Four ways to write the same molecule
Take butan-1-ol, a four-carbon alcohol:
| Formula | What it shows | Butan-1-ol |
|---|---|---|
| Molecular | Just the atoms | C₄H₁₀O |
| Structural (condensed) | Each carbon and what's on it | CH₃CH₂CH₂CH₂OH |
| Displayed | Every atom and every bond | all C–H, C–C, C–O and O–H bonds drawn |
| Skeletal | Carbon chain as lines, functional groups written out | three zig-zag lines ending in OH |
Rules for skeletal structures
- Every corner and every line end is a carbon, unless another atom is written there.
- Hydrogens on carbon are not drawn. Each carbon has enough hydrogens to make four bonds in total.
- Draw chains as zig-zags, with bond angles of about 120°. Straight lines hide where the carbons are.
- Write every atom that isn't carbon (O, N, Cl, Br…) and the hydrogens on it: OH, NH₂, CHO.
- Double and triple bonds are two or three parallel lines. Draw a C≡C and the atoms on either side of it in a straight line.
Examples
Counting hydrogens
To find the hydrogens on any carbon in a skeletal structure, count its bonds and subtract from four. An end carbon with one line is CH₃. A corner with two lines is CH₂. A branch point with three lines is CH. A carbon with four lines has no H. A double bond counts as two.
Showing 3D shape: wedges and hashes
For a tetrahedral carbon, draw two bonds as plain lines in the plane of the page, a wedge for the bond pointing towards you, and a hashed bond for the one pointing away. You need this for stereocentres and R/S. See R and S configuration.
Common mistakes
- Five bonds to a carbon. Count the lines at every corner. Four is the maximum.
- Writing "C" at every corner. In a skeletal structure, carbons are corners, not letters.
- Forgetting the H on heteroatoms. An alcohol is OH, not just O.
- Drawing straight chains, which hide the number of carbons.
Quick answers
What is a skeletal formula?
A skeletal formula shows the carbon skeleton as zig-zag lines. Every corner and line end is a carbon, hydrogens on carbon are left out, and every other atom is written in.
How do you count carbons in a skeletal structure?
Count every corner and every end of a line. Each one is a carbon unless another atom is written there.
What do wedges and dashes mean?
A solid wedge is a bond pointing towards you, out of the page. A hashed (dashed) wedge is a bond pointing away from you, behind the page.
Draw structures in Bondline: it names them as you goFree, works offline, no account needed.
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