example, the molecule CHClBrI, in which the carbon (C) atom is the central atom with the other four atoms attached in a tetrahedral arrangement around the carbon, can be prepared in two distinct ways such that the two molecules are non-superimposable.
One of the subcategories within structural isomerism is chain isomerism which is due to different arrangements of atoms in a chain molecule. Chain isomerism is commonly encountered in organic chemicals. For example one molecule may be a straight chain, whereas an isomer of this molecule would be a chain with a branching of molecules at the end, such as n-butane, CH3CH2 CH2 CH3 and 2- methylpropane, CH3CH(CH3) CH3. Positional isomerism is an example of side chain groups being found on different atoms of the backbone or central chain molecule. An example of this would be the difference between the molecule 2-chloropentane (CH3CHClCH2CH 2 CH3) and 3-chloropentane (CH3CH2 CHClCH2 CH3).Functional group isomerism (metamerism) is due to a rearrangement of atoms to produce a different functional group, for example many organic alcohols are isomers of aldehydes. A special case of this type of isomerism is dynamic isomerism or tautomerization. As the name suggests the structure can move from one form to another and back; it is reversible between the two isomers. This is caused by the movement of an atom or group of atoms from one position to another. This is often due to the presence of double bonds in the molecule, which can rearrange. These isomers usually exist in equilibrium. Structural isomers often exhibit significant differences in physical properties such as boiling points and refractive index.
The second major type of isomerism is stereoisomerism. Stereoisomers have the same atoms connected to each other in the same order but in different spatial arrangements. Optical isomerism is due to different arrangements of asymmetric molecules. Optical isomers are mirror images that cannot be superimposed on each other. Such isomers are called enantiomers and molecules that have enantiomers are said to be chiral or to show chirality. The most common examples of chiral molecules are enantiomers. These isomers have the ability to rotate the plane of polarization of polarized light. Isomers that rotate the plane of polarization to the right are dextrorotory isomers, those that rotate to the left are laevorotory isomers. A mixture of both types of isomer is known as a racemic mixture.
Geometrical isomerism is unique to molecules in which there is a double bond. The presence of the double bond restricts the motion of the two carbon atoms that contribute to the double bond. The nature of the double bond is such that the two carbon atoms that form the double bond and the two atoms attached to each of those carbon atoms lie in the same plane. For example, the accompanying figure shows two molecules which have the same chemical formula, CHClCHCl. However, in one molecule, the two H atoms are on the same side of the double bond (as are the two Cl atoms), while in the other molecule, there is one H and one Cl atom on each side of the double bond. These two molecules cannot be easily interconverted because of the high energy required to rotate a double bond. The naming of geometrical isomers reflects the position of the atoms or groups on each of the carbon atoms in the double bond. If both groups are on the same side of the double bond, that is known as the cis arrangement. Where they are on opposite sides of the double bond, the arrangement is the trans arrangement.
Different properties are exhibited by groups of isomers. With functional isomerism the reason is obvious, a different functional group is present. With other types of isomerism the change in properties may be more subtle. Steric hindrance may occur where the new arrangement of atoms makes it harder for other chemicals to get close to the part of the molecule where the reaction would take place.
Isomers are compounds that have the same molecular formula, but different structural formulas which may lead to different physical and chemical properties.
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