Chapter 3. ELECTRONIC STRUCTURE OF ORGANIC MOLECULES

There are many organic compounds whose molecules contain more than one multiple (double or triple) bonds. According to mutual arrangement, the multiple bonds of polyunsaturated compounds are classified as being cumulated, isolated, and conjugated. These will be considered by the simplest examples, namely, alkadienes that are usually referred to simply as dienes.

The term cumulated means that one carbon participates in two double bonds; in other words, these bonds follow one after another. This type of bonding occurs very rarely among the natural products. If at least one saturated carbon atom intervenes between the double bonds of a diene, these bonds are regarded as isolated. Thus, 1,2-butadiene is an example of cumulated dienes, and 1,4-pentadiene represents isolated dienes.

3.1. CONJUGATION AS STABILIZING FACTOR OF MOLECULES

The most interesting are dienes with conjugated double bonds, i. e. 1,3-dienes where the double and single bonds alternate in the chain. There are many polyunsaturated compounds with conjugated double bonds, which play an important role in nature and biology. For example, β-carotene is a yellow-orange pigment in carrots that involves eleven conjugated double bonds:

Other examples of highly conjugated systems among biologically active compounds are retinol (vitamin A) and retinal, the latter being a substance responsible for vision.

3.1.1. π,π Conjugation

Conjugated dienes are similar to nonconjugated dienes (and alkenes) in many but not all of their chemical properties. The former are somewhat more stable than nonconjugated dienes. An explanation for the higher stability of conjugated dienes can be given in describing molecular orbitals of the conjugated systems by example of 1,3-butadiene, a simplest conjugated diene (Fig. 3.1, a).