Related Systems Ketenes and Cumulenes

WORKED PROBLEM 12.2 Two isomers of 1,3-dimethylallene do exist. What are they  

ANSWER There are two isomers of 1,3-dimethylallene, because it is a chiral molecule.The mirror image of 1,3-dimethylallene is not superimposable on the original therefore a pair of enantiomers exists. 

AUenes are high-energy molecules, just as are alkynes. The sp hybridization of the central carbon of this functional group is not an efficient way of using orbitals to make bonds, because 7t bonds are much weaker than o bonds. The 7t bond energy of about 66 kcal/mol is substantially lower than most o bond energies. This inefficiency is reflected in relatively positive heats of formation values). Table 12.1 

TABLE 12.1 Heats of Formation for Some Small Hydrocarbons 

In allenes, the double bonds share a central carbon atom. There are other cumulated molecules in which heteroatoms (noncarbon atoms such as oxygen or nitrogen) take the place of one or more carbons of an allene and/or in which there are more than two double bonds in a row. When one end carbon of allene is replaced with an oxygen, the compound is called ketene (Fig. 12.6). If both end atoms are oxygens, the molecule is carbon dioxide. Cumulated hydrocarbons with three double bonds attached in a row are called cumulenes, although they can also be named as derivatives of 1,2,3-butatrienes (Fig. 12.6). Cumulenes are very reactive and most difficult to handle. 

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