3.2: Constitutional Isomers of Alkanes

Organic compounds of the same molecular formula can have different structural formulas called constitutional isomers, and the phenomenon is known as constitutional isomerism. Alkanes with four or more carbons showing multiple structures with the same molecular formula thereby exhibit constitutional isomerism.

The linear isomer of an alkane is prefixed by the term “n”; hence a linear isomer of pentane is known as n-pentane. Based on the type of branching, some of the branched isomers are given the prefixes “iso” and “neo”. For example, pentane has two-branched isomers, iso-pentane and neo-pentane. Iso-pentane has a four-carbon straight chain with a single methyl substituent, whereas neo-pentane has a three-carbon straight chain with two methyl groups as substituents.

The possibility of branching increases with the growing number of carbon atoms in the chain across the homologue, leading to the exponential rise in the number of constitutional isomers. For example, the six-carbon alkane hexane has five isomers, whereas the number of isomers increases to 75 for the 10-carbon alkane decane.

Despite having the same molecular formula, isomers exhibit significant variation in their physical properties, such as melting points and boiling points.

Alkanes contain two types of single bonds: carbon-carbon and carbon-hydrogen. 

Simple alkanes such as methane, ethane, and propane have only one way of arranging their atoms, implying that they have a single structural formula.

However, alkanes with four or more carbons can have their atoms arranged in multiple ways for the same molecular formula; they can show constitutional isomerism or structural isomerism.

For example, the four-carbon alkane, butane, has two structural isomers. The straight-chain form has four carbon atoms that are linearly arranged.

In comparison, the branched form has a three-carbon chain with the fourth carbon attached as a side-chain, or branch, to the second carbon in the chain.

Each isomer of butane has ten hydrogen atoms bonded to four carbon atoms. Butane, therefore, has two structural isomers with the same molecular formula.

Furthermore, isomers vary in their physical properties: branched butane has a boiling point of −11.6 °C, whereas straight-chain butane boils at −0.5 °C.

Compared to butane, which has only one branched isomer, higher alkanes can branch at any suitable carbon. Thus, higher alkanes can have many distinct branched isomers but only one straight-chain isomer.

For example, pentane, C₅H₁₂, has one straight-chain isomer and two branched isomers. The straight-chain isomer, also known as n-pentane, has all five carbon atoms in an unbranched chain.

One of the branched isomers has a four-carbon chain, with the fifth carbon branching from the carbon at the second position. This isomer is called iso-pentane because of the −CH(CH₃)₂ group at the end of the chain.

The other branched isomer has a three-carbon chain with the fourth and fifth carbons branching from the central carbon. This isomer is called neo-pentane due to the terminal −C(CH₃)₃ group.