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Reading and drawing skeletal structures

How skeletal (line-angle) formulas encode carbon atoms, hydrogens and bonds, and how to draw them quickly and unambiguously.

Updated

Organic chemists rarely draw every atom. A skeletal structure (also called a line-angle or bond-line formula) shows the carbon framework as lines and leaves most hydrogens to the reader. Once you can read one fluently, you can draw a structure from a name in seconds, and every exercise on this site expects you to.

The four conventions

  1. Every line is a bond. Two parallel lines are a double bond and three are a triple bond.
  2. Every end of a line and every corner is a carbon atom, unless another element is written there.
  3. Hydrogens on carbon are not drawn. Each carbon has as many hydrogens as it needs to make four bonds in total.
  4. Every other element is written out, together with the hydrogens attached to it: OH, NH2, SH, and so on.

Example: propan-2-ol

There are three carbons: the two line ends and the corner where the OH is attached. The middle carbon has three bonds drawn (to two carbons and to oxygen), so it carries 4 − 3 = 1 hydrogen. Each end carbon has one bond drawn, so each is a CH3. The molecular formula is therefore C3H8O.

Counting hydrogens

For a neutral carbon, the number of implied hydrogens is

hydrogens on C = 4 − (number of bonds drawn to that carbon, counting a double bond as 2 and a triple bond as 3)

A carbon with four single bonds already drawn has no hydrogens; a carbon at the end of a chain with one single bond is a CH3. The same idea works for heteroatoms if you remember their usual bond counts: neutral nitrogen forms 3 bonds and neutral oxygen 2, but their hydrogens are always written explicitly in a skeletal structure.

Drawing chains: why the zigzag?

Chains are drawn as zigzags with angles of about 120° because that makes every carbon visible as a corner. A straight line of three bonds would hide the carbons in the middle. The exact angles do not matter chemically, and the checker on this site ignores the shape and orientation of your drawing: only which atoms are bonded to which, and with what bond order, is compared.

The exception to the zigzag is a triple bond: the two carbons of a C≡C and the atoms attached to them lie on a straight line, so draw them linearly.

Example: but-1-ene and but-2-yne

In but-1-ene the double bond is between C1 and C2, the end of the chain. In but-2-yne the triple bond is in the middle and the four carbons are drawn in a straight line.

Converting a condensed formula

Textbooks often give condensed formulas such as CH3CH2CH(CH3)CH2OH. Read it left to right: each carbon group is bonded to the next one, and a group in brackets is a branch on the carbon just before it.

  1. CH3–CH2–CH–CH2–OH is the main chain: four carbons and an OH.
  2. (CH3) is a methyl branch on the third carbon from the left.
  3. Draw a four-carbon zigzag, add OH at one end, and add a one-carbon branch to the carbon next to the CH2OH.
CH3CH2CH(CH3)CH2OH is 2-methylbutan-1-ol

Rings

A ring is drawn as a polygon: a triangle for cyclopropane, a square for cyclobutane, a hexagon for cyclohexane. Each corner is a CH2unless something is attached. Benzene is usually drawn as a hexagon with three alternating double bonds; the two ways of placing those double bonds (and the circle-in-a-hexagon notation) all describe the same molecule, and the checker treats them as identical.

Common mistakes

  • Counting a bend as nothing. Every corner is a carbon. A zigzag with three segments has four carbons, not three.
  • Adding a "methyl" to the end of a chain. A one-carbon branch on the first carbon is not a branch at all; it simply makes the chain one carbon longer.
  • Forgetting that a double bond uses up two bonds. A carbon in C=C with two other carbons attached has no hydrogens left.
  • Five bonds to carbon. If a carbon ends up with more than four bonds, something is wrong; the editor marks the atom and the checker will tell you which element is over its limit.

Using the editor

In the practice editor, press on a dot and drag to a neighbouring dot to draw a bond; keep dragging from dot to dot to draw a chain, and start a new drag from any atom to add a branch. Tap the middle of a bond to make it double or triple, and tap an atom to change it into O, N, a halogen or another element. Hydrogens on carbon are implied, so you never draw them yourself, and on other atoms the editor shows them for you. TheHelp button in the editor toolbar shows these steps with pictures, and an advanced editor (Ketcher) is available for anyone used to ChemDraw-style tools. You can try it on the practice page.