What is a delocalized chemical bond

Experiment: Examination of various foods for-carotene
Attempt: Where does the carrot get its color from?
Experiment: bromination of tomato juice

Our environment appears colored to us. This is due to the ability of some types of molecules to absorb light of certain wavelengths, but not other wavelengths of the visible range of light. The non-absorbed wavelengths are perceived by our eyes as color. Such substances are the dyes. We will only deal with the colored hydrocarbons here.

The ability of hydrocarbons to absorb light is often based on the presence of extended ones π-electron systems. In long-chain alkenes, which contain alternating single and double bonds, these electrons can be distributed over all perpendicular p orbitals of the neighboring carbon atoms. Such double bonds, which are separated from each other by only one single bond, are called conjugated Double bonds. One example is 1,3-butadiene. The numbers indicate at which carbon atoms the double bond attaches.

The π electrons are in a system of conjugated double bonds delocalized. So you stay in a common space that is formed from all p orbitals.

π electrons can be excited much more easily than the σ electrons of saturated hydrocarbons. But there should already be a lot of π electrons. While 1,3-butadiene still appears colorless because it only absorbs in the UV range, molecules with many conjugated double bonds can also absorb lower-energy, longer-wave light in the visible range and thus shine for us again colored. There is a for the excitation of π-electron systems and colors acoustic model. Click here.

The carotenoids - colored by 11 conjugated double bonds
Carotenoids belong to the class of tetraterpenes (C40-Body). They have a very extensive π-electron system delocalized over 30 carbon centers, which is particularly easy to excite.

The most famous carotenoid is that -carotenethat not only gives the carrot its orange color. It is also found in all green leaves as an important photosynthesis pigment. It appears orange to us, i.e. it absorbs light in the blue to green spectral range of visible light between 415 and 500 nm.


As a pure hydrocarbon, it is completely non-polar and can therefore only be dissolved in non-polar liquids such as acetone, gasoline or various oils (lipophilic = "oil-loving") (-> experiment).

Reactions at conjugated, delocalized double bonds
Tomato juice can be discolored with bromine water (-> try). This is because the red pigment in tomatoes also has a delocalized, unsaturated π-electron system.

This deep red dye is that Lycopene. It is also a carotenoid.


The conjugated double bonds add bromine just like individual double bonds. The bromine atoms can, however, dock at many different points on the molecule. This interrupts the delocalization of the electron system in a random way. The molecule then first absorbs light of other wavelengths. So you can see all the colors of the spectrum from red to blue. After a while all double bonds are brominated; then no visible light is absorbed at all and the material appears colorless.

Different layers of the tomato juice treated with bromine water.
The bromine slowly migrates from the aqueous layer into the lycopene solution
(Photo: Andreas)

Lycopene forms radicals
With sulfuric acid, -carotene forms blue to blue-black complexes (-> experiment). Here, short-lived radicals are formed on the hydrocarbon chain. This tendency to form radicals can be the reason why lycopene is supposed to protect against radicals.

Further texts on the topic of `` hydrocarbons ''