Structural, Condensed, and Skeletal Formulas

Organic molecules can be written as molecular formulas, expanded structural formulas, condensed structural formulas or skeletal (line-angle) formulas, which show progressively less detail and are faster to draw. In brief: butane can be written as C₄H₁₀ (molecular), CH₃CH₂CH₂CH₃ (condensed) or as a zigzag line with four vertices (skeletal), in which each end and each corner is a carbon atom and the hydrogen atoms are not drawn.

Because the same molecular formula can stand for several different compounds, organic chemists use structural formulas to show how atoms are connected. Choosing the right type of formula makes it possible to communicate clearly without cluttering the page. This article explains each type, gives the rules for skeletal formulas, shows how to convert between them and compares them with 3D representations.

What are the types of organic formulas?

• Molecular formula: the number of atoms of each element, such as C₂H₆O. It does not show how the atoms are joined.
• Empirical formula: the simplest whole-number ratio of atoms; glucose, C₆H₁₂O₆, has the empirical formula CH₂O.
• Expanded (full) structural formula: every atom and every bond is drawn, including each C–H bond.
• Condensed structural formula: the atoms are written in sequence, with groups such as CH₃ and CH₂ grouped, and the bonds usually omitted.
• Skeletal formula: only the carbon skeleton is drawn as lines, and carbon and hydrogen atoms are not labeled.

How are condensed formulas written?

The main rules are:

1. Write each carbon atom together with the hydrogen atoms attached to it: CH₃, CH₂, CH.
2. List the atoms in the order in which they are bonded along the main chain.
3. Show branches in parentheses after the atom they are attached to: (CH₃)₂CHCH₃ or CH₃CH(CH₃)CH₃.
4. Show double and triple bonds with = and ≡, for example CH₃CH=CH₂.
5. Write identical repeated groups with a subscript: CH₃(CH₂)₃CH₃ is pentane.

What are the rules for skeletal formulas?

1. Every vertex (bend) and every free end of a line is a carbon atom.
2. Each line is a covalent bond between atoms; double and triple bonds are drawn with two or three parallel lines.
3. Hydrogen atoms attached to carbon are not drawn. The number of hydrogen atoms on a carbon atom is whatever is needed to give it four bonds in total.
4. Atoms other than carbon and hydrogen are labeled with their symbols (O, N, Cl), and the hydrogen atoms attached to them are drawn (OH, NH₂).
5. Rings are drawn as polygons: a triangle is cyclopropane, a hexagon is cyclohexane. Benzene is a hexagon with a circle or alternating double bonds.

For example, in the skeletal formula of ethanol there are two lines at an angle: one end is CH₃, the vertex is CH₂ and the other end is labeled OH. The structure of 2-methylpropane is a Y-shape: a central carbon with three lines going out, to three carbon atoms.

Comparison of formula types

How do you convert between formulas?

Example 1: from condensed to molecular

Find the molecular formula of CH₃CH₂CH(CH₃)CH₂CH₃. Count the carbon atoms: 1 + 1 + 1 + 1 (the branch) + 1 + 1 = 6. Count the hydrogen atoms: 3 + 2 + 1 + 3 + 2 + 3 = 14. The molecular formula is C₆H₁₄, and the compound is 3-methylpentane.

Example 2: from a skeletal description to a condensed formula

A skeletal formula shows a zigzag of five vertices, with a double bond between the first and second vertices. The first carbon has one double bond and so needs 2 H, the second carbon has a double bond and one single bond and needs 1 H, the third and fourth carbons need 2 H each and the last carbon needs 3 H. The compound is CH₂=CHCH₂CH₂CH₃, pent-1-ene, with the molecular formula C₅H₁₀.

Example 3: counting hydrogens in a skeletal formula

A hexagon has six vertices, each with two bonds to neighboring carbon atoms, so each carbon atom needs two H atoms: C₆H₁₂ (cyclohexane). If the hexagon has one double bond, the two carbon atoms of the double bond have only one H each, and the formula is C₆H₁₀ (cyclohexene).

How are three-dimensional shapes shown?

To show the 3D arrangement around a carbon atom, chemists use wedge and dash notation. A solid wedge indicates a bond pointing toward the viewer, a dashed wedge a bond pointing away from the viewer, and ordinary lines are bonds in the plane of the paper. Fischer projections and Newman projections are used for sugars and for conformations. For large molecules, ball-and-stick and space-filling models are generated by computer.

Where did these notations come from?

Archibald Couper (1858) and August Kekulé (1857–1858) were among the first to draw lines between atoms to represent bonds. Alexander Crum Brown introduced his graphical formulas, with atoms in circles, in 1864. The drawing of structures with carbon atoms and hydrogen atoms hidden was gradually adopted in the 20th century as molecules became larger. Skeletal formulas are now the standard in organic chemistry papers and textbooks.

Which mistakes are common?

• Forgetting the hidden hydrogens: a carbon in a skeletal formula always has four bonds, and the rest are hydrogens.
• Mixing styles in one skeletal drawing: either leave out all hydrogens on carbon or show all of them, but do not draw only some.
• Leaving out the hydrogen on O and N: OH and NH₂ must be drawn.
• Miscounting the carbons in a ring: each vertex counts, and the ring has as many carbons as vertices.
• Reading a condensed formula wrongly: CH₃COOH has two carbon atoms, not three.