
The molecular properties of alkadienes depend on the relationship between the double bonds, that is, whether they are cumulated, conjugated, or isolated:

$\ce{CH_3CH=C=CHCH_3}$

$\ce{CH_2=CH-CH=CHCH_3}$

$\ce{CH_2=CH-CH_2-CH=CH_2}$
The properties of a compound with isolated double bonds, such as 1,4-pentadiene, generally are similar to those of simple alkenes because the double bonds are essentially isolated from one another by the intervening $$\ce{CH_2}$$ group. However, with a conjugated alkadiene, such as 1,3-pentadiene, or a cumulated alkadiene, such as 2,3-pentadiene, the properties are sufficiently different from those of simple alkenes (and from each other) to warrant separate discussion. Some aspects of the effects of conjugation already have been mentioneed, such as the influence on spectroscopic properties (Section 9-9B). The emphasis here will be on the effects of conjugation on chemical properties. The reactions of greatest interest are addition reactions, and this chapter will include various types of addition reactions: electrophilic, radical, cycloaddition, and polymerization.