Solved Practice Questions on UV Visible Spectroscopy | Organic Chemistry PDF Download

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Q.1. Which of the following comparison is correct for solvent shift on the n –>π* transition of acetone?
(a) H₂0 = CH₃0H = C₂H₅0H = CHC1₃ = C₆H₁₄
(b) H₂0 > CH₃0H > C₂H₅0H > CHC1₃ > C₆H₁₄
(c) H₂0 < CH₃0H < C₂H₅0H < CHC1₃ < C₆H₁₄
(d) H₂0 > CH₃0H < C₂H₅0H < CHC1₃ < C₆H₁₄

Correct Answer is Option (c)
H-bonding with ground state in n–> π* results in increase in energy gap & decrease in wavelength.
And as polar solvents show strong H-bonding. So, the correct option is H₂0 < CH₃0H < C₂H₅0H < CHC1₃ < C₆H₁₄.

Q.2. What is the correct order of λmax for n –> π* transition for the R-CN, R-NO₂, and R-N=N-R?
(a) R-CN < R-NO₂ < R-N=N-R
(b) R-CN = R-NO₂ = R-N=N-R
(c) R-CN > R-NO₂ > R-N=N-R
(d) R-CN > R-NO₂ < R-N=N-R

Correct Answer is Option (c)
λmax for n –> π*

Q.3. What is the wavelength range for UV spectrum of light?
(a) 400 nm – 700 nm
(b) 700 nm to 1 mm
(c) 0.01 nm to 10 nm
(d) 10 nm to 400 nm

Correct Answer is Option (d)
Ultraviolet (UV) is an electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays (the visible region fall between 380-750 nm and X- rays region fall between 0.01 to 10nm).

Q.4. What is the correct order of λmax for n –> σ* transition?
(a) R-OH > R-NH₂ > R-SH
(b) R-OH < R-NH₂ < R-SH
(c) R-OH > R-SH > R-NH₂
(d) R-OH < R-SH < R-NH₂

Correct Answer is Option (b)
According to molecular orbital energy diagram for R-OH, R-NH₂, R-SH (shown below), energy level will decrease respectively so λmax will increase.

As energy level for E₁ > E₂ > E₃. So, λ_max---> λ₁ < λ₂ < λ₃.