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.

Short answer

The Lewis structure of allene (C3H4) consists of a central carbon atom connected to two other carbon atoms, each forming a double bond. The central carbon is connected to two hydrogen atoms, and each terminal carbon atom is connected to a hydrogen atom. Allene is not a planar molecule as the geometry is non-planar. It has no net dipole moment as the dipole moments of the polar double bonds cancel each other out. The bonding in allene is not delocalized, as the pi bonds are formed using p orbitals on the carbon atoms which are perpendicular to each other and the electrons involved are not delocalized.

Step-by-step solution

(Draw the Lewis Structure of Allene)

: First, identify the molecular formula for allene. Allene is an organic compound with the formula C3H4. To draw the Lewis structure: 1. Count the total number of valence electrons: Carbon (C) contributes 3 x 4 = 12 electrons, and Hydrogen (H) contributes 4 x 1 = 4 electrons, giving a total of 12 + 4 = 16 electrons. 2. Arrange the atoms: Place one carbon atom in the center, connect another carbon atom on each side on the central carbon atom. 3. Distribute the remaining electrons to complete the octets of the carbon atoms and create double bonds with neighboring carbons. 4. The central carbon atom is connected to two hydrogen atoms, and terminal carbon atoms are connected to one hydrogen atom each.

(Determine if the molecule is planar)

: Now, let us determine the molecular geometry and check if the molecule is planar. The central carbon (C) atom is bonded to 2 other carbon atoms and 2 hydrogen atoms, following the linear molecular geometry with a bond angle of 180° between atoms. Terminal carbon atoms are connected to a carbon atom and a hydrogen atom each, following linear geometry as well. Considering all the atoms, the geometry is non-planar. Thus, allene is not a planar molecule.

(Evaluate the Dipole Moment)

: Each of the double bonds between carbon atoms is polar, as the electrons will not be evenly distributed. However, due to the symmetric distribution of electrons and 180° bond angle between carbon atoms, the dipole moments cancel each other out. Therefore, allene has a net-zero dipole moment.

(Analyze the Bonding as Delocalized or Not)

: In allene, the double bonds (pi bonds) within the molecule are formed using p orbitals on the carbon atoms, lying perpendicular to each other. The electrons involved in these pi bonds are not delocalized, as they are confined to the bond between the respective neighboring carbon atoms. Hence, the bonding in allene is not considered delocalized.

Key concepts

Lewis Structure

The Lewis structure of any molecule helps to visualize its electronic structure and bonding. For allene, which has the chemical formula C₃H₄, it is essential first to determine the total number of valence electrons. Carbon, being a group 14 element, contributes four valence electrons per atom, while hydrogen, from group 1, provides one valence electron each. Utilizing this, the total comes to 16 valence electrons, calculated as follows:

• Carbon: 3 atoms × 4 electrons = 12 electrons
• Hydrogen: 4 atoms × 1 electron = 4 electrons

Distribute these electrons to fulfill the octet rule for the central atoms, forming double bonds between the carbon atoms. The central carbon atom will bond linearly with two other carbon atoms, while the terminal carbon atoms connect to individual hydrogen atoms. Each terminal hydrogen atom completes the duplet rule, while the terminal carbon satisfies the octet.

Molecular Geometry

Understanding the molecular geometry of allene requires a look into how the atoms are spatially arranged. The molecular geometry of allene is described by looking at the central carbon atom, which is linked to two other carbon atoms by double bonds and two hydrogen atoms. This arrangement provides a linear geometry around the central carbon with a bond angle of 180°.

• Central carbon atom: linear with a bond angle of 180°
• Terminal carbon atoms: also follow a linear geometry

While this may appear to be planar, the alignments of p orbitals create a perpendicular arrangement for the pi bonds, making the overall structure non-planar. Hence, allene is not a planar molecule.

Dipole Moment

The concept of dipole moment involves the distribution of charge within a molecule. In allene, the symmetrical arrangement around the central carbon atom with linear geometries leads to an interesting case of polar bonds but non-polar molecule overall. Each carbon-carbon double bond is polar due to uneven electron sharing. However,

• The symmetric linear structure means that the polarities cancel each other out, leading to a zero net dipole moment.
• The 180° bond angles ensure that any dipole caused by polar bonds is effectively neutralized.

Hence, despite having polar covalent bonds, allene exhibits no overall dipole moment, classifying it as a non-polar molecule.

Delocalized Bonding

Delocalized bonding involves electrons spread over several atoms, contributing to structures like aromatic rings. In the case of allene, the double bonds involve pi electrons, which are confined to p orbitals situated perpendicular to the main axis of the molecule.

• Each double bond between carbon atoms shares electrons only between two specific atoms, keeping the pi electrons localized.
• The construction of the allene molecule thus prevents delocalization of these electrons.

As such, the bonding in allene does not exhibit delocalized character. The electron distribution remains within the vicinity of the respective carbon atoms, characterizing the electrons as localized.