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If white light is spread out by a prism,
we can see that it is composed of different colors. Each color corresponds
to a different wavelength. The color of a compound depends on the
wavelength of light which it absorbs. If a compound does not absorb any
visible light it will be colorless. If a compound absorbs light we will
perceive the complementary color, because the light which reaches our eyes
is missing the wavelengths which have been absorbed.
Color and wavelength |
| Wavelength of light, nm |
Color |
Complementary color |
| 400-430 |
violet |
green-yellow |
| 430-480 |
blue |
yellow |
| 480-490 |
green-blue |
orange |
| 490-510 |
blue-green |
red |
| 510-530 |
green |
purple |
| 530-570 |
yellow-green |
violet |
| 570-580 |
yellow |
blue |
| 580-600 |
orange |
green-blue |
| 600-680 |
red |
blue-green |
The ability of a compound to absorb light
depends on the presence of certain kinds of structural features called
chromophores. "Chromophore" is one of those useful but sloppy words whose
meaning depends somewhat on the context.
If we are talking about color, then a
chromophore will be an extended conjugated system, particularly one where there are different resonance
contributing forms differing in charge distribution.
If there is simply a conjugated system,
with nothing but C=C, it takes quite a long chain of conjugated bonds to
give a colored system:
H-(CH=CH)n-H |
| n = |
wavelength (nm) |
| 1 |
180 |
| 2 |
217 |
| 3 |
268 |
| 4 |
310 |
| 5 |
335 |
However if the chain of double bonds is
long enough quite intense colors can result; the colors of carrots and
tomatoes result from carotenoids, which have such chromophores.
Similarly, although benzene is colorless
(it does not absorb visible light) compounds with fused rings absorb at
longer and longer wavelengths. The ultimate is graphite, which contains
large sheets of fused rings and is black because it absorbs all
wavelengths of visible light.
Much smaller molecules can be colored if they have polar functional groups.
An example is indigo, the dye used in blue
jeans.
If you write the various resonance
contributing forms you will see that there can easily be separation of
charge, with the charge spread over the molecule. Ancient Mediterranean
civilizations prized a variant of indigo, obtained from a species of
snail. 6,6'-Dibromoindigo or Tyrian purple was so expensive that only
royalty could afford it.
Many natural pigments are based on the quinone chromophore,
The parent quinone shown, benzoquinone,
is yellow; with larger conjugated systems, and especially with hydroxyl groups to modify the chromophore, they can be bright red.
Cochineal is a pigment obtained from dried
female insects which live on a cactus. The pigment is used for food and
cosmetics; the principal ingredient is carminic acid, which is a C-glucoside
of a polyhydroxyanthraquinone.
Before the invention of synthetic dyes, it
was an important dye for wool and silk. The natural dye consists of the
dried bodies of the insects. The insect, and the cactus it feeds on were
brought back to the Old World from Mexico, and extensively cultivated.
Since cactus species are only found in the New World, both the insect and
its food had to be brought back.
The colors of many flowers are based on
chemicals with the flavone chromophore or variations of it.
A complicating feature is that the color
changes with pH, and the same chemical can be, and is, responsible for the
red color of roses and the blue of cornflower; the pigment is cyanidin.
This was once attributed to different cytoplasmic pH for the different
flowers, but this difference is nowhere near large enough to cause the
color change.
Only recently has it been discovered that
the actual pigments are aggregates of several cyanin type molecules with
metal ions and other pigments. A complication which should be mentioned is
that the actual species in the flowers is a glycoside, with a sugar
attached to at least one of the hydroxyls. For some of the hydroxyl
groups, loss of a proton would give a molecule for which at least some
resonance contributing forms would not have a charge on oxygen, although
there would still be contributing forms with charge separation. Similar
behavior is responsible for the change in color of morning glories as the
bright blue of morning becomes purple or pink in the evening as the flower
dies and becomes acidic.
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