Cumulated Alkadienes

2-dienes, which have cumulated double bonds, commonly are called allenes. The simplest example is 1,2-propadiene, 

Verification of the chirality of such allenes (originally proposed by van t Hoff in 1875) was slow in coming and was preceded by many unsuccess- 

13 Polyfunctional Compounds. Alkadienes. Approaches to Organic Synthesis 

Examples of Chiral Substances Resulting from Restricted Rotation About Double or Single Bonds8 

The chirality observed in this kind of substituted allene is a consequence of dissymmetry resulting from restricted rotation about the double bonds, not because of a tetrahedral atom carrying four different groups. Restricted rotation occurs in many other kinds of compounds and a few examples are shown in Table 13-3, which includes trans-cycloalkenes (Section 12-7), cycloalkyli-denes, spiranes, and ort/zo-substituted biphenyl compounds. To have enantiomers, the structure must not have a plane or center of symmetry (Section 5-5). 

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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 CH2 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 mentioned, such as the influence on spectroscopic properties (see 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. 

Which IS the most stable arrangement of double bonds m an alkadiene—isolated con jugated or cumulated ... 

Thus the order of alkadiene stability decreases m the order conjugated diene (most stable) isolated diene cumulated diene (least stable) To understand this ranking we need to look at structure and bonding m alkadienes m more detail... 

Our discussion of chemical reactions of alkadienes will be limited to those of conju gated dienes The reactions of isolated dienes are essentially the same as those of individual alkenes The reactions of cumulated dienes are—like their preparation— so specialized that their treatment is better suited to an advanced course m organic chemistry... 

A hydrocarbon that contains two double bonds is called an alkadiene, and the relationship between the double bonds may be described as isolated, conjugated, or cumulated. Isolated diene units are those in which two carbon-carbon double bond units are separated from each other by one or more 5/r -hybridized carbon atoms. 1,4-Pentadiene and 1,5-cyclooctadiene have isolated double bonds ... 

Which is the most stable aiiangement of double bonds in an alkadiene—isolated, conjugated, or cumulated ... 

A further classification is used for the relationships of the double bonds to each other. Thus 1,2-alkadienes and similar substances are said to have cumulated double bonds ... 

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

Alkadienes, or dienes, contain two C—C double bonds that can be cumulated (next to each other), conjugated (separated by one C—C single bond), or nonconjugated (separated by more than one C—C single bond). 

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