Alkadienes and Allylic Systems

Not all the properties of aUcenes are revealed by focusing exclusively on the functional group behavior of the double bond. A double bond can affect the properties of a second functional unit to which it is directly attached. It can be a substituent, for example, on a positively charged carbon in an allylic carbo-cation, on a carbon that bears an unpaired electron in an allylic free radical, on a negatively charged carbon in an allylic anion, or it can be a substituent on a second double bond in a conjugated diene. 

17 A TT Molecular Orbital Analysis of the Diels-Alder Reaction 417 

Descriptive Passage and Interpretive Problems 10 Intramolecular and Retro Diels-Alder Reactions 425 

A train can do what its component parts can t do separately. Conjugation (joining together) of chemical structural units alters their properties too. 

Conjugare is a Latin verb meaning to link or yoke together, and allylic carbocations, allylic free radicals, allylic anions, and conjugated dienes are all examples of conjugated systems. In this chapter weTl see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone. 

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