CHM138H1 Chapter all: Conjugated Pi Systems and Pericyclic Reactions
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CHM136H1 Full Course Notes
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Document Summary
Dienes, or alkadienes, possess two double-bonded carbons. These bonds consist of one and one bond. There are three types of dienes: cumulated, conjugated, and isolated. Cumulated dienes, or allenes, have adjacent bonds connected to a mutual carbon. Conjugated dienes have bonds separated by one bond. Isolated dienes have their bonds separated by two or more bonds. The reactivity of dienes depends on the proximity of their bonds. In cumulated dienes, the two bonds are oriented in mutually perpendicular planes so that there can be no continuous molecular orbital that ranges from one end of the molecule to the other. Conjugated dienes contain one continuous system of four overlapping p orbitals. Conjugated dienes can be prepared from allylic halides and from dihalides by an elimination mechanism. Conjugated dienes have shorter bond lengths than the normal single-bonded carbons. This is because they have an overlap of two sp2-hybridized orbitals which have their electron density closer to the nucleus.
