Constitutional isomers

Introduction

Constitutional isomerism is a significant characteristic of organic molecules. The earliest observation of isomerism was made by Friedrich Woehler in 1827. Isomerism in organic molecules governs the property of products formed during an organic reaction. The concept of isomerism builds the foundation of organic chemistry and must be understood before moving on to complex organic reactions. There are various categories of isomerism. Constitutional isomerism is one of the two broad categories of isomerism. The other is stereoisomers, which have their branch of chemistry dedicated to them (called stereochemistry). Constitutional isomers are products of various important reactions. Generally, reactions that follow Markovnikov's rule or Anti- Markovnikov's rule or Kharasch effect or peroxide effect can produce a mixture of constitutional isomers.

What are Constitutional Isomers?

Isomers with the same chemical formula but different atomic constitutions and organisation are called constitutional isomers. The concept of constitutional isomerism helps in constructing a relationship between molecules. This is done to identify the differences and similarities in the properties of molecules. The three-dimensional structure of constitutional isomers helps in understanding the regioselectivity mechanism of some important reactions. For example, the addition of hydrogen chloride, hydrogen bromide or hydrogen iodide to an alkene can yield constitutional isomers as products. The constitution of the hydrogen-bonded during the reaction can help determine the mechanism of the addition reaction. If the halogen atom is attached to the site that is more substituted (or hydrogen is attached to the carbon with a larger number of hydrogen atoms), the reaction progresses with Markonikov's rule. If the halogen atom is attached to the site that is less substituted (or hydrogen is attached to the carbon with less number of hydrogens), the reaction progresses with the peroxide effect or Kharasch effect.

Here the halogen atom is attached itself to more substituted carbon and thus produces A

Here the halogen atom is attached itself to less substituted carbon and thus produces B.

A and B are constitutional isomers.

The molecule can have different functional groups, distinctions in positions of functional groups and different chain lengths.

Chain Isomers or Skeletal Isomers

On comparing the organisation of bonds and atoms of two molecules that constitute the framework or skeleton of the molecules is different the isomer is identified as skeletal isomerism. Skeletal isomerism is a subcategory of constitutional isomerism. This type of isomerism is most commonly observed in simpler molecules like alkanes. For example, a pentane molecule can have 3 skeletal isomers. n-pentane is a straight-chain isomer of pentane while isopentane is a branched-chain isomer of pentane. Neopentane is another isomer of pentane that has a carbon attached to 4 methyl groups. For acyclic systems, skeletal isomers are termed chain isomers.

An example of skeletal isomerism in the cyclic system is cyclopentane. Cyclopentane has 5 cyclic isomers that have different ring sizes. Including cyclopentane, the skeletal isomers of cyclopentane are 1-methyl cyclobutane, 1,2-dimethyl cyclopropane, 1,1- dimethyl cyclopropane and lastly 1-ethyl-1-methyl cyclopropane. This type of isomer is commonly seen in products of reactions involving ring expansion and ring contraction. It can also be observed in rearrangement reactions in which a substituent rearranges itself to produce a much more stable intermediate.

Positional Isomers or Regioisomers

The molecules that show this type of isomerism generally have an equivalent parent framework. The difference in the chemical and physical properties arises due to the difference in the position of the functional group or the substituent. Positional isomers are shown by almost every functional group. It may not be possible in bulky functional groups or strained functional groups. An example of this type of isomerism can be observed in pentanol. Pentanol has 3 positional isomers namely pentan-1-ol, pentan-2- ol and pentan-3-ol. The functional group present here is the hydroxyl group. The position of the hydroxyl group differs in all three isomers. The hydroxyl group is present on the first carbon in pentan-1-ol. Similarly, the hydroxyl group is present on the second carbon in pentan-2-ol and the hydroxyl group is present on the first carbon in pentan-3-ol.

Another example of positional isomerism is butene. In butene, the functional group is a double bond or pi bond. There are 2 positional isomers of butene namely 1-butene and 2-butene. In 1-butene, the double bond is present between the first and second carbon of the parent chain. In 2-butene, the double bond is present between the second and third carbon of the parent chain.

Functional Isomers

This subcategory of constitutional isomerism is most important. Functional isomerism is often confused with positional isomerism. But it should be noted that the molecules showing this type of isomerism have the most distinction than any other constitutional isomer. This is because in this isomerism the functional groups present in the molecules are different. The entire framework of the parent chain of the molecule might also differ. Generally, aldehydes and ketones often exist as functional isomers. An example of such occurrence is acetone $\mathrm{CH_{3}COCH_{3}}$ which is a molecule with a ketone functional group.

It is a functional isomer of propanal$\mathrm{(CH_{3}CH_{2}CHO)}$ which is a molecule with an aldehyde functional group. Both molecules have the same molecular formula but very distinct structures and carbon chain frameworks. The physical and chemical properties of both molecules have appreciable differences. Another pair of functional groups that generally show positional isomerism is ether and alcohol. As observed in aldehydes and ketones, the molecular formula of the isomers is the same. The difference in physical and chemical properties arises due to the presence of distinct functional groups. Ethanol $\mathrm{(CH_{3}CH_{2}OH)}$, alcohol is a function isomer of an ether named dimethyl ether $\mathrm{(CH_{3}OCH_{3})}$.

Conclusion

In a nutshell, the concept of constitutional isomerism is necessary to understand the chemical and physical properties of organic molecules. This practice helps in better application of the concept of constitutional isomers in industries and research and development of practically significant molecules. Constitutional isomerism is categorised into skeletal isomerism, positional isomerism and functional isomerism. Each of these subcategories is important to identify the regioselectivity of certain reagents and molecules which will help in developing better, safe and low waste- producing methods of product synthesis. Constitutional isomerism is the part of the basics that construct the fundamentals of organic chemistry. Thus, a sound understanding of constitutional isomerism is necessary to understand organic chemistry.

FAQs

1. Functional isomerism in molecules can be identified by which physical method?

Functional isomerism in molecules can be identified by infrared (IR) spectroscopy. Infrared spectroscopy is based on the vibrational frequency of molecules and atoms. Functional groups have different vibrational modes because they have different sets of atoms adjoined by different connectivity.

2. Which category of isomerism has the same connectivity between atoms?

Unlike constitutional isomers, stereoisomers have the same connectivity between atoms. They are sub-categorised into enantiomers and diastereomers.

3. Name the constitutional isomers of cyclohexane ?

Including cyclohexane, the other constitutional isomers are 1-hexene, 2-hexene, 3- methyl-1-pentene and 1-ethyl-2-methyl cyclopropane.

4. How can constitutional isomers be determined?

The best method to determine constitutional isomerism in molecules is to calculate the number of carbons present and the degree of unsaturation.

5. $\mathrm{C_{3}H_{6}O}$ represent which subcategory of constitutional isomerism?

$\mathrm{C_{3}H_{6}O}$ represent functional isomerism. The chemical formula represents aldehyde and ketone. The aldehyde is propanal $\mathrm{(C_{3}CH_{2}CHO)}$ and the ketone is propanone $\mathrm{(CH_{3}COCH_{3})}$.