Complete, Condensed and Bond-line Structural Formulas

Representing Organic Structures: From Dots to Lines

Organic compounds are built from carbon skeletons with hydrogen and other atoms attached. Chemists have developed several ways to draw these structures, each offering a different balance between completeness and convenience. The progression from Lewis structures to bond-line formulas is a story of increasing abstraction — you trade away visual detail for speed and clarity.

Complete Structural Formulas (Dash Structures)

The Lewis dot structure shows every valence electron, but this becomes cumbersome for even small molecules. The first simplification replaces each pair of bonding electrons with a dash. A single dash ($-$) represents a single covalent bond, a double dash ($=$) a double bond, and a triple dash ($\equiv$) a triple bond. Lone pairs on heteroatoms — oxygen, nitrogen, sulphur, halogens — may be shown or omitted depending on the context.

This representation is called a complete structural formula or dash structure. It still shows every atom and every bond explicitly. For example:

• Ethane ($\ce{C2H6}$): $\ce{H3C-CH3}$ (each dash is a single bond)
• Ethene ($\ce{C2H4}$): $\ce{H2C=CH2}$ (the double dash indicates a double bond)
• Ethyne ($\ce{C2H2}$): $\ce{HC#CH}$ (the triple dash indicates a triple bond)
• Methanol ($\ce{CH3OH}$): $\ce{H3C-OH}$

The dash structure focuses attention on the electrons involved in bonding. Every bond is a pair of electrons, and every dash is a visual shorthand for that pair.

Condensed Structural Formulas

Complete structural formulas still require drawing every bond. The next level of abbreviation is the condensed structural formula, where some or all of the dashes are omitted, and identical groups attached to the same atom are indicated by a subscript.

The same four compounds become:

• Ethane: $\ce{CH3CH3}$
• Ethene: $\ce{H2C=CH2}$ (the double bond is still shown because it is chemically significant)
• Ethyne: $\ce{HC#CH}$ (the triple bond is still shown)
• Methanol: $\ce{CH3OH}$

Notice that single bonds between carbons are simply implied by writing the atoms next to each other. The subscript tells you how many identical groups are present — $\ce{CH3}$ means one carbon with three hydrogens attached.

For longer chains, condensation can go further. The straight-chain alkane with eight carbons can be written as $\ce{CH3CH2CH2CH2CH2CH2CH2CH3}$, but this is repetitive. A more compact form is $\ce{CH3(CH2)6CH3}$. The $( \ce{CH2} )_6$ tells you there are six methylene ($\ce{-CH2-}$) groups in a row between the two terminal methyl groups.

Bond-Line Structural Formulas

The most abstract and widely used representation in organic chemistry is the bond-line structural formula (also called skeletal formula). Here, carbon and hydrogen atoms are not written at all. Only lines representing carbon-carbon bonds are drawn, and they are arranged in a zig-zag fashion.

The rules are simple:

• Each line segment represents a carbon-carbon bond.
• Every junction (where two lines meet) and every line terminus is a carbon atom.
• Hydrogen atoms are not drawn; they are assumed to fill the remaining valency of each carbon (four bonds total per carbon).
• Atoms other than carbon and hydrogen — oxygen, nitrogen, chlorine, bromine, etc. — are written explicitly.

Example: 3-Methyloctane

3-Methyloctane has a main chain of eight carbons with a methyl group ($\ce{-CH3}$) attached to the third carbon. It can be represented in several ways:

• Complete structural formula: all bonds and atoms drawn explicitly.
• Condensed formula: $\ce{CH3CH2CH(CH3)CH2CH2CH2CH2CH3}$ — the branching is shown by placing the $\ce{CH3}$ group in parentheses after the carbon it attaches to.
• Bond-line formula: a zig-zag line of eight segments (the main chain) with a short line branching off the third carbon.

In the bond-line drawing, the terminus of the main chain on the left is a $\ce{CH3}$ group, the terminus on the right is another $\ce{CH3}$ group, and the branch is a $\ce{CH3}$ group attached to the third carbon. No hydrogen atoms are drawn anywhere.

Example: 2-Bromobutane

2-Bromobutane has four carbons in a chain with a bromine atom on the second carbon. Its representations:

• Complete: all atoms and bonds shown.
• Condensed: $\ce{CH3CHBrCH2CH3}$ — the bromine is written next to the carbon it attaches to.
• Bond-line: a zig-zag of three segments (four carbons) with "Br" written at the second carbon. The carbon at that junction now has one bond to Br, two bonds to adjacent carbons, and therefore one hydrogen (not drawn). …