Isomerism

Isomers having different connection points are called constitutional isomers, these have same parts, but are attached to each other in a different way, for example if we consider the simplest hydrocarbons such as methane (CH4), ethane (CH3CH3), and propane (CH3CH2CH3), then we can say that they have no constitutional isomers, as the carbons and hydrogens of these molecules consistent with the tetravalency of carbon and the uni-valency of hydrogen can’t be connected.

But if we consider butane, then C4H10, which are called butane and isobutane, are constitutional isomers. These are different molecules with properties. Butane has its four carbon atoms bonded in a structure of continuous chain and Isobutane has a branched structure.

Stereoisomers 

Stereoisomers are isomers that have the similar composition but these differ in the orientation of groups in space. There are two types of stereoisomers and that are enantiomers and diastereomers. Enantiomers are mirror images and, Diastereomers are defined as non-mirror image non-identical stereoisomers. However, timescale and energy are important factors.

Methane (CH4) is a molecule which is a perfect tetrahedron, and so it is commonly said that no isomerism is possible with methane. We know that, the carbon-hydrogen bonds of methane are constantly vibrating and bending, so that on very short timescales an apparent isomerism may exist. But these structures are not at energy minima point, and thus they do not qualify as isomers. As complexity increases, the isomerism which is induced by rotations about bonds becomes a bigger affecting factor, now in ethane (CH3CH3), for example, both carbons are known to be tetrahedral, there are two limiting structures, first is staggered ethane, in which the carbon-hydrogen bonds are present as far as possible, and second is eclipsed ethane, in which the bonds are situated as close as possible. These two structures are not the same, to see the difference we need “Newman projection” it was named after Melvin Newman. In Newman projection, it is a rule to sight down the carbon-carbon bond and focus on the positions of the six hydrogens. In a Newman projection, the front carbon is located at the intersection of the bonds to the three attached hydrogen atoms, and the back carbon is an exploded circle, with the attached bonds emanating from the circumference of the circle, this is how it is visualised.

Chirality in natural and synthetic materials

The function of biologically active molecules depends on the fit, on an exquisite lock-and-key connection between molecules that allows some biochemical activity to turn on or off. In the evolutionary part, the chirality came to be a critical part of the lock-and-key fit. The principle is simple, A left-hand glove does not fit a right hand, and, in the same way, one member of an enantiomeric pair of molecules might fit another molecule whereas the other member would not. 

Example

An example of the penalties of specificity is thalidomide, a compound which was originally marketed as a sedative in Europe in 1956. Thalidomide contains a stereogenic carbon, and therefore the compound can exist in both R and S forms. Most synthesizing procedures generate equal mixtures of enantiomers, means equal amounts of the R and S forms which is often referred as a racemic mixture; special care must be taken to make a pure enantiomer, and the company involved in the original promotion of thalidomide saw no reason to bear the cost of this process. The result was the marketing of a racemic mixture of the R and S forms and the (S)-Thalidomide turned out to be a powerful teratogen and it caused all kinds of external and internal abnormalities in foetuses if it was given to pregnant women in the first trimester. The R enantiomer is far more benign, although even it is dangerous, as the R form racemates under physiological conditions and thus produces some of the dangerous S enantiomer.

Optical isomers 

Optical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity), and different spatial arrangements of the atoms, but these have non-super imposable mirror images. There are two types of optical isomers – D & L type isomers. The D-type isomer rotates plane-polarized light clockwise and the L-type isomer rotates counter-clockwise. A racemic mixture exists when there is an equal proportion (50%) of each isomer type. Optical isomers can occur when there is an asymmetric carbon atom present. An asymmetric carbon atom is one which is bonded to four different groups as it forms a chiral centre of the molecule. There are two ways optical isomers can be determined: first is using mirror images, second is using planes of symmetry. Optical isomers do not exhibit symmetry and do not have identical mirror images. It is a type of isomerism in which molecules have the same molecular and structural formulae, but are non-super imposable mirror images of each other. An example is butan-2-ol. It has four different groups attached to its second carbon atom.

Structural isomerism

Structural isomerism, or constitutional isomerism, is a type of isomerism where isomers (each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties) have same molecular formula but have different arrangements of atoms within the molecule, for examples for example: Butane and isobutane have the same number of carbon (C) atoms and hydrogen (H) atoms, so their molecular formulas are the same another such example could be  n-pentane, iso-pentane and neopentane are structural isomers.

Two main forms of isomerism

• Structural isomerism
• Stereo isomerism

Isomerism is classified into two main types that are structural or constitutional isomerism and stereoisomerism, in structural isomerism the bonds between the atoms differ; and stereoisomerism or spatial isomerism, the bonds are the same but the relative positions of the atoms differ.

Some common examples of Structural Isomers

• Butane and Isobutane (C4H10).
• Pentan-1-ol, pentan-2-ol, and pentan-3-ol are structural isomers that exhibit position isomerism.
• Cyclo-hexane and hex-1-ene are examples of functional group structural isomers.

Displacement reaction

A chemical reaction in which a more reactive element is displaced by the less reactive element from its compound is called a displacement reaction; a displacement reaction can be written as AB + C → AC + B. For an example, when the iron is added to a copper sulphate solution, it displaces the copper metal.

Conclusion

Isomers are compounds with identical chemical formulae, but different structures. They are very important in air pollution chemistry, because even slightly different structures can bring some dramatic differences in their chemical and physical properties. Optical isomerism is extremely important in biological systems. The chemical properties of optical isomers are identical other than when reacting with other optical isomers.