CHEM 12B
FOOTHILL COLLEGE
WADE CHAPTER TWELVE OUTLINE
CHAPTER TWELVE: INFRARED SPECTROSCOPY AND MASS SPECTROMETRY
1. The Electromagnetic Spectrum
a. Absorption of em radiation in Infrared region induces changes in vibrational
energy states
b. Energy of photon E=h; mole of photons E=(6.02•1023)h
c. Wavenumbers as a measure of resonant frequency 1/;
IR spectrum 400-4000 cm-1
2. The frequencies and shapes of IR signals are characteristic of the functional groups in
the molecule
a. Stretching frequency is generally proportional to bond order as in Hookes law:
k
n = const
m
where k is force constant for bond and  is reduced mass m1m2/(m1+m2)
b. Signals are complex combinations of various stretching and bending
vibrational modes
c. To be visible in an IR spectrum, a bond vibration must induce a change in the
dipole moment of the molecule.
3. Interpreting IR spectra:
a. The OH/NH stretch≈ 3500 cm-1
b. The C-H stretch ≈ 3000 cm-1
o
Sp2C-H >3000 cm-1; sp3C-H <3000 cm-1
c. The C=O stretch ≈ 1700 cm-1
d. The C-O stretch ≈ 1100 cm-1
e. C=C stretching frequencies ≈ aromatic C=C @ 1600 cm-1; alkene C=C @
1650 cm-1
f. Aldehydes/Nitriles/Alkynes≈ 2920/2200/2200 cm-1 respectively
MASS SPECTROMETRY
4. Schematic of electron ionization mass spectrometer
5. The molecular ion M+
i.
odd mass = 1 or odd number N
ii. even mass = 0 or even number N
6. Estimating #C’s from spectroscopic evidence:
(Molecular weight – weight of heteroatoms) ÷ 13
7. The base peak: most stable cation/ radical cation
common points of cleavage (13.3)
 -cleavage (next to CO)
 carboxylic acids -m/z 45
 alkylbenzenes –m/z 91
8. Relative Abundance and Isotopes: M/Z peaks larger than molecular ion are due to
presence of heavy isotopes
a. M+1
o Carbon 100:1.11
CHEM 12B
FOOTHILL COLLEGE
WADE CHAPTER TWELVE OUTLINE
o Calculating #C’s from ratio M: M+1:
%(M+1)/%(M) ≈ .011 • #C’s
b. M+2
o Chlorine 3:1
o Bromine 1:1
o Oxygen 99.8:0.2
o High Resolution MS
9. Proposing a Molecular Formula using Combination Spectroscopy
LEARNING OUTCOMES:
 Identify functional groups present in an organic molecule from its Infrared Spectrum.
 Assign characteristic signals in an Infrared Spectrum to specific functional groups.
 Calculate the energy of Infrared radiation from its frequency or wavelength and viceversa.
 Use Hooke’s law to predict the stretching frequency of a given bond.
 Find molecular ion and base peak in mass spectrum.
 Interpret relative abundance of M and M+1 or M+2 signals in order to determine
presence of halogen or number of carbon or oxygen atoms, when appropriate.
 Predict stable points of cleavage for a given molecule.
 Use Mass Spectrometry in conjunction with other spectra to elucidate the structure of
an organic molecule whose molecular formula is unknown.
SAMPLE EXAM PROBLEMS:
1) Which of the following is true about the compound whose Mass Spectrum is shown
below:
A) The compound contains bromine.
B) The compound contains chlorine.
C) The compound contains nitrogen.
D) The compound has a molecular weight of 86.
E) None of these can be ascertained from the Mass Spectrum alone.
CHEM 12B
FOOTHILL COLLEGE
WADE CHAPTER TWELVE OUTLINE
2) Propose a stucture for the base peak in the following mass spectrum of 2-heptanone:
3) Consider the spectrum below and tell whether it is consistent with any or all of the
following :
a. Benzoic acid (PhCOOH)
e. Cyclohexylamine (C6H13NH2)
b. Toluene (PhCH3)
f. Aniline (PhNH2)
c. Styrene (PhCH=CH2)
g. Methylaniline (CH3PhNH2)
d. Cyclohexanol (C6H13OH)
h. Cyclohexene
4) Consider the spectrum below and tell whether it is consistent with any or all of the
following :
A) Pentanoic acid (C4H9COOH)
E)
N-methylcyclopentane
B) 3-hydroxycyclohexene
F)
Methylaniline (CH3PhNH2)
C) Styrene (PhCH=CH2)
G)
3-hexyne
D) Cyclohexanol (C6H13OH)
H)
Phenol (PhOH)
CHEM 12B
FOOTHILL COLLEGE
WADE CHAPTER TWELVE OUTLINE
5) Tell which of the following would be expected to show a lower frequency carbonyl
stretching band in its IR spectrum? Explain.