Q.22: UV Spectroscopy Transitions Analysis
Question Statement
The UV spectrum of 2-butanone and the UV spectrum of methyl vinyl ketone (MVK) are independently recorded and compared. Among the various λmax, 185 nm, 219 nm, 277 nm and 324 nm, the absorption at λmax = 324 nm is due to
(A) n→π* transition in 2-butanone
(B) n→π* transition in MVK
(C) π→π* transition in 2-butanone
(D) n→σ* transition in 2-butanone
2-Butanone shows UV absorptions around 185 nm (π→π*) and 277 nm (n→π*), while methyl vinyl ketone (MVK), an α,β-unsaturated ketone, exhibits a bathochromic shift to 219 nm (π→π*) and 324 nm (n→π*) due to conjugation.
Option Analysis
Option (A) n→π transition in 2-butanone*
Simple saturated ketones like 2-butanone have n→π* transitions near 270-280 nm, not 324 nm, as the carbonyl lacks conjugation to lower the energy gap.
Option (B) n→π transition in MVK*
MVK’s conjugated enone system extends the chromophore, shifting the weak n→π* band to longer wavelengths around 324 nm.
Option (C) π→π transition in 2-butanone*
The intense π→π* transition in 2-butanone occurs below 190 nm (around 185 nm), far shorter than 324 nm.
Option (D) n→σ transition in 2-butanone*
n→σ* transitions in ketones appear at much shorter wavelengths (<150 nm), not in the visible UV region like 324 nm.
UV Transitions Explained
Carbonyl compounds undergo three main UV transitions: π→π* (intense, ~180-200 nm), n→π* (weak, ~270-290 nm for saturated ketones), and n→σ* (very short wavelength). Conjugation in α,β-unsaturated ketones like MVK delocalizes π electrons, reducing the HOMO-LUMO gap and red-shifting both bands: π→π* to ~220 nm and n→π* to ~320 nm.
Compound π→π* (nm) n→π* (nm)
2-Butanone 185 277
MVK 219 324
Introduction to UV Spectroscopy in Carbonyl Compounds
UV spectroscopy transitions reveal electronic structures in organic molecules like 2-butanone and methyl vinyl ketone (MVK). Saturated ketones absorb at shorter λmax, while conjugation in MVK causes bathochromic shifts, key for identifying n→π* at 324 nm.
Key Absorption Bands Compared
185 nm: π→π* in 2-butanone’s isolated carbonyl.
219 nm: π→π* in MVK’s conjugated enone system.
277 nm: n→π* in 2-butanone.
324 nm: n→π* in MVK due to extended conjugation.
Implications for CSIR NET Life Sciences
Understanding these shifts aids in spectral interpretation for exams. Conjugation lowers transition energy, making α,β-unsaturated ketones detectable at longer λmax.
