6.3: Structural Isomers- Branched-Chain Alkanes

Structural Isomers

The structure of C₄H₁₀ can be written as a string of four carbon atoms in a row,

–C–C–C–C–

and then adding enough hydrogen atoms to give each carbon atom four bonds:

The compound butane has this structure, but there is another way to put 4 carbon atoms and 10 hydrogen atoms together. Place 3 of the carbon atoms in a row and then branch the fourth one off the middle carbon atom:

Now we add enough hydrogen atoms to give each carbon four bonds:

Compare the 3D models of butane and 2-methylpropane in Figure \(\PageIndex{1}\).

The structure of 2-methylpropane shows a continuous chain of three carbon atoms only, with the fourth attached as a branch off the middle carbon atom of the continuous chain, which is different from the structures of butane (compare the two structures in Figure \(\PageIndex{1}\).

Unlike C₄H₁₀, the compounds methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) do not exist in isomeric forms because there is only one way to arrange the atoms in each formula so that each carbon atom has four bonds.

Next beyond C₄H₁₀ in the homologous series is pentane. Each compound below has the same molecular formula: C₅H₁₂. The compound at the far left is pentane because it has all five carbon atoms in a continuous chain. The compound in the middle is 2-methylbutane (also called isopentane); it has a one CH₃ branch off the second carbon atom of the continuous chain. The compound at the far right, discovered after the other two, is 2,2-dimethylpropane. Its common name is neopentane (from the Greek neos, meaning “new”). Although all three have the same molecular formula, they have different properties, including boiling points: pentane, 36.1°C; isopentane, 27.7°C; and neopentane, 9.5°C. You will learn more about naming in the next few sections and throughout the next few chapters of this book.

The ultimate condensed formula is a skeletal structure, in which carbon atoms are implied at the corners and ends of lines, and each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds. For example, we can represent pentane (CH₃CH₂CH₂CH₂CH₃) and isopentane [(CH₃)₂CHCH₂CH₃] as follows: