Question

Acetylene has ......... bonds: (a) \(2 \sigma, 2 \pi\) (b) \(2 \sigma, 3 \pi\) (c) \(3 \sigma, 2 \pi\) (d) \(3 \sigma, 3 \pi\)

Short answer

The correct option is (a) 2 \(\sigma\), 2 \(\pi\).

Step-by-step solution

Understanding Acetylene's Structure

Acetylene (C₂H₂) is a molecular compound with a linear structure. It consists of two carbon atoms triple-bonded to each other and each carbon atom also forms a single bond with a hydrogen atom.

Counting Sigma Bonds

Sigma bonds (\(\sigma\)) are the strongest type of covalent bonds. In acetylene, there is one \(\sigma\) bond in each C-H bond and one \(\sigma\) bond within the C≡C bond. Therefore, acetylene has 2 \(\sigma\) bonds: one for each C-H and one for the C-C.

Counting Pi Bonds

Pi bonds (\(\pi\)) occur when two orbitals overlap side-to-side. In acetylene, the triple bond between the carbon atoms consists of one \(\sigma\) bond and two \(\pi\) bonds. Hence, there are 2 \(\pi\) bonds in acetylene.

Determining the Correct Answer

Based on the above analysis, acetylene contains 2 \(\sigma\) bonds and 2 \(\pi\) bonds. Thus, the correct option is (a).

Key concepts

Molecular Structure

When examining the molecular structure of acetylene, we notice that this compound, with the chemical formula \( \text{C}_2\text{H}_2 \), is quite unique. Acetylene has a linear structure due to the triple bond between the two carbon atoms. Each carbon atom also forms a single covalent bond with a hydrogen atom. This linear shape is characteristic of molecules with triple bonds, as it allows for the triple-bonded atoms to be arranged in a straight line.
Throughout this structure, each atom is sp hybridized, meaning that the carbon atoms use one s orbital and one p orbital to form these bonds, leaving two p orbitals to form the triple bond. Understanding this linear and symmetrical structure is crucial when analyzing the types of bonds present in acetylene.

Sigma Bonds

Sigma bonds, denoted as \( \sigma \) bonds, are covalent bonds that are formed by the head-to-head overlap of atomic orbitals. These are typically the strongest type of covalent bond and are found in all single bonds.
In acetylene, two \( \sigma \) bonds can be identified: one between each carbon and hydrogen (C-H) pair and one within the carbon-carbon (C≡C) triple bond. In a triple bond, only one of the bonds is a \( \sigma \) bond. This is because it forms a direct overlap between the sp hybridized orbitals of the carbon atoms.

• One \( \sigma \) bond from C-H (for each hydrogen)
• One \( \sigma \) bond in C≡C (the primary bond)

Hence, acetylene has two \( \sigma \) bonds, which are foundational to its stability.

Pi Bonds

Pi bonds, represented by \( \pi \) bonds, result from the side-to-side overlap of p orbitals. They are generally weaker than \( \sigma \) bonds but play a crucial role in the formation of double and triple bonds. Within acetylene, the triple bond between the carbon atoms consists of one \( \sigma \) bond and two \( \pi \) bonds.
The \( \pi \) bonds arise from the lateral overlap of the remaining p orbitals that were not used in the formation of the \( \sigma \) bond. This side-to-side overlap offers additional bond strength and helps maintain the linearity and rigidity of the acetylene molecule. Each of these \( \pi \) bonds is vital for the structural integrity and chemical properties of acetylene.

Acetylene

Acetylene, with the formula \( \text{C}_2\text{H}_2 \), is a simple alkyne; a family of hydrocarbons known for their carbon-carbon triple bonds. It is an important raw material in the chemical industry and serves many practical purposes. Because of its triple bond, acetylene is highly unsaturated, which makes it a very reactive molecule.
The presence of both \( \sigma \) and \( \pi \) bonds in acetylene not only contributes to its unique molecular structure but also affects its reactivity and bonding strength. This makes acetylene useful in various applications such as welding (due to combustion), synthesis of other organic compounds, and even in manufacturing plastics. Its industrial relevance and distinctive bonding make acetylene a fascinating compound to study and understand.