Previous years questions from Spectroscopy

1. Previously asked Questions
from Spectroscopy
Presented by
Dr. Abiram

2. 1. The absorbance of solution having 20%
transmittance is
a. 0.301
b. 0.699
c. 1.301
d. 1.699

3. 2. In a broad band decoupled 13C NMR
Spectrum, the number of signals appear for
 1. Catechol
 2. Resorcinol
 3. Hydroquinone are:
a) Six, four and two
b) Six, six and four
c) Three, four and four
d) Three, four and two

4. 3. The order of carbonyl stretching frequency in
the IR spectra of ketone, amide and anhydride
a. Anhydride > Amide> Ketone
b. Ketone > Amide > Anhydride
c. Amide > Anhydride > Ketone
d. Anhydride > Ketone > Amide

5. 4. The absorption at λmax 279 nm for Acetone
is due to
a) n __> π*
b) π __> π*
c) n __> σ*
d) σ __> σ*

6. 5. A liquid compound gave a mass spectrum
showing a strong molecular ion at m/z = 156.
The only fragment ions are seen at m/z = 127
& 29.
 What are the fragments and what is
the structure?

7. 6.
 A liquid compound gave a mass
spectrum in which the molecular ion
appears as a pair of equal intensity
peaks at m/e = 122 & m/z = 124.
Small fragment ion peaks are seen at
m/z = 107 & 109 (equal intensity),
and at m/z = 79, 80, 81, & 82 (all
roughly the same size). Large
fragment ions are seen at m/z = 43
(base peak), 41 & 39. Suggest the
name of the compound

8. a. 2-chloro propane
b. 1-bromo propane
c. 2-chloro butane
d. 1-bromo butane

9. 7. The number of normal modes
of vibration for C6H6 would be
a. 20
b. 30
c. 40
d. 50

10. 8. Which classes of compounds are found in
the highly deshielded region of an NMR
spectrum?
a. Sp3 hybridised alkanes
b. Alkenes and SP hybridised alkynes
c. TMS
d. Carboxylic acids and Aldehydes

11. 9. A carbon atom has three hydrogens attached to it,
and its nearest neighbor has one hydrogen. The NMR
spectrum will contain at least -
 a) a triplet with an area of 3 and a
singlet with an area of 1
 (b) a singlet with an area of 3 and a
singlet with an area of 1
 (c) a quartet with an area of 1 and a
doublet with an area of 3
 (d) a quartet with an area of 3 and a
doublet with an area of 1

12. 10. How many absorptions will the
following compound have in its carbon
NMR spectrum?
a. 3
b. 4
c. 5
d. 6

13. 11. The CMR spectrum of an unknown compound
shows 6 absorptions and the PMR spectrum
shows 5 absorptions. Which of the following
compounds is the unknown compound?

14. 12. The CMR spectrum of an unknown compound
shows 4 absorptions and the PMR spectrum
shows 4 absorptions. Which of the following
compounds is the unknown compound?

15. Answer Explanantion
 There should be equal numbers of
absorptions in both spectra and there
are 4 unique positions in the molecule.
Note that the molecule is symmetrical
with respect to the red line...

17. 13.In 2,2,4-trimethyl pentane the base
peak is observed at m/z
a. 43
b. 41
c. 28
d. 57

18. 14.The base peak formed for the friedal-crafts
alkylation of bromobenzene is observed at
m/z
a. 156 & 158
b. 171 & 173
c. 77 & 79
d. 91 & 93

19. 15. Which of the following bonds would
show the strongest absorption in the IR?
a. carbon-oxygen
b. Oxygen-hydrogen
c. Nitrogen-Hydrogen
d. Sulfur- hydrogen

20. 16. The compound that exhibit sharp bands at
3300cm-1and 2150cm-1 in IR Spectra is
a. 1-Butyne
b. 2-Butyne
c. Butyronitrile
d. Butylamine

21.  Nitrile C=N Stretch 2260 – 2120
 =C-H Stretch : 3100-3300 cm-1

23. 17. The H NMR spectrum of a dilute solution of a
mixture of Acetone and dichloromethane in CDCl3
exhibits two singlets of 1: 1intensity. Molar ratio of
Acetone to dichloromethane is
a. 3:1
b. 1:3
c. 1:1
d. 1:2

24. 18. Intense band generally observed for a
carbonyl group in a IR spectrum is due to
a. The force constant of CO bond is large
b. The force constant for CO bond is small
c. There is no change in dipole moment for
CO bond stretching
d. The dipole moment change due to CO
bond stretch is large

25. Explanation
 Bond Vibration The energy of the
infrared light can interact with the
resonant vibrational frequency of the
bond Since different bonds have
different energies, they require
different energy to cause vibration.

27.  Intensity of Absorbance
 Intensity of light absorbed by a
molecule is related to the dipole of the
bond
 The greater the dipole, the greater the
absorbance intensity C-O bond
stretches are therefore more intense
than C-C stretches

28. 19. The mass of metastable ion produced due
to decomposition of F1in the following mass
fragmentation sequence is
Diethylphthalate F1 + F2 + CO
222 177
a. 141.2
b. 125.4
c. 45
d. 210.2

29. M1 = Molecular ion mass
m1 = daughter ion peak

30. Answers:
1. B
2. D
3. D
4. C
5. A
6. B
7. B
8. D

31. 9. C 17. A
10. D 18. D
11. B
12. A
13. D
14. A
15. B
16. A