Allene Chemistry

The simplest cumulated diene is 1,2-propadiene, CH₂=C=CH₂, also known as allene. Indeed, cumulated dienes are often called allenes. The central carbon in such compounds is sp-hybridized (it has only two bonding partners), and the double bond array is linear as a result. Since the π-bonds of allenes are orthogonal, the planes defined by the end carbon substituents are also orthogonal. As shown in the following diagram, the overall configuration of allenes resembles that of an elongated tetrahedron. An interesting consequence of this configuration is that allenes having two different substituents on each of the terminal carbon atoms are chiral.

The above diagram shows an allene with different substituents (A & B) on each of the terminal (sp²) carbon atoms. The enantiomeric configurations are displayed relative to a mirror plane placed to illustrate their mirror-image relationship. To assign a stereochemical prefix, i.e. R or S, to these configurations we must view them from one end (it doesn't matter which), as shown in the Newman-like projection on the right. If the sequence order of substituents is A > B, then the two substituents nearest the viewer are assigned a ranking of 1 (A) and 2 (B), while the remote substituents are given rankings of 3 (A) and 4 (B). Applying the viewing rule then leads to the configurational notation shown above. This procedure may be used even when the A & B substituents on one sp² carbon are different from those on the other sp² carbon.

More than two double bonds may have a cumulated structure, as we find in 1,2,3-butatriene (CH₂=C=C=CH₂) and 1,2,3,4-pentatetraene (CH₂=C=C=C=CH₂). The carbon atoms in such cumulenes all have a linear configuration, but the configuration of the terminal substituents depends on the number of cumulated double bonds. For an even number of double bonds, an orthogonal configuration of terminal substituents (as in allene) will be observed. For an odd number of double bonds, the terminal substituents and all the carbons between them will lie in a plane. If the terminal substituents at each end are different, the even double bond compounds will have enantiomeric stereoisomers; whereas, the odd double bond compounds will exist as cis-trans diastereoisomers.

Some instructive physical properties of a simple cumulated diene, 1,2-butadiene, compared with its conjugated diene and alkyne isomers are presented in the following table. From the heats of hydrogenation we see that the methylallene is thermodynamically the least stable of these isomers, with the conjugated diene being most stable. The ionization potential is intermediate between the alkyne and the conjugated diene (note than an electron volt is equivalent to 23.05 kcal/mol), suggesting that the pi-electrons in the allene are less strongly bound than in the alkyne. Finally, the gas phase basicity or proton affinity is close to that of the conjugated diene, and slightly greater than that of the alkyne.