Which of the molecules shown above will have its lamda max the longest wavelength?

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To determine which compound has the longest wavelength absorption, we consider the extent of conjugation and the presence of a carbonyl or other electron-accepting group that lowers the energy gap.
Option A: Acetophenone (phenyl-CO-CH3) has a benzene ring conjugated with a carbonyl group, but the conjugation is relatively limited to the immediate benzene–carbonyl system. This provides a moderate pi→pi* transition, but not the most extended system among the options.
Option B: The molecule is simply a styrene-like structure (phenyl–CH=CH2) without a carbonyl or other strongly conjugating acceptor. Although there is some conjugation in the double bond, the lack of a second strong acceptor group keeps the energy gap relatively high, so the absorption occurs at shorter wavelengths.
Option C: This structure extends conjugation across the phenyl ring, the intervening double bond, and the carbonyl-containing fragment (a conjugated enone system). The enlarged pi-system lowers the HOMO–LUMO gap more than in A or B, shifting absorption to longer wavelengths.
In summary, the size and extent of conjugation increase from B to A to C, with C offering the most extended conjugation and thus the longest lambda max. The reasoning hinges on how extended pi delocalization stabilizes the excited state, resulting in absorption of lower energy (longer wavelength) light.

CHEM 110 Lecture Quiz Week 4

Which of the molecules shown above will have its lamda max the longest wavelength?

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