Electronic Structure and Reactions of Alkenes

The simplest reaction of alkenes, the Z - isomerization, was already mentioned in the previous section. The transformation in which one isomer (the reactant) is rearranged to give another isomer (the product) by rotation of 180° requires heating to temperatures above 200 °C in most cases. 

However, many cis-trans (Z-E) isomerizations of natural products are known to occur under mild conditions in living cells. These reactions proceed via different mechanism, utilizing light rather than heat. The well known example is the reaction which occurs in the eye retina. This reaction represents a fundamental step in the process of visual perception. 

The molecule of the derivative retinal (see figure below) is an alkene with conjugated double bonds. As we have already mentioned the conjugated molecules have the property to absorb visible light. Upon the absorption of a photon the retinal molecule is transformed from the Z- to the -isomer. This change in the molecular stereochemistry triggers the signal by which the information about the absorption of a photon is transferred to the nervous system. 

Such photoisomerizations are typical of alkenes and can be explained by a shift of one electron from the bonding % to the antibonding jt orbital. This process is shown in following diagram. 

A molecule in which electrons, after absorption of a photon, become shifted to higher orbitals (for instance into Jt or o orbital) is said to be in an electronic excited state. In the example represented in the figure above the excited molecule is described with the structure A. Since the % orbital has an antibonding character as we have already discussed, the double bond in A is broken and in A only the single o bond remains. Because of the almost free rotation around the single bond (see chapter about conformations) the molecule can be converted to the new cis (or Z) configuration in which all atoms lie in the same plane and the tt-bond can be reformed. 

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