CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS

Not all the properties of alkenes 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 carbocation, or on a carbon that bears an unpaired electron in an allylic free radical, or it can be a substituent on a second double bond in a conjugated diene. 

Conjugare is a Latin verb meaning to link or yoke together, and allylic carbocations, allylic free radicals, and conjugated dienes are all examples of conjugated systems. In this chapter we ll 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. 

The group CH2=CHCH2— is known as allyl, which is both a common name and a permissible lUPAC name. It is most often encountered in functionally substituted derivatives, and the following compounds containing this group are much better known by their functional class lUPAC names than by their substitutive ones  

The tenn allylic refers to a C=C—C unit. Its -hybridized carbon is called the allylic carbon, and an allylic substituent is one that is attached to an allylic carbon. Conversely, the yp -hybridized carbons of a carbon-carbon double bond are called vinylic carbons, and substituents attached to either one of them are referred to as vinylic substituents. 

Allylic carbocations are carbocations in which the positive charge is on an allyhc carbon. Allyl cation is the simplest aUyhc carbocation. 

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. 

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