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Constitutional Isomers

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Isomeric compounds are those which have the same empirical formula but different structures. Two non-identical molecules with the same molecular formula are called isomers. If these isomers have different atomic connectivity, then they are called constitutional isomers. 

Constitutional isomers are chemical compounds that differ with regard to their constitution, such as in the location of one or more atoms. Tautomers one specific type of constitutional isomer, differ with regard to the placement of a hydrogen atom. 

Definition

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Constitutional isomers of compounds are defined as the atoms of those compounds are differently connected. The three different types of constitutional isomers are skeletal, functional and positional isomers. 

Constitutional Isomers

These isomers are unique compounds because of their structural differences and they have different physical and chemical properties. Constitutional isomers have the same molecular formula but different connectivity means that their atoms are connected in a different order.

Constitutional Isomers vs Stereoisomers

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Isomers are compounds that contain the same atoms bonded together in different ways. If the connectivity of the atoms in the two isomers is different, they are constitutional isomers. If the connectivity of the atoms in the two isomers is the same, they are stereoisomers. Enantiomers are stereoisomers and so are E and Z double bonds. Compounds such as butane and isobutane that have the same molecular formula but differ in the order in which the atoms are connected are called constitutional isomers. 

Constitutional isomers also include "positional isomers" they represent a particular type of constitutional isomer frequently encountered in pharmacological studies. 

Constitutional isomers: the way the atoms are connected up means their connectivity differs. 

Constitutional Isomers

Stereoisomers: the atoms have the same connectivity, but are arranged differently. For example, 

Stereoisomers

Constitutional Isomers of Pentane

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Pentane is an example of alkenes that are called straight chain alkanes. Each molecule has its own distinct set of properties. Even though isomers are made from the same set of atoms, their differing arrangements into molecules makes them different substances. As the number of carbon atoms in an alkane increases, the number of isomers increases dramatically. Pentane C5H12 exists as three isomers as shown below. 

Pentane

Pentane has three isomers, n-pentane, isopentane and neopentane.

Examples 

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Constitutional isomers can differ in their carbon backbones. Consider the structural differences in the two isomers of C4H10, butane and isobutane. Butane has an uninterrupted chain of carbon atoms, but isobutane has only three carbon atoms connected in sequence. The fourth carbon atom is bonded to the chain as a "branch".

Constitutional isomer can also have different functional groups. For example, both ethyl alcohol and dimethyl ether have the same molecular formula C2H6O. Although the molecule formulas of the two compounds are identical, their functional groups differ. The atomic connectivity is C-C-O in ethyl alcohol and the oxygen atom is part of an alcohol. In contrast, the C-O-C connectivity in the isomer forms an ether.

Constitutional isomers can have the same functional groups, but they are located at different points on the carbon skeleton. Some of the examples are given below.

Propanol 1 Propanol
2 Propanol
Butane N Butane
Isobutane
Ethanol and dimethyl ether Ethanol
Dimethyl Ether

Practice Problems

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Some of the solved problems based on constitutional isomers are given below.

Solved Examples

Question 1: Are the two alkanes constitutional isomers or are they identical? 

Methyl Pentane

Solution:
Constitutional isomers have the same molecular formula but different structures. The molecules given above are identical molecules. They both have the same formula (C6H14) and the same name 2-methylpentane.

Question 2: Are cyclopropane and propene constitutional isomers, or are they stereoisomers?
Solution:
Rearrange the atoms of cyclopropane in several ways, but all of them require moving at least one hydrogen atom from one carbon atom to another. Since we must reconnect the hydrogen to a different carbon atom, these two molecules are constitutional isomers. 

Constitutional Isomer 2