Benzene, a flat ring

Section 3 of 4

Benzene, C₆H₆, is a ring of 6 carbons with just one hydrogen on each. It is perfectly flat, and it shares some of its electrons round the whole ring.

Each carbon in benzene is bonded to 2 carbons and 1 hydrogen: 3 directions. You have seen that 3 directions make a flat triangle with bonds 120° apart, and 120° is exactly the corner of a regular hexagon. So the ring stays flat, and all 12 atoms lie in one plane.

Benzene, C₆H₆Six carbon atoms in a flat ring, drawn with alternating single and double bonds, each carbon also bonded to one hydrogen atom.CCCCCCHHHHHH
Benzene, C₆H₆, drawn with every atom. Open the 3D model and tip it: all 12 atoms lie flat.

Count the bonds at one carbon, though: 2 to carbons and 1 to hydrogen make 3, and carbon makes 4. The usual drawing gives every carbon one double bond, so double and single bonds take turns round the ring. Each carbon then has 4 lines.

A C=C double bond is shorter than a C–⁠C single bond, so that drawing suggests three short bonds and three long ones. Chemists have measured benzene's ring very precisely, and all six bonds are the same length, in between a single and a double bond.

Carbon to carbon bond lengths, drawn to scale Three pairs of carbon atoms drawn to scale. The single bond in ethane is the longest, 0.154 nanometres. The double bond in ethene is the shortest, 0.134 nanometres. The bond in benzene's ring, highlighted, is 0.140 nanometres, in between, and all six bonds in the ring have this length. single bond, ethane 0.154 nm benzene ring bond 0.140 nm double bond, ethene 0.134 nm
Carbon-carbon bonds drawn to scale. All six bonds in benzene's ring are 0.140 nm long, between ethane's single bond and ethene's double bond. A nanometre (nm) is a millionth of a millimetre.

Here is why. Each carbon uses 3 of its 4 outer electrons in its bonds to two carbons and a hydrogen. The fourth electron of every carbon is shared by the whole ring. Those 6 electrons spread evenly all the way round, above and below the flat ring, so every carbon-carbon bond gets the same share. The flat sheets of graphite share their fourth electrons in the same way.

Benzene's six spare electrons are shared round the ring Two ball-and-stick drawings of benzene, a flat hexagon of six carbon atoms, each with one hydrogen pointing outward. Left, each carbon has one electron drawn as a dot just inside the ring: the electron left after its three bonds. Right, the six dots are replaced by one ring of colour inside the hexagon, because the six electrons are shared by all six carbons. one spare electron on each carbon all six shared round the whole ring
Each carbon has one outer electron left after its three bonds (left). The six of them are shared by the whole ring (right).

Chemists draw benzene in two ways. The drawing with double bonds makes it easy to count bonds. The drawing with a circle inside the hexagon shows the 6 shared electrons, spread evenly round the ring. Both stand for the same molecule.

Benzene, C₆H₆Six carbon atoms in a flat ring, drawn with alternating single and double bonds, each carbon also bonded to one hydrogen atom.
Benzene with double and single bonds taking turns.
Benzene, C₆H₆, drawn with a circle A hexagon of six single lines with a circle inside it. Each corner is a carbon atom with one hidden hydrogen. The circle stands for six electrons shared evenly round the ring.
Benzene with a circle for the 6 shared electrons.

Quick check

How long are the six carbon-carbon bonds in benzene's ring?