The key difference between allene and cumulene is that allene contains two double bonds, whereas cumulene contains three double bonds.
Both allene and cumulene are organic compounds. These are alkenes having double bonds between carbon atoms. Allene has three carbon atoms, and there are two double bonds between these three carbon atoms. Cumulene, on the other hand, contains four carbon atoms and there are three double bonds between them.
What is Allene?
Allene is an organic compound having two double bonds between three carbon atoms. In other words, there are three carbon atoms per allene molecule, and there is a C=C=C- chemical bond. The terminal carbon atoms are attached to two hydrogen atoms per carbon atom. The general structure is as follows:
The central carbon atom in this allene molecule shows sp hybridization and the two terminal carbon atoms have sp2 hybridization. Since the bond angle of C=C=C bond is 180 degrees, the molecule appears as a linear molecule. The two-terminal carbon atoms show planar geometry. We can observe this molecule in an extended tetrahedral structure.
Allene can be synthesized using specialized methods. The parent molecule of allene is propadiene, and it can be produced industrially on a large scale as an equilibrium mixture with methylacetylne. This mixture is called MAPP gas, which is commercially available. There are some laboratory methods we can use to synthesize allene; from the reaction of certain terminal alkynes with formaldehyde, copper(I) bromide, and added base, from the dehydrohalogenation of certain dihalides, etc.
What is Cumulene?
Cumulene is an organic compound having three double bonds between four carbon atoms. In other words, this compound has four carbon atoms bonded in a linear pattern having double bonds with each other, and the two terminal carbon atoms have two hydrogen atoms bonded to the carbon atom. This structure is called a cumulative double bonded structure. Rather than naming these compounds as alkenes, we categorize them as cumulenes. Unlike most of the alkanes and alkenes, cumulenes are rigid substances and are comparable with alkynes.
The well-known method for the synthesis of cumulenes is the reductive coupling of a geminal dihalovinylidene. The first reported synthesis of a cumulene compound was in 1921, the synthesis of a butatriene.
The cumulene compounds are rigid because of the presence of two central carbon atoms that carry two double bonds. These carbon atoms have sp hybridization which results in two pi bonds one to each neighbouring carbon atom. Therefore, cumulene compounds have linear geometry. If there are nonequivalent substituents at the terminal carbon atoms rather than hydrogen atoms, then we can observe isomerism there.
What is the Difference Between Allene and Cumulene?
Allene and cumulene are organic hydrocarbon compounds that we can categorize as alkenes. The key difference between allene and cumulene is that allene contains two double bonds, whereas cumulene contains three double bonds. However, both are linear compounds because there are double bonds at the centre of the molecules.
The following table lists more differences between allene and cumulene.
Summary – Allene vs Cumulene
Allene and cumulene are organic hydrocarbon compounds that we can categorize as alkenes. The key difference between allene and cumulene is that allene contains two double bonds whereas cumulene contains three double bonds.
1. “Allene.” An Overview | ScienceDirect Topics, Available here.
1. “Allene” (Public Domain) via Commons Wikimedia
2. “Butatriene-3D-balls” By Jynto and Ben Mills – Derived from File:Ethylene-CRC-MW-3D-balls.png. (Public Domain) via Commons Wikimedia