Download Chapter 12: Infrared Spectroscopy

Chemistry 241
Chapter 12: Mass Spectrometry and Infrared Spectroscopy
HW: 28, 30, 43(c, d, i, j), 45, 49(ignore 1400-1000 region), 54(a,b), 56, 60, 63, 65, 66
I.
Review Index of Hydrogen Deficiency (Sometimes called Degrees of
Unsaturation)
A. Rules
1. every “n” carbons, 2n+2 hydrogens should be present
2. every group V element (N, P, etc), add 1 additional hydrogen
3. every group VI element (O, S, etc.), no change in H required
4. every halogen, reduce the number of H’s required by 1
5. the difference between the number of H’s required using these
rules and the actual # H’s present in formula and divide that
number by two
6. this is the number of pi bonds and/or rings present
7. if four or more, possibility of an aromatic
B. Practice
C5H10ClBr
II.
C6H7N
C8H16
C3H5NO
C4H10S
Mass Spectrometry (MS)
A. How does it work? (not for memorization!)
1. Molecule bombarded with high energy electrons and falls apart
2. charged pieces collected
3. analyzed for mass to nearest whole amu
4. fragments may continue to break up (more than once)
5. some very expensive MS machines do more significant figures
B. Terms
1. fragments: charged pieces collected
2. m/z: mass/charge charge is typically +1
3. molecular ion peak (M): mass of the molecule
4. M+1 peak: small peak 1 higher mass unit than M due to 13C
5. M+2 peak: peak 2 higher mass units than M due to isotopes of
Br and Cl
6. heterolytic cleavage: both electrons go with more
electronegative atom as bond breaks
7. hemolytic cleavage: electrons split between two atoms
C. Fragmentation
1. After having an electron removed, molecule is unstable and
fragments into a cation and a radical
2. more stable fragments usually formed in higher abundance
3. weaker bonds usually break more often
4. lone pair electron (e-) usually dislodged over bonding e-
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5. C-C, C-H bonds break heterolytically
6. C-O, C-Br, C-Cl bonds break homolytically
7. M+1 peak of increasing intensity as # carbons increase
a. intensity of M+1 = #C x 0.011 x intensity of M
b. not necessary to memorize!
8. Example
D. By functional group
1. Bromides
a. M+2 peak of equal intensity
b. 79Br abundance = 81Br
c. C-Br bond is weakest, so usually breaks first
2. Chlorides
a. M+2 peak = a third of M peak
b. 35Cl abundance = three times 37Cl
c. C-Cl bond str. ~ equal to C-C, sometimes you get α
cleavage (C-C bond next to C-Cl breaks)
III.
3. Ethers
a. usually broken heterolytically, with O taking both e-,
leaving behind carbocation
b. or homolytically at α C-C bond
c. Examples: p492
4. Alcohols
a, pretty unstable, typically have small M peaks
b. typically break at alpha C-C
c. if gamma (γ) H, may break off H2O, mw 18
5. Ketones
a. typically break at α C-C
b. McLafferty rearrangement (p494-495) with γ H
i.
beta C-C bond breaks after ketone snags γ H
Infrared (IR) Spectroscopy
A. Terms
1. bending vibrations
2. stretching vibrations
3. fingerprint region (16004. functional group region (4000-1400 cm-1)
B. Intensity and Position of absorptions
C. Characteristic IR absorptions (see charts on p534 and 541)
D. Conclusion: concentrate on functional group region, mine what
information you can from IR, then move on to other info sources
IV.
Functional Groups (theoretically review from Gen Chem)
A. Table 12.1
V.
Percent Composition (review from Gen. Chem.)
A. Steps to get empirical formula
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1.
2.
3.
4.
5.
Assume 100g of substance
How many grams of each atom
convert grams to moles
divide each by the smallest number of moles to get mole ratio
convert to whole numbers
Example:
C 52.12%
H 13.13%
O 34.75%
52.12 g C x 1 mole/12.00g = 4.343 moles C
13.13 g H x 1 mole/1.008 g = 13.026 moles H
34.75 g O x 1 mole/16.00 g = 2.17 moles O
2.17 moles O/ 2.17 = 1 mole O
13.026 moles H/ 2.17= 6 moles H
4.343 moles C/2.17 = 2 moles C
empirical formula =
Exercises:
C2H6O
C 83.91% H 16.09% (when done, convert to whole number ratio!)
C 77.38% H 7.58% N 15.04%
Objectives
Knowledge
Define the terms electromagnetic radiation, frequency, wavelength, wavenumber,
spectroscopy, absorbtion spectrum, vibrational modes and stretching frequencies
Describe m/z (mass to charge), base peak, parent or molecular ion (M) peak, M +1, M+2
peaks
Memorize functional groups on table 12.1 p514
Memorize rules for calculating index of hydrogen deficiency (unsaturation index)
 For every “n” carbons, 2n+2 hydrogens should be present
 For every group V element (N, P, etc), add 1 additional hydrogen
 For every group VI element (O, S, etc.), no change in H required
 For every halogen, reduce the number of H’s required by 1.
 Find the difference between the number of H’s required using these rules
and the actual # H’s present in formula and divide that number by two
 This is the number of pi bonds and/or rings present
Comprehension
Understand what a McLafferty rearrangement is and its implications are for mass spec
Application
Use index of hydrogen deficiency to determine how many  bonds/rings present in
molecule
Determine the presence or absence of Br or Cl from M+2 peak
Analysis
Examine IR spectra to find functional groups (using table given– no need to memorize)
Interpret mass spectrum and find molecular fragments
Inventory data available from the formula, IR, and mass spec to determine molecular
structure
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