Alkadienes and Polyunsaturated Hydrocarbons

The following enol (an sSkene-sAchool) and keto (a fetone) forms of C2H4O differ in the positions for their electrons, but they are not resonance structures. Explain why they are not. 

Many hydrocarbons are known that contain more than one double or triple bond. A hydrocarbon that contains two double bonds is called an alkadiene one that contains three double bonds is called an alkatriene, and so on. Colloquially, these compounds are often referred to simply as dienes or trienes. A hydrocarbon with two triple bonds is called an alkadiyne, and a hydrocarbon with a double and triple bond is called an alkenyne. 

The following examples of polyunsaturated hydrocarbons illustrate how specific compounds are named. Recall from lUPAC rules (Sections 4.5 and 4.6) that the numerical locants for double and triple bonds can be placed at the beginning of the name or inune-diately preceding the respective suffix. We provide examples of both styles. 

The multiple bonds of polyunsaturated compounds are classified as being cumulated, conjugated, or isolated. 

Hydrocarbons whose molecules have cumulated double bonds are called cumulenes. The name allene (Section 5.18) is also used as a class name for molecules with two cumulated double bonds  

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