Rank each set of compounds in order of decreasing lmax: a.

Verified step by step guidance

1. Identify the conjugated systems in each compound. Conjugation generally increases the wavelength of maximum absorption (λmax).
2. Compound A has two phenyl rings connected by a single bond, allowing for some conjugation.
3. Compound B has a phenyl ring conjugated with a vinyl group, which is a smaller conjugated system compared to A.
4. Compound C has a single phenyl ring with no additional conjugation, likely resulting in the lowest λmax.
5. Compound D has an extended conjugated system with two phenyl rings and a vinyl group, likely resulting in the highest λmax.

Key Concepts

Lambda Max (λmax)

Lambda max (λmax) refers to the wavelength at which a compound exhibits maximum absorbance in UV-Vis spectroscopy. It is influenced by the electronic structure of the molecule, particularly the presence of conjugated systems, which allow for greater delocalization of π electrons. Compounds with extended conjugation typically show higher λmax values due to lower energy gaps between the ground and excited states.

Conjugation

Conjugation is the overlap of p-orbitals across adjacent double bonds or lone pairs, leading to a delocalization of π electrons. This delocalization stabilizes the molecule and lowers the energy required for electronic transitions, resulting in a higher λmax. The more extensive the conjugated system, the greater the shift in λmax towards longer wavelengths (red shift).

Substituent Effects

Substituents on a benzene ring can significantly influence the electronic properties of the compound, affecting its λmax. Electron-donating groups (EDGs) can increase electron density in the conjugated system, leading to a higher λmax, while electron-withdrawing groups (EWGs) can decrease electron density, resulting in a lower λmax. Understanding these effects is crucial for predicting the order of λmax in different compounds.