What Is Constitutional Isomerism in Alkanes

Constitutional isomers are compounds that have the same molecular formula, but different structural formulas. By different structural formulas, we mean that these compounds differ in the kinds of bonds they have (single, double, or triple) or in their cormectivity (the order of attachment among their atoms). 

For the molecular formulas CH4, C2H0, and C3H8, only one order of attachment of atoms is possible. For the molecular formula C4H10, two orders of attachment of atoms are possible. In one of these, named butane, the four carbons are bonded in a chain in the other, named 2-methylpropane, three carbons are bonded in a chain, with the fourth carbon as a branch on the middle carbon of the chain. 

Butane and 2-methylpropane are constitutional isomers they are different compounds and have different physical and chemical properties. Their boiling points, for example, differ by approximately 11°C. We will discuss how to name alkanes in the next section. 

In Section 1.7, we encountered several examples of constitutional isomers, although we did not call them that at the time. We saw that there are two alcohols with the molecular formula C3H8O, two aldehydes with the molecular formula C4H8O, and two carboxylic acids with the molecular formula C4H8O2. 

To find out whether two or more structural formulas represent constitutional isomers, write the molecular formula of each and then compare them. All compounds that have the same molecular formula, hut different structural formulas, are constitutional isomers. 

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