Structural representations

Organic structures can be written in several ways, from the fullest to the shortest:

• A complete (Lewis or dash) formula shows every atom and every bond. Ethanol drawn out fully shows all its C–H, C–C, C–O and O–H bonds.
• A condensed formula groups the atoms on each carbon: ethanol is CH₃CH₂OH. Repeated units can be bracketed: HO(CH₂)₃CH(CH₃)CH(CH₃)₂ has three CH₂ groups in a row.
• A bond-line formula is the shortest: a zigzag line. Each corner and each end is a carbon atom, with as many hydrogens as it needs to make four bonds; the hydrogens on carbon are not drawn. Atoms other than C and H (O, N, Cl…) are written in.

For example, butane, CH₃CH₂CH₂CH₃, is a zigzag of three lines with four points (two ends and two corners), one for each carbon.

Rings are drawn as polygons: join the two ends of propane's chain and you get cyclopropane, a triangle. Cyclopentane is a pentagon; cyclohexane a hexagon.

To show the 3-D shape on flat paper, chemists use wedges:

• a solid wedge: a bond coming out of the page, towards you;
• a dashed wedge: a bond going into the page, away from you;
• a plain line: a bond lying in the plane of the page.