Q55E - Organic Chemistry (Mcmurry)

Question

A cumulene is a compound with three adjacent double bonds. Draw an orbital picture of a cumulene. What kind of hybridization do the two central carbon atoms have? What is the geometric relationship of the substituents on one end to the substituents on the other end? What kind of isomerism is possible? Make a model to help see the answer.

${\mathbf{R}}_2\mathbf{C}\mathbf{=}\mathbf{C}\mathbf{=}\mathbf{C}\mathbf{=}{\mathbf{CR}}_2$

    A cumulene

Step-by-Step Solution

Answer

The two central carbon atom has ${\mathrm{sp}}^{\mathrm{2}}$ hybridization.

The substituents on one end to the substituent on the other end shows cis trans isomerism.

Step 1: Allene structure

Allenes are the organic compound in which one carbon atom is attached to the adjacent carbon atom by double bond.

Cumulative double bond is seen in allenes. Allenes does not show planar structure because of steric hindrance of the overlapping unoccupied p-orbitals of double bonded carbon atoms.

Step 2: Cumulene structure

The given structure of cumulene shows that it has three double bonds between four carbon atoms.

Carbon atoms of cumulene shows ${\text{sp}}^{\text{2}}{\text{\&sp}}^{\text{3}}$ hybridization. 

Step 3: Stereochemistry of cumulene

As we see, that cumulene has three double bond hence it is capable of showing geometrical isomerism cis and trans. In the allene system if the same priority group lies on the same side of double bond then it shows cis isomerism.