Question

For each of the questions, four choices have been provided. Select the correct alternative. The number of sigma and pi bonds in benzene are (a) \(6 \sigma\) and \(3 \pi\) bonds (b) \(12 \sigma\) and \(3 \pi\) bonds (c) \(9 \sigma\) and \(3 \pi\) bonds (d) \(6 \sigma\) and \(6 \pi\) bonds

Short answer

Answer: In a benzene molecule, there are 12 sigma bonds and 3 pi bonds.

Step-by-step solution

Understand the structure of benzene

The structure of benzene is a planar hexagonal molecule, where each carbon atom is bonded to two other carbon atoms and one hydrogen atom. The carbon-carbon bonds in benzene are not all single bonds (sigma bonds) but rather a mix of single and double bonds (pi bonds). This planar arrangement is also known as aromatic since it undergoes special types of reactions. Let's count the number of sigma and pi bonds in this molecule.

Count the number of sigma bonds

In benzene, there are six carbon atoms and six hydrogen atoms. Each carbon atom is bonded to two other carbon atoms and one hydrogen atom. The carbon-carbon and carbon-hydrogen bonds are sigma bonds. Therefore, there are 6 carbon-carbon sigma bonds (for a total of 6 C-C single bonds) and 6 carbon-hydrogen sigma bonds (for a total of 6 C-H single bonds). This sums up to \(6 + 6 = 12\) sigma bonds.

Count the number of pi bonds

In addition to the sigma bonds, benzene also has pi bonds between carbon atoms. Pi bonds form the second bond in double bonded carbon atoms. In benzene, there are 3 double bonds between adjacent carbon atoms, which accounts for 3 pi bonds.

Select the correct alternative

Based on our analysis, we can now select the correct alternative, which is the one that states benzene has \(12 \sigma\) bonds and \(3 \pi\) bonds. Therefore, the correct answer is (b) \(12 \sigma\) and \(3 \pi\) bonds.

Key concepts

Sigma and Pi Bonds

Let's take a closer look at sigma and pi bonds to really grasp their roles in molecules like benzene. Sigma (\bgreek{\text{\(\text{\bgsigma}\)}}) bonds are single bonds formed by the head-on overlap of atomic orbitals, resulting in the creation of a bonding orbital that lies directly between the two atoms. These bonds are the strongest type of covalent bonds because of this direct overlap, and they allow for free rotation around the bond axis.

On the other hand, pi (\bgreek{\text{\(\text{\bgpi}\)}}) bonds are formed when parallel orbitals overlap and create a bonding orbital above and below the plane of the atoms. These bonds generally form after a sigma bond has been established, and they do not allow for rotation; this is what gives double bonds their rigid structure. In benzene, pi bonds contribute to the 'delocalized' nature of electrons that flow above and below the plane of the molecule, creating a stable structure known as the benzene ring.

Aromatic Compounds

Aromatic compounds, like benzene, have a unique stability due to the circular sharing of electrons within the structure; this is often referred to as aromaticity. Aromatic compounds must follow certain rules, such as having a planar ring structure and a certain number of \bgreek{\text{\(\text{\bgpi}\)}} electrons that adhere to Hückel's rule (which is (4n + 2) \bgreek{\text{\(\text{\bgpi}\)}} electrons, where n is a nonnegative integer).

Benzene, with its six carbon atoms each contributing one \bgreek{\text{\(\text{\bgpi}\)}} electron, conforms to Hückel's rule and thus is aromatic. This aromaticity provides inherent stability and distinct chemical properties, like resistance to certain types of reactions that readily occur in non-aromatic compounds.

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules, and it's classified in several ways, such as ionic, covalent, and metallic bonding. Benzene showcases covalent bonding, where atoms share pairs of electrons.

Covalent bonding comes in two primary forms: sigma (\bgreek{\text{\(\text{\bgsigma}\)}}) bonds and pi (\bgreek{\text{\(\text{\bgpi}\)}}) bonds. As illustrated in the structure of benzene, covalent bonds are essential for the formation of stable molecules with complex geometries, and they explain how atoms can form molecules by sharing electrons to achieve a more stable electron configuration, such as the noble gases.