A structural formula shows which atoms are bonded to which, and that is the information a plain formula such as C2H6O leaves out. There are three working notations: the full drawing with every bond, the condensed line such as CH3CH2OH, and the skeleton drawing in which each corner is a carbon and hydrogens are implied. Learn all three, because each is used for a different job and you need to convert between them without losing a hydrogen.
Part of Organic Chemistry
The drawing conventions used to write down and read organic molecules, from every-bond diagrams to compact condensed lines.
One Formula Can Mean Two Compounds
C2H6O is a legitimate formula for two substances. One is ethanol, CH3CH2OH, in which the oxygen sits at the end of the carbon chain with a hydrogen on it. The other is dimethyl ether, CH3OCH3, in which the oxygen sits between the two carbons. They share a formula and differ in structure, which is the subject of Isomers. A formula that cannot separate them is not enough for a community deciding what it is holding.
Three Notations, Three Jobs
Full structural formula. Every atom and every bond is drawn. Use it when learning, when checking a bond count against Bonding Rules, and whenever a reaction needs you to see which bond breaks.
Condensed formula. Atoms are written in order along the chain with their hydrogens grouped: CH3CH2OH for ethanol, CH3COOH for acetic acid. It fits on one line, so use it in notes and batch records.
Skeleton drawing. Each line end and corner is a carbon, hydrogens on carbon are not drawn, and only atoms other than carbon and hydrogen are labelled. It is the fastest to draw by hand and the easiest to misread, because a missing hydrogen is invisible.
Method: Converting A Skeleton To A Formula
Step 1: Count the corners and line ends. Each is a carbon.
Step 2: Give each carbon enough hydrogens to reach four bonds. A carbon with one drawn single bond gets three hydrogens; with two, two hydrogens; a double bond counts as two.
Step 3: Add the labelled atoms such as O and N, and add one hydrogen for each oxygen or nitrogen bond-end not otherwise filled.
Step 4: Add up carbons, hydrogens and the rest.
Worked Example: Acetic Acid From A Drawing
Vinegar’s active compound has a two-carbon skeleton. The left carbon has one single bond (to the second carbon), so it takes 4 − 1 = 3 hydrogens. The right carbon has one single bond to the first carbon, one double bond to an oxygen (counting 2) and one single bond to an OH oxygen, so 1 + 2 + 1 = 4 bonds and takes no hydrogens. The OH carries one hydrogen. Total hydrogens: 3 + 1 = 4. Atoms: 2 carbons, 4 hydrogens, 2 oxygens, so C2H4O2, condensed CH3COOH. The mass, with C = 12, H = 1, O = 16, is 2 × 12 + 4 × 1 + 2 × 16 = 24 + 4 + 32 = 60.
Common Mistakes
- Drawing a carbon with five bonds. It means a hydrogen was added twice or a double bond was counted as one.
- Reading CH3OCH3 as an alcohol. An oxygen between two carbons is an ether, not an alcohol. See Functional Groups.
- Losing track of which notation a record uses. In a batch notebook, write the condensed formula and a name beside it.
Safety
A drawing is not an identification
Two compounds with the same formula can differ enormously in hazard. Do not conclude that a liquid is safe because its formula resembles something familiar. Identify by the field properties listed in the carrier (boiling point, solubility, smell, colour, flame behaviour) and by the container’s provenance, and treat unknowns as poisonous.
What’s Next
- Learn next: Isomers — same atoms, different arrangement
- Combine with: Naming Conventions — turning a drawing into a name