Difference between Allotropes and Isomers: An Introduction
Today, one can easily determine the image of a diamond and the orientation of chemical compounds even by sitting in a room. This has become possible due to rapid advancement in medical science and technology and intensifying research in chemistry, one can clearly distinguish between isomers and allotropes. Isomers and allotropes have become integral parts of chemistry, the major field of research in science. Isomers have different chemical and physical properties due to different arrangements of atoms and bonds with respect to central (parent) carbon. On the other hand, allotropes show the same chemical but different physical properties due to the presence of individual elements. The difference in the physical properties of allotropes accounts for the different melting points, boiling points and other important parameters. Though, their chemical properties remain the same due to the presence of the same atom throughout the structure. Both allotropes and isomers have individual importance and should be dealt with individually.
What are Allotropes or What is Allotropy
Allotropy is the property exhibited by certain elements that help them to make two or more forms containing the same element, and it is ensured that the physical state of the element remains same, i.e. if the element is in solid it will remain in solid even when they bear different structures. Compounds exhibiting this property are termed allotropes. For e.g., whether carbon exists as graphite, diamond, or buckminsterfullerene, all of them will exist in a solid state only.
Major Forms of Allotropes
We learned about diamond and graphite as allotropes of carbon in class 10th. But many different elements exhibit allotropism. These elements are mostly found under groups 13 - 16 in the modern periodic table. The Highest proportion of allotropes is shown by metals rather than non-metals.
Phosphorus - Phosphorus exists in six allotropic forms. Some allotropes are as follows:
White Phosphorus - Also known as P4.
Black Phosphorus - Formed by heating white Phosphorus.
Oxygen - Different molecular forms of oxygen like O3, O2, and O4. Here, all of them have the same element, that is oxygen but exist in different forms.
Sulfur - Sulfur exists as cyclo hexasulfur O6 heptasulfur O7, octasulfur O8 or Pentasulfur O5. Thus, show allotropes.
Isomers
Isomers are chemical compounds that are similar in molecular formula but different in chemical structures. They exhibit different spatial arrangements as per the resonance stability and steric hindrance. Steric hindrance is the hindrance or difficulty created by the larger molecules or compounds with bulky side chains or side groups in the chemical reactions. It decreases the reactivity process or sometimes even reverses the reaction in unwanted directions, leading to unwanted products.
The property shown by isomers is known as isomerism.
Types of Isomers
Isomers are broadly classified into structural and stereoisomers.
Structural Isomerism is exhibited by compounds that are different in the position of bonds and atoms.
Various types of structural Isomers are as follows:
Chain Isomerism - Here, the position of the double or triple bond is changed. For e.g, But-1-ene and 2-Methylprop-1-ene.
Functional Isomerism - Isomers differ from each other on the basis of the position of functional groups. For e.g, ethanol and methoxymethane.
Position Isomerism - Isomers differ from each other based on the position of double bonds. For e.g, But-1-ene and but-2-ene.
Metamerism - Here, methyl groups of compounds are at different positions with respect to alkyl groups. For e.g, Ethoxyethane1-Methoxypropane.
Tautomerism - This isomerism is maintained by fluctuating position of the H atom and double bonds. For e.g., in HCN (1) and HNC (2), here hydrogen cyanide (1) has triple bond between C and N but hydrogen isocyanide (2) has a double bond between N and C atom of the compound.
Stereoisomers
These isomers have different arrangements based on spatial arrangements giving rise to conformational, geometrical, and optical isomers. These are imaginary structures made according to the convenience of studying the compounds in an elaborated way.
What is the Difference between Allotropes and Isomers
Significance of Isomers and Allotropes
Isomers - can contribute to environmental damage as slight changes in structures give rise to another compound. It's different chemical properties and causes harm to the environment. Different positions of atoms lead to different compounds. For e.g. in N-pentane and neopentane, both isomers have a wide range of differences in boiling points. Neopentane having low boiling points and low vapour pressure are flammable. If they enter the environment, they can cause massive explosions and damage.
Allotropes - Compounds formed from the same elements have different importance in the market. Single elements are exploited according to needs to form the desired products. Examples of allotropes are the allotropes of carbon, diamond is used for cutting rocks and making jewellery, whereas graphite formed from the same element carbon is used as lubricant or lead for pencils.
Key Features of Isomers and Allotropes
Isomers have the same number of different atoms. Whereas allotropes have the same number of individual elements.
Both isomers and allotropes have different melting and boiling points.
Allotropes have the same chemical properties, but isomers do not show the same chemical properties always.
FAQs on Difference Between Allotropes and Isomers for JEE Main 2024
1. How can one determine whether pure water is an allotrope or not?
Water is a molecule formed by the combination of H and C atoms in a fixed proportion. Here, two elements are involved in the molecule formation, which does not fulfil the criteria of allotropes; that is, only one element must be present in an allotrope. Hence, water is a molecule that cannot be considered an allotrope.
2. Why do allotropes have the same chemical properties but different physical properties?
Allotropes have similar electronic configurations because of an equal number of protons and neutrons due to the presence of the same elements which is why they have similar chemical properties. But they exist in different forms though their physical state remains the same because of stability reasons. Hence, exhibits different structural properties and reactivity, which is why they show different physical properties. For e.g diamond has a tetrahedral structure, and graphite has parallel structures which lead to different physical properties but the same chemical properties.
3. What are the Essential criteria for terming two compounds as isomers, or one can say that it is an isomer?
Isomers are compounds with the same molecular formula and molecular weight but different structures. Special focus is given to a number of bonds and functional groups present in a compound. If one of the compounds contains one triple bond irrespective of the position, then another compound of the isomer should also have one triple bond present in a molecule. The same rule applies to the Functional groups, the Functional groups present in a molecule should contain the same number of atoms as another compound. Hence, bonds and atoms present in a compound should be the same to characterize it as an isomer.
