3.2 : Isomères constitutionnels des alcanes

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.