Alkanes and cycloalkanes: Drawing structures

DRAWING STRUCTURES

Key Notes

C–H Bond omission

Alkanes can be drawn more quickly and efficiently if the C–H bonds are omitted.

Skeletal drawings

Skeletal drawings show only the C–C bonds. Each bond junction is assumed to have a carbon atom with sufficient hydrogens present to make up four bonds.

Alkyl groups

Alkyl groups (C_nH_2n1) are alkane portions of a more complicated structure. They can be drawn as a skeletal drawing, or as CH₃, CH₂CH₃, et cetera

C–H Bond omission

There are several ways of drawing organic molecules. A molecule such as ethane can be drawn showing every C–C and C–H bond. However, this becomes tedious, especially with more complex molecules, and it is much easier to miss out the C–H bonds (Fig. 1).

Skeletal drawings

A further simplification is often used where only the carbon–carbon bonds are shown. This is a skeletal drawing of the molecule (Fig. 2). With such drawings, it is understood that a carbon atom is present at every bond junction and that every carbon has sufficient hydrogens attached to make up four bonds.

Straight chain alkanes can also be represented by drawing the C–C bonds in a zigzag fashion (Fig. 3).

Alkyl groups

Alkyl groups (C_nH_2n1) are alkane substituents of a complex molecule. Simple alkyl groups can be indicated in skeletal form (Fig. 4a), or as CH₃, CH₂CH₃, CH₂CH₂CH₃, et cetera. (Fig. 4b).

Notice how the CH₃ groups have been written in Fig. 5. The structure in Fig. 5a is more correct than the structure in Fig. 5b since the bond shown is between the carbons.