Which parts of the molecule benzophenone are chromophores?
is it the two aromatic rings?
what types of electronic transitions are responsible for the molecule absorbing the light?
To determine which parts of the molecule benzophenone are chromophores, we need to understand the concept of chromophores and the electronic transitions responsible for light absorption.
A chromophore is a group or atom within a molecule that absorbs certain wavelengths of light and imparts color to the molecule. In organic molecules, chromophores typically contain conjugated π-systems. A conjugated π-system refers to a series of alternating single and multiple bonds, usually involving carbon atoms with unhybridized p orbitals.
In the case of benzophenone, it consists of two aromatic rings connected by a carbonyl (C=O) group. The two aromatic rings in benzophenone, which are composed of alternating double bonds (π-bonds), form a conjugated π-system. This conjugated system is responsible for the molecule's chromophoric properties.
Regarding the electronic transitions responsible for light absorption, conjugated π-systems can undergo certain types of transitions. The primary types of electronic transitions involved in the absorption of light by conjugated systems are π → π* transitions and n → π* transitions.
1. π → π* transitions: In this type of transition, an electron from a π-bond (usually the highest occupied molecular orbital, ) is excited to an adjacent π*-antibonding orbital (lowest unoccupied molecular orbital, LUMO) within the same conjugated system. This transition commonly occurs in the ultraviolet (UV) region, resulting in the absorption of light at shorter wavelengths.
2. n → π* transitions: In this type of transition, an electron from a non-bonding (n) orbital or a lone pair orbital of an atom adjacent to the conjugated system is excited to the π*-antibonding orbital. This transition generally involves longer-wavelength light absorption in the visible region.
In benzophenone, electronic transitions primarily occur within the conjugated π-system. The absorption of light by benzophenone is mainly attributed to π → π* transitions, which occur in the UV region. However, it's important to note that the carbonyl group (C=O) can also participate in electronic transitions, specifically n → π* transitions, resulting in some absorption in the visible region.
Overall, in benzophenone, the two aromatic rings serve as the chromophores, and the electronic transitions responsible for light absorption are primarily π → π* transitions within the conjugated π-system, predominantly occurring in the UV region.