Problem 99 - Chemistry The Central Science

Question

The Lewis structure for allene is Make a sketch of the structure of this molecule that is analogous to Figure \(9.25 .\) In addition, answer the following three questions: (a) Is the molecule planar? (b) Does it have a nonzero dipole moment? (c) Would the bonding in allene be described as delocalized? Explain.

Step-by-Step Solution

Answer

The allene molecule (C3H4) is nonlinear and not planar due to its hybridization and geometries of the carbon atoms. It has a net dipole moment of zero as the symmetric non-planar structure results in the cancellation of individual dipole moments. The bonding in allene is not delocalized since the electrons are not shared by more than two atoms in the molecule, and all bonds are localized between neighboring carbon atoms.

Step 1: Draw the Lewis structure of allene

To draw the Lewis structure of allene, we start with the central carbon atom (C), and then we continue connecting its neighbors according to the chemical structure C = C = C. Next, we add the hydrogen (H) atoms, two bonded to each of the outer carbon atoms. \[ \text{H}\text{C}=\text{C}=\text{C}\text{H}\] Now that we have the Lewis structure for allene, we can answer the given questions.

Step 2: Is the molecule planar?

The molecule allene is not planar. This is because the central carbon atom is sp-hybridized, with linear geometry, and forms two sigma bonds with the two outer carbon atoms. The outer carbon atoms are sp2-hybridized, which makes a trigonal planar geometry, but they are in perpendicular planes to each other. Therefore, the allene molecule adopts a non-planar arrangement in a bent shape.

Step 3: Determine if the molecule has a nonzero dipole moment

To determine if the molecule has a nonzero dipole moment, we need to analyze the arrangement of electron densities in the molecule. Given the Lewis structure of allene, the positive end of the molecule is at one end with hydrogen atoms, and the negative end is at the other end with the carbon atoms. However, due to the symmetric non-planar structure of the molecule, the dipole moments from each side cancel out each other, resulting in a net dipole moment of zero. So, allene does not have a nonzero dipole moment.

Step 4: Would the bonding in allene be described as delocalized?

The bonding in allene would not be described as delocalized. Delocalized bonding refers to electrons that are shared by more than two atoms in a molecule, like in benzene. In allene, the bonds between carbon atoms are double bonds, where one sigma bond and one pi bond are formed. The sigma bond is localized between the two carbon atoms, and the pi bond is between the two neighboring carbon atoms only. No electrons are shared by all three carbon atoms or delocalized across the entire molecule. Therefore, the bonding in allene is not delocalized.