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 allene molecule is non-planar, with two perpendicular planes intersecting at the central carbon atom. It has no nonzero dipole moment, as the symmetric structure causes the dipole moment vectors to cancel out, resulting in a net dipole moment of zero. The bonding in allene is not described as delocalized because electrons are localized in the carbon-carbon double bonds and carbon-hydrogen single bonds, and not shared across multiple bonded atoms like in benzene.

Step-by-step solution

Count Valence Electrons

In order to draw the Lewis structure for allene, we first need to count the total number of valence electrons available for bonding. Allene has three carbon atoms and four hydrogen atoms. The valence electrons for each carbon atom are 4, and the valence electrons for each hydrogen atom are 1. So, the total number of valence electrons in allene is: (3 Carbon atoms x 4 valence electrons) + (4 Hydrogen atoms x 1 valence electron) = 12 + 4 = 16 valence electrons

Create the Skeletal Structure

Next, we will create the basic skeletal structure of allene by connecting the atoms with single bonds (2 electrons per bond). The central carbon atom is bonded with the two other carbon atoms, and each terminal carbon atom is bonded to two hydrogen atoms. - C = C = C - Each hydrogen atom is connected to one terminal carbon atom.

Complete the Octets

Now, we need to complete the octets of all the atoms in the structure by adding the remaining valence electrons as lone pairs. Place the remaining 8 electrons (16 - 8 = 8) around the carbon atoms to give them a complete octet. C=C=C Each carbon atom has a complete octet, and the hydrogen atoms satisfied with the two electrons they share with the carbon atoms. The Lewis structure for allene is now complete. Now that we have the Lewis structure, we can sketch the allene molecule and answer the three questions.

Sketch the Allene Molecule

Based on the Lewis structure and according to the Figure 9.25 style, we can draw the allene molecule like this: H \ C = C = C \ H

Answering Questions

(a) Is the molecule planar? No, the molecule is not planar. The two carbon atoms at the terminal ends and the hydrogen atoms attached to them form two perpendicular planes intersecting at the central carbon, forming a non-planar allene molecule. (b) Does it have a nonzero dipole moment? No, allene does not have a nonzero dipole moment. Allene is symmetric, meaning that the dipole moment vectors caused by the carbon-hydrogen and carbon-carbon bonds cancel each other out, resulting in a net dipole moment of zero. (c) Would the bonding in allene be described as delocalized? Explain. No, the bonding in allene cannot be described as delocalized. In allene, the electrons are localized in the carbon-carbon double bonds and carbon-hydrogen single bonds. In a delocalized bonding system, electrons are shared and spread out across multiple bonded atoms. This occurs in molecules like benzene where electrons are distributed over more than two atoms. Allene does not have such a bonding system, indicating that its bonding is not delocalized.

Key concepts

Allene Molecule

The allene molecule is an interesting compound featuring a unique arrangement of atoms. It is a small organic molecule comprising three carbon atoms connected in a linear arrangement with two double bonds. Each of the terminal carbon atoms ( C ) forms a bond with two hydrogen ( H ) atoms. The structure can be depicted as:

• C=C=C

The central carbon is connected to the other two carbons by double bonds, forming a straight line, which is crucial for defining its geometric shape and electronic properties. With its distinctly linear form, the allene molecule presents various unique properties that are worth exploring.
Despite its simple appearance, it exhibits fascinating molecular behavior, such as interactions between the orbitals of its carbon atoms, contributing to its structural characteristics. Understanding the molecular structure and behavior of allene helps in grasping basic concepts in organic chemistry, such as hybridization and bonding.

Valence Electrons

Valence electrons are vital in determining how atoms bond to form molecules, and they are key components in understanding the chemical behavior of molecules such as allene. In the case of allene, the total number of valence electrons is determined by considering the electrons available from both carbon and hydrogen atoms.

• Carbon: Each carbon atom has 4 valence electrons.
• Hydrogen: Each hydrogen atom has 1 valence electron.

Consequently, when calculating for allene's valence electrons, we find: \[ \text{Valence electrons = (3 \times 4) + (4 \times 1) = 12 + 4 = 16} \]
This number of electrons helps in forming the bonds seen in allene, namely the two carbon-carbon double bonds and the carbon-hydrogen single bonds.
Understanding valence electrons allows chemists to predict the arrangement of atoms and the types of bonds formed in a molecule, establishing the foundation for Lewis structure creation and molecular geometry analysis.

Molecular Geometry

Molecular geometry is crucial for understanding the physical properties and reactivity of a molecule. When examining allene, it is important to consider that its geometry is not exactly straightforward despite its linear electron arrangement.
Allene is formed by a central carbon atom double-bonded to two terminal carbons. Due to sp hybridization of the central carbon atom, allene's molecular geometry is linear around the central carbon. However, the two outer carbons exhibit sp² hybridization, leading to an interesting three-dimensional structure. These characteristics give rise to two planes in which the terminal carbon atoms' substituents, the hydrogen atoms, are perpendicular to each other. The resulting non-planar structure affects the overall geometry:

• The central carbon and the two terminal carbons form a straight line.
• The planes formed by hydrogen atoms connected to each terminal carbon are perpendicular.

Understanding molecular geometry aids in predicting molecule behaviors, such as polarity and potential interactions with other substances.

Dipole Moment

The concept of a dipole moment is essential for understanding molecular polarity, which affects how a molecule interacts with electric fields and other molecules. A dipole moment arises when there is an uneven distribution of electron density, leading to partial charges within the molecule.
For allene, however, this is not the case. Despite its complex geometry, allene remains a nonpolar molecule. Due to its symmetrical structure, specifically the arrangement and equality in the pull of electrons around the linear central axis, any dipole moments arising from the carbon-hydrogen or carbon-carbon bonds cancel each other out.
The key points of dipole moment in allene:

• Symmetry: The linear symmetry of allene ensures that polar bonds do not result in a polar molecule.
• No net dipole: Because of the cancellation of molecular dipole moments, allene has a zero net dipole moment.

This absence of a net dipole moment reflects why allene doesn't exhibit polar behavior, influencing its solubility and interactions with other compounds in a range of environments.