Organic Chemistry MCAT Practice Exam

How does UV/Vis spectroscopy indicate the presence of a conjugated π system?

1. By measuring the boiling point

2. By observing color changes

3. By the red shift of light

4. By the conductivity of the compound

The correct answer is: By the red shift of light

The detection of a conjugated π system via UV/Vis spectroscopy is primarily indicated through the phenomenon of a red shift in light. In this context, a red shift refers to the absorption of wavelengths that are longer (lower energy) than might be expected for non-conjugated systems or those with fewer π bonds. Conjugated systems, characterized by alternating single and double bonds, allow for the delocalization of π electrons, which lowers the energy gap between the ground state and the excited state. As the number of conjugated double bonds increases, the energy required to promote an electron from the ground state to the excited state decreases. This causes a shift in the light absorbed from the UV region into the visible range, which can often result in a color change depending on the extent of conjugation. This is what leads to the observed red shift — as conjugation increases, the absorption shifts to longer wavelengths. The boiling point and conductivity of a compound do not provide direct information about the presence of a conjugated system, making them unsuitable indicators for this purpose. Observing color changes, although potentially related to the presence of conjugated systems, does not provide the specific spectroscopic data that the red shift does, as it lacks the quantitative and comparative basis