Question

What are the approximate bond angles (a) about carbon in an alkane, (b) about a doubly bonded carbon atom in an alkene, (c) about a triply bonded carbon atom in an alkyne?

Short answer

The approximate bond angles about carbon in a) an alkane are \(109.5^{\circ}\) due to its sp3 hybridization, b) an alkene are \(120^{\circ}\) due to its sp2 hybridization, and c) an alkyne are \(180^{\circ}\) due to its sp hybridization.

Step-by-step solution

Determine the Molecular Geometry of the Carbon Atom

For each type of hydrocarbon, examine the hybridization and bonding around the carbon atom. In an alkane, the carbon atom forms single bonds with four other atoms, which makes it sp3 hybridized. In an alkene, the carbon atom forms a double bond with another carbon atom and two single bonds with other atoms, which makes it sp2 hybridized. Lastly, in an alkyne, the carbon atom forms a triple bond with another carbon and a single bond with another atom, making it sp hybridized.

Determine Bond Angles for Alkanes, Alkenes, and Alkynes

Using the VSEPR theory, we can now determine the bond angles for each type of hydrocarbon: (a) Alkane: The sp3 hybridized carbon forms a tetrahedral geometry around the carbon atom, which gives us an approximate bond angle of \(109.5^{\circ}\). (b) Alkene: The sp2 hybridized carbon forms a trigonal planar geometry around the carbon atom, which gives us an approximate bond angle of \(120^{\circ}\). (c) Alkyne: The sp hybridized carbon forms a linear geometry around the carbon atom, which gives us an approximate bond angle of \(180^{\circ}\). So, the approximate bond angles for the three types of hydrocarbons are: (a) Alkane: \(109.5^{\circ}\) (b) Alkene: \(120^{\circ}\) (c) Alkyne: \(180^{\circ}\)

Key concepts

Hybridization

Hybridization is a concept that helps us understand how atomic orbitals mix to form new hybrid orbitals. These hybrids then form bonds in a molecule. This concept is particularly important in understanding the bonding nature of carbon atoms in hydrocarbons.
In an alkane, the carbon atom undergoes sp3 hybridization. Here, one s and three p orbitals combine to form four equivalent sp3 hybrid orbitals. This allows the carbon to form single bonds with four other atoms. This is a typical feature of alkanes.

• Alkanes: 4 single bonds (sp3 hybridization)

Alkenes, however, display sp2 hybridization. In this case, one s and two p orbitals mix to create three sp2 hybrid orbitals, leaving one unhybridized p orbital. This setup permits the formation of a double bond.

• Alkenes: 1 double bond + 2 single bonds (sp2 hybridization)

Lastly, alkynes involve sp hybridization. Here, one s and one p orbital hybridize, resulting in two sp orbitals, while two p orbitals stay unhybridized. This configuration enables the formation of a triple bond.

• Alkynes: 1 triple bond + 1 single bond (sp hybridization)

VSEPR Theory

VSEPR stands for Valence Shell Electron Pair Repulsion theory. This theory is used to predict the shapes of molecules by considering the repulsions between electron pairs in the valence shell of an atom.
According to VSEPR theory, the geometry of a molecule is determined by repulsions between all electron pairs surrounding a central atom, including both bonding and nonbonding pairs. This theory helps us understand how molecules arrange themselves in space to minimize repulsion.
For example, an sp3 hybridized carbon in an alkane adopts a tetrahedral geometry. This shape minimizes the repulsion between the four orbitals. An sp2 hybridized carbon, common in alkenes, arranges the atoms in a trigonal planar fashion. In alkynes, the sp-hybridized carbon forms a linear geometry with a 180-degree bond angle to minimize repulsion between its bonding pairs.

sp3: Tetrahedral (Alkanes)

sp2: Trigonal Planar (Alkenes)

sp: Linear (Alkynes)

Thus, VSEPR theory provides a simple way to understand the 3D arrangement of atoms in a molecule.

Bond Angles

Bond angles are a crucial aspect in molecular geometry for understanding the spatial organization of atoms within a molecule.
In hydrocarbons, bond angles vary depending on the hybridization of the carbon atoms involved.
For instance, in alkanes, where the carbon is sp3 hybridized, the geometry is tetrahedral, leading to bond angles of approximately 109.5 degrees. This tetrahedral angle arises because sp3 orbitals equally repel each other in a symmetric shape.
In alkenes, the sp2 hybridized carbon causes a trigonal planar arrangement with bond angles of about 120 degrees. This occurs because three sp2 orbitals spread out in a plane to minimize repulsion.
Meanwhile, in alkynes, the sp hybridized carbon assumes a linear shape, resulting in bond angles of 180 degrees. Here, the linear arrangement minimizes repulsive forces aligning in opposite directions.
Understanding bond angles through hybridization and molecular shapes is essential for comprehending the diverse structures of different hydrocarbons and their chemical properties. Depending on the molecule's hybridization, these angles determine how it reacts and interacts with other molecules.

• sp3: 109.5 degrees (Alkanes)
• sp2: 120 degrees (Alkenes)
• sp: 180 degrees (Alkynes)