Q4.26P - Chemistry: Molecular Nature Of Matter And Change

Question

The allene molecule has the following Lewis structure:

Must all four hydrogen atoms lie in the same plane? If not, what is the spatial relationship among them? Why?

Step-by-Step Solution

Answer

The four hydrogens do not lie in the same plane. They are perpendicular to each other.

Step 1 : Bonding between the central atom and the terminal atom

We know that the  $\mathrm{\pi }$  bond is formed due to sidewise overlapping of the p orbital. Thus for the formation of  $\mathrm{\pi }$  bond the p orbitals need to be in the same plane. The central carbon in the allene molecule is sp hybridized. This means that it has two p orbitals for forming $\pi$   bond.  But the p orbitals will be perpendicular to each other. As a result when the terminal carbons will form $\pi$  bond with the p orbitals of the central carbon, the bonds formed will be perpendicular to each other.

Step 2 :Spatial arrangement of the four hydrogens

The $\mathrm{\pi }$   bonds formed will be perpendicular to each other. The sp hybrid orbitals will be perpendicular to the p orbitals. The terminal carbons are  ${\text{sp}}^{\text{2}}$  hybridised and will have trigonal planar geometry. Therefore the two hydrogen on the left will be perpendicular to the two hydrogen on the left.