5.9 Conjugation
Molecules that have alternating single and double bonds have unusual properties. Such bonds are called conjugated, and conjugated systems do two things: they conduct electricity and they absorb light.
A conjugated hydrocarbon.
We can see the effect of conjugation on light absorption by examining molecules of compounds called polycyclic aromatic hydrocarbons (PAHs):
benzene colorless |
naphthalene colorless |
anthracene colorless/pale yellow |
naphthacene orange |
pentacene dark purple |
hexacene dark blue-green |
All organic molecules absorb light, but most of them absorb deep in the ultraviolet and look colorless to our eyes. Benzene (single ring), naphthalene (2 fused rings), and anthracene (3 fused rings) all absorb entirely in the UV, but at progressively longer wavelengths closer to visible light.
Naphthacene (4 fused rings) absorbs blue light and appears orange. Pentacene (5 rings) absorbs green and yellow light and appears purplish black; hexacene (6 rings) absorbs almost all visible light and appears very dark blue-green. The larger a conjugated system, especially longer straight chains, the longer the wavelength of light it will absorb.
Cis bonds absorb light at a longer wavelength than trans bonds. That is part of why azobenzene is red in its cis form and yellow in its trans form.
cis-azobenzene (red) |
trans-azobenzene (yellow) |
A chalcogen or pnictogen, single-bonded to a conjugated system, is itself also conjugated, for example the hydroxy group of phenol. A molecule like phenol, or vanillin, or eugenol will tend to keep its conjugated atoms close to coplanar with the ring if it can. For some molecules like salicylic acid, this is not possible since the substitutions are too bulky.
phenol |
vanillin |
eugenol |
salicylic acid |