UV - Visible Spectroscopy - Organic Chemistry Questions and Answers

This set of Organic Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “UV – Visible Spectroscopy”.

1. 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
View Answer

Answer: d
Explanation: 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).

2. 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
View Answer

Answer: c
Explanation: 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.

The comparison for solvent shift transition is H20 < CH30H < C2H50H < CHC13 < C6H14

3. 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
View Answer

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

The correct order of λmax for n –> σ* transition is R-OH < R-NH2 < R-SH” width=”630″ height=”309″ class=”alignnone size-full wp-image-224171″ /></a></div>

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4. 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
View Answer

Answer: a
Explanation: λmax for n –> π*

The order for n –> π* transition for R-CN, R-NO2, & R-N=N-R is R-CN < R-NO2 < R-N=N-R” width=”319″ height=”84″ class=”alignnone size-full wp-image-224172″ /></a><br />
Electronegative difference order: 3 < 2 < 1<br />
If different is less than energy gap between n –> π* is also less hence more wavelength.<br />
So, energy gap between n –> π* 3 < 2 < 1<br />
λ<sub>max</sub>–> 3 > 2 > 1<br />
</div>
<p>5. What is the correct order of λ<sub>max</sub> for π –> π* transition for the following three compounds?<br />
a) R-C<span style=

=C-R > R₂C = CR₂ > R-CHO
b) R-C=C-R < R₂C = CR₂ < R-CHO
c) R-C=C-R = R₂C = CR₂ = R-CHO
d) R-C=C-R < R₂C = CR₂ >R-CHO
View Answer

Answer: b
Explanation: MO diagram (C = C), (C = C) & (C = O) and shows the energy gap between π – π*, increases among these three molecules. So λ_max will give the reverse trend.
R-C=C-R < R₂C = CR₂ < R-CHO

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6. What is the correct order of λ_max for n–> π* transition for the following three compounds?
a) RCOOH > RCOOR’ > RCONH₂
b) RCOOH = RCOOR’ = RCONH₂
c) RCOOH = RCOOR’ < RCONH₂
d) RCOOH = RCOOR’ > RCONH₂
View Answer

Answer: b
Explanation:

The order of λmax for n–> π* transition for three compounds is RCOOH = RCOOR’ = RCONH2″ width=”536″ height=”112″ class=”alignnone size-full wp-image-224173″ /></a><br />
As we can see from above diagram: +m1 > +m2; +m3<br />
Here since mesomeric effect shows Bathochromic shift: i.e. a change of spectral band position in the absorption, reflectance, transmittance, or emission spectrum of a molecule to a longer wavelength (lower frequency)<br />
Here, λ<sub>max</sub> –> 3 ≈ 2 < 1.</div>
<p>7. Which of the following is the correct order of λ<sub>max</sub> for n–> π* transition for the given compounds?<br />
<a href=

Find the order of λmax for n–> π* transition for the given compounds” width=”663″ height=”84″ class=”alignnone size-full wp-image-224174″ /></a><br />
a) 1>2>3>4>5<br />
b) 1<2<3<4<5<br />
c) 1>2<3>4<5<br />
d) 1>2>3<4>5<br />
<span class=

View Answer

Answer: a
Explanation: λ_max depends upon +m & I effect, which will lead to a shift to longer wavelength accompanied by increased intensity of absorption represents an electronic level (considering inductive and mesomeric effect) of the ground and excited states and thus a decrease in transition energy.
The correct order of λ_max is 1 > 2 > 3 > 4 > 5.

8. The ultraviolet spectrum of benzonitrile shows a primary absorption band at 224 nm. If a solution of benzonitrile in water, with a concentration of 1x 10^-4 molar, is examined at a wavelength of 224 nm, the absorbance is determined to be 1.30. The cell length is 1 cm. What is the molar absorptivity of this absorption band?
a) 2.3 x 10⁴
b) 3.3 x 10⁴
c) 1.3 x 10⁴
d) 4.3 x 10⁴
View Answer

Answer: c
Explanation:
Benzonitrile in water with:
C = 1 x 10^-4 M
A = 1.30
l = 1 cm
We know, A = ℇCl
ℇ = A/C I = 1.30/ 1X 10^-4 X 1 = 1.34 X 10⁴.

9. The ultraviolet spectrum of benzonitrile shows a secondary absorption band at 271 nm. If a solution of benzonitrile in water, with a concentration of 1×10^-4 molar solution is examined at 271 nm, what will be the absorbance reading (ℇ = 1000) and what will be the intensity ratio, I_O/I, respectively?
a) 0.1, 1.26
b) 0.2, 2.26
c) 0.3, 3.26
d) 0.4, 4.26
View Answer