The energy-level diagram in Figure 9.36 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding π orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the p2p to the p*2p molecular orbital. (c) Is the C¬C bond in ethylene stronger or weaker in the excited state than in the ground state? Why?

Verified step by step guidance

1. Identify the type of bonding in ethylene: Ethylene (C2H4) has a double bond between the two carbon atoms, which consists of one sigma (σ) bond and one pi (π) bond. The σ bond is formed by the head-on overlap of sp2 hybrid orbitals, and the π bond is formed by the sideways overlap of p orbitals.
2. Understand the molecular orbitals involved: In the ground state, the π bond is formed by the bonding molecular orbital (π), which is lower in energy and more stable. The antibonding molecular orbital (π*) is higher in energy and empty in the ground state.
3. Consider the effect of photon absorption: When a photon is absorbed, an electron in the bonding π orbital is excited to the antibonding π* orbital. This transition from a lower energy state to a higher energy state affects the electron density in the π bond.
4. Analyze the impact on the C-C bond strength: In the excited state, the presence of an electron in the antibonding π* orbital reduces the overall electron density in the bonding region between the carbon atoms. This reduction in electron density weakens the π bond.
5. Conclude the effect on the overall bond strength: Since the π bond contributes to the overall bond strength between the carbon atoms in ethylene, a weaker π bond in the excited state implies that the overall C-C bond is weaker in the excited state compared to the ground state.

Key Concepts

Molecular Orbitals

Molecular orbitals (MOs) are formed by the linear combination of atomic orbitals (LCAO) when atoms bond. In ethylene, the sideways overlap of p orbitals creates bonding (π) and antibonding (π*) molecular orbitals. The bonding orbital stabilizes the molecule, while the antibonding orbital destabilizes it. Understanding the nature of these orbitals is crucial for analyzing the strength of the C-C bond in different electronic states.

Excited State vs. Ground State

The ground state of a molecule refers to its lowest energy configuration, where electrons occupy the lowest available molecular orbitals. In contrast, the excited state occurs when an electron is promoted to a higher energy orbital, such as from a bonding π orbital to an antibonding π* orbital. This transition can affect the bond strength, as the promotion of an electron can weaken the bond due to the presence of an antibonding orbital.

Bond Strength and Electron Configuration

Bond strength is influenced by the electron configuration of the involved atoms. In ethylene, the C-C bond strength is determined by the occupancy of the bonding and antibonding orbitals. When an electron is excited from a bonding orbital to an antibonding orbital, the effective bond order decreases, leading to a weaker bond in the excited state compared to the ground state. This relationship is essential for understanding the stability of the molecule under different conditions.