Question

Which of the following statements is not correct for \(\sigma\) and \(\pi\) bonds which is formed between two carbon atoms? (a) Free rotation of atoms about s bond is allowed but not in case of a \(\pi\) bond (b) \(\sigma\) bond determines the direction between carbon atoms but a \(\pi\) bond has no primary effect in this regard (c) \(\sigma\) bond is stronger than a \(\pi\) bond (d) Bond energies of \(\sigma\) bond and \(\pi\) bond are of the order of \(264 \mathrm{~kJ} / \mathrm{mol}\) and \(347 \mathrm{~kJ} / \mathrm{mol}\), respectivel

Short answer

Statement (d) is incorrect; F bonds should have higher energy than D bonds.

Step-by-step solution

Understanding Sigma and Pi Bonds

Sigma (FE) bonds are formed by the head-on overlapping of atomic orbitals and allow free rotation of the bonded atoms. On the other hand, pi (DE) bonds result from the side-by-side overlap of orbitals and restrict rotational movement.

Analyzing Statement (a)

Statement (a) asserts that there is free rotation about the F bond, which is correct, and that this rotation is restricted in a D bond, which is also correct.

Analyzing Statement (b)

Statement (b) indicates that the F bond determines the direction of atoms and that the D bond has no effect, which is also true because F bonds are responsible for the orientation, whereas D bonds do not influence the directional alignment directly.

Analyzing Statement (c)

Statement (c) mentions that F bonds are stronger than D bonds, which is generally correct because F bonds involve stronger overlap between orbitals.

Analyzing Statement (d)

Statement (d) provides bond energies, claiming that a D bond has a higher energy than a F bond with values of 264 kJ/mol and 347 kJ/mol, respectively. This is incorrect since F bonds are generally stronger and have higher energy than D bonds.

Key concepts

Bond Rotation

In the world of chemistry, the nature of bonds is crucial in determining molecular structures and properties. Bond rotation is a widely discussed aspect, especially when comparing sigma (\(\sigma\)) and pi (\(\pi\)) bonds.

• Sigma bonds allow for free rotation of the atoms they connect. This is because they form through a head-on overlap of orbitals, resembling a straight line between the nuclei of two atoms. Due to this alignment, the bond can rotate without breaking the overlap, enabling flexibility in molecular shapes.
• Conversely, pi bonds restrict rotation. Pi bonds occur due to the side-by-side overlap of orbitals. This results in electron clouds laying above and below the plane of the atoms, forming a "barrier" that holds the atoms in place. Rotation around this bond disturbs the overlap, which makes such motion energetically unfavorable.

Thus, while sigma bonds, with their rotational freedom, provide molecules with structural flexibility, pi bonds offer rigidity, crucial for maintaining specific molecular geometries.

Bond Strength

Bond strength is a fundamental concept that explains how robust a bond is between two atoms. It inherently affects the stability and reactivity of molecules.

• Sigma bonds are generally stronger than pi bonds. This strength comes from the direct overlap of orbitals, allowing electrons to maximize the area of interaction between the atomic nuclei. The head-on overlap often results in stronger bonds, providing stability and resistance to breaking.
• On the other hand, pi bonds have a weaker nature due to their side-by-side overlap. The electron interaction is not as extensive as in sigma bonds, which makes them generally weaker and more reactive.

Understanding the strength differences is key for predicting how a molecule will behave in reactions. It also affects how molecules absorb energy and change under different conditions.

Bond Energy

Bond energy refers to the amount of energy required to break a bond between two atoms in a molecule. This energy reflects the bond's strength and stability.

• In general, sigma bonds have higher bond energies compared to pi bonds. This corresponds to their stronger interaction and overlap between atomic orbitals. The higher energy required to break sigma bonds ensures they contribute to the molecule's structural integrity.
• Pi bonds, being weaker, demand less energy to break. As a result, they are more susceptible to being broken during chemical reactions. The lower energy requirement reflects their secondary role in bonding, usually appearing in addition to sigma bonds.

Thus, knowing the bond energies of sigma and pi bonds helps in understanding the energetic profile of chemical reactions and molecular stability.

Molecular Orientation

Molecular orientation describes how atoms are spatially arranged in a molecule, which profoundly affects the molecule's chemical properties and reactivity.

• The sigma bond plays a critical role in determining the direction of the bonded atoms. This is due to its linear and direct overlap of orbitals, which sets the atoms' alignment along the bond axis. Such control impacts the overall shape and orientation of the molecule.
• Pi bonds, however, do not primarily influence molecular orientation. Since they occur in addition to sigma bonds (in double and triple bonds), their role is more about adding rigidity by locking the molecule in its place rather than directing its geometry.

Molecular orientation influences how molecules interact with each other, affecting everything from reactivity to physical properties like boiling and melting points. Understanding these bonding principles enriches knowledge about the three-dimensional nature of chemical compounds.