Structural Representations of Organic Compounds

Organic compounds can be drawn in several ways, each offering a different balance between completeness and convenience. The choice of representation depends on what you need to show — whether it is every electron, every bond, or just the skeleton of the molecule.

Lewis Structures and Dash Structures

The most fundamental representation is the Lewis structure (or dot structure), where each valence electron is shown as a dot around the atomic symbol. Covalent bonds appear as pairs of dots shared between two atoms. This method is thorough but quickly becomes cumbersome for larger molecules.

A natural simplification replaces each shared pair of electrons (each covalent bond) with a single dash ($-$). A single dash represents a single bond (one shared pair), a double dash ($=$) represents a double bond (two shared pairs), and a triple dash ($\equiv$) represents a triple bond (three shared pairs). Lone pairs of electrons on heteroatoms — atoms like oxygen, nitrogen, sulphur, or halogens that are not carbon or hydrogen — may be shown or omitted depending on the context.

These dash-based drawings are called complete structural formulas because they show every atom and every bond explicitly.

Examples of Complete Structural Formulas

Consider four simple organic compounds:

Ethane ($\ce{C2H6}$) — two carbon atoms joined by a single bond, each carbon carrying three hydrogen atoms:

(Each carbon is tetrahedral; the planar drawing is a convention.)

Ethene ($\ce{C2H4}$) — two carbon atoms joined by a double bond, each carbon carrying two hydrogen atoms:

Ethyne ($\ce{C2H2}$) — two carbon atoms joined by a triple bond, each carbon carrying one hydrogen atom:

Methanol ($\ce{CH3OH}$) — a carbon atom bonded to three hydrogens and an oxygen atom, which in turn is bonded to a hydrogen:

In methanol, the oxygen atom has two lone pairs of electrons (not shown here), which are often omitted in dash structures unless they are relevant to the reaction or property being discussed.

The Progression of Simplification

The complete structural formula is the starting point. From it, you can derive more compact representations:

1. Condensed structural formula — atoms are written in a line, with subscripts showing how many of each atom are attached to a given carbon. For example, ethane becomes $\ce{CH3CH3}$, ethene becomes $\ce{CH2=CH2}$, ethyne becomes $\ce{CH#CH}$, and methanol becomes $\ce{CH3OH}$. …