Difference Between Constitutional Isomers and Stereoisomers

Constitutional Isomers vs Stereoisomers

In general, isomer is a term used in Chemistry, specifically in Organic Chemistry, to refer to molecules with the same molecular formula but have different chemical structures. Due to the variance in the chemical structures, these molecules also exhibit different chemical and physical properties from each other in general, yet having the same molecular formula.

What are Constitutional Isomers?

Constitutional isomers are also known as structural isomers because these molecules having the same molecular formula only differ from each other in the way the individual atoms are connected.  The name structural isomers itself clearly suggests this idea. There are three sub-divisions under constitutional isomers; they are skeletal, positional and functional group isomers.

Skeletal isomers are isomers where the main chain in the compound gets branched out in different ways through different forms of connectivity. For example, if a compound has six Carbon atoms, let’s assume it’s composed of only Carbon and Hydrogen atoms for convenience; if these elements are placed in a straight chain, the compound can be named as the alkane ‘Hexane’. A typical hexane molecule would have six Carbon atoms and fourteen Hydrogen atoms. Now let’s look at other ways of connectivity. Suppose the Carbon atom at the end of the chain was removed and fixed on to the second Carbon atom. Then the main chain would be shortened to five Carbon atoms with the additional Carbon atom at a branch point. This new compound can be named as the alkane ‘2-methylpentane’. Likewise, other branch points can be created by adding methyl groups to different places along the chain. Some other ways of connectivity include; 2,3-dimethylbutane, 2,2-dimethylbutane, 3-methylpentane etc.

If the compound that one is dealing with has functional groups in it such as alcohol, amine, ketone/aldehyde etc., by displacing the functional groups onto various Carbon atoms along the main Carbon chain, several different molecules can be created; yet each having the same molecular formula. This type of isomerism is called positional isomerism. At times, when attempting to re-arrange elements ordered in a molecular formula, one could create molecules having different functional groups, yet sticking to the same elemental composition given in the molecular formula; this is known as a functional group isomerism. Alcohols and ethers can be comfortably interchanged this way (e.g. CH3-O-CH3 and CH3-CH2-OH) and with the right amount of unsaturation present, it can be exchanged with ketones and aldehydes, as well. Another common example is a straight chain hexene and a cyclohexane compound. Changes in functional groups vastly affect the chemical properties of the compound and also its physical characteristics.

What are Stereoisomers?

Stereoisomers are isomeric compounds with the same molecular formula and also have the same connectivity of atoms, but only differ in the 3 dimensional arrangements of atoms in space, hence also known as spatial isomers. There are different kinds of stereoisomers namely; enantiomers, diastereomers, cis-trans isomers, conformational isomers etc.

Enantiomers are molecules that are mirror images of each other; hence these molecules are non-superimposable. The magic is created by centres called chiral centres. These are Carbon atoms that have four different groups connected to it. Chiral centres are responsible for creating enantiomers, and these molecules have almost identical properties, but they can be identified from the way they rotate plane polarized light. Therefore, these are also called optical isomers. There are also stereoisomers that are not enantiomers, that is they are not mirror images of each other, and some such molecules are; diastereomers, cis-trans isomers and conformers. There’s a special class of diastereomers called meso compounds, which have a mirror plane within the molecule, but the molecule taken as a whole, its mirror image doesn’t form another molecule, but instead results the same molecule. Conformers are molecules that have the same connectivity but takes different shapes; e.g. various conformations of cyclohexane; chair, boat, half boat etc.

What is the difference between Constitutional Isomers and Stereoisomers?

• Constitutional isomers have atoms connected in different orders whereas, in stereoisomers, the connectivity in atoms is similar but the 3D arrangement of atoms in space is different

• Chirality is seen in stereoisomers and not in constitutional isomers.

• Constitutional isomers can have very different chemical names from each other, whereas stereoisomers will usually have the same chemical name with a letter or symbol of identification of orientation in front of the name.

• The chemical and physical properties of constitutional isomers differ more rapidly than between stereoisomers.