Transcript Chemdraw B&W - Pennsylvania State University

12. Structure Determination: Mass Spectrometry
and Infrared Spectroscopy
Based on
McMurry’s Organic Chemistry, 6th edition
Determining the Structure of an
Organic Compound
• The analysis of the outcome of a reaction requires that
we know the full structure of the products as well as
the reactants
• In the 19th and early 20th centuries, structures were
determined by synthesis and chemical degradation
that related compounds to each other
• Physical methods now permit structures to be
determined directly. We will examine:
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mass spectrometry (MS)
infrared (IR) spectroscopy
nuclear magnetic resonance spectroscopy (NMR)
ultraviolet-visible spectroscopy (VIS)
12.1 Mass Spectrometry (MS)
• Measures molecular weight
• Sample vaporized and subjected to bombardment by
electrons that remove an electron
– Creates a cation-radical
• Bonds in cation radicals begin to break (fragment)
• Charge to mass ratio is measured (see Figure 12-1)
The Mass Spectrum
• Plot mass of ions (m/z) (x-axis) versus the intensity of
the signal (roughly corresponding to the number of
ions) (y-axis)
• Tallest peak is base peak (100%)
– Other peaks listed as the % of that peak
• Peak that corresponds to the unfragmented radical
cation is parent peak or molecular ion (M+)
MS Examples: Methane and
Propane
• Methane produces a parent peak (m/z = 16) and
fragments of 15 and 14 (See Figure 12-2 a)
• The MS of propane is more complex (Figure 12-2 b)
since the molecule can break down in several ways
12.2 Interpreting Mass Spectra
• Molecular weight from the mass of the molecular ion
• Double-focusing instruments provide high-resolution
“exact mass”
– 0.0001 atomic mass units – distinguishing specific atoms
• Example MW “72” is ambiguous: C5H12 and C4H8O but:
– C5H12 72.0939 amu exact mass C4H8O 72.0575 amu exact mass
– Result from fractional mass differences of atoms 16O = 15.99491,
12C = 12.0000, 1H = 1.00783
• Instruments include computation of formulas for each
peak
Other Mass Spectral Features
• If parent ion not present due to electron bombardment
causing breakdown, “softer” methods such as
chemical ionization are used
• Peaks above the molecular weight appear as a result of
naturally occurring heavier isotopes in the sample
– (M+1) from 13C that is randomly present
12.3 Interpreting Mass-Spectral
Fragmentation Patterns
• The way molecular ions break down can produce
characteristic fragments that help in identification
– Serves as a “fingerprint” for comparison with known
materials in analysis (used in forensics)
– Positive charge goes to fragments that best can stabilize it
12.5 Spectroscopy of the
Electromagnetic Spectrum
• Radiant energy is proportional to its frequency
(cycles/s = Hz) as a wave (Amplitude is its height)
• Different types are classified by frequency or
wavelength ranges
Absorption Spectra
• Organic compound exposed to electromagnetic
radiation, can absorb energy of only certain
wavelengths (unit of energy)
– Transmits, energy of other wavelengths.
• Changing wavelengths to determine which are
absorbed and which are transmitted produces an
absorption spectrum
• Energy absorbed is distributed internally in a distinct
and reproducible way (See Figure 12-11)
12.6 Infrared Spectroscopy of
Organic Molecules
• IR region lower energy than visible light (below red –
produces heating as with a heat lamp)
• 2.5  106 m to 2.5  105 m region used by organic
chemists for structural analysis
• IR energy in a spectrum is usually measured as
wavenumber (cm-1), the inverse of wavelength and
proportional to frequency
• Specific IR absorbed by organic molecule related to its
structure
Infrared Energy Modes
• IR energy absorption corresponds to specific modes,
corresponding to combinations of atomic movements,
such as bending and stretching of bonds between
groups of atoms called “normal modes”
• Energy is characteristic of the atoms in the group and
their bonding
• Corresponds to vibrations and rotations
12.7 Interpreting Infrared Spectra
• Most functional groups absorb at about the same
energy and intensity independent of the molecule they
are in
• Characteristic higher energy IR absorptions in Table
12.1 can be used to confirm the existence of the
presence of a functional group in a molecule
• IR spectrum has lower energy region characteristic of
molecule as a whole (“fingerprint” region)
• See samples in Figure 12-13
Regions of the Infrared
Spectrum
• 4000-2500 cm-1 N-H, C-H,
O-H (stretching)
– 3300-3600 N-H, O-H
– 3000 C-H
• 2500-2000 cm-1 CC and
C  N (stretching)
• 2000-1500 cm-1 double
bonds (stretching)
– C=O 1680-1750
– C=C 1640-1680 cm-1
• Below 1500 cm-1
“fingerprint” region
Differences in Infrared Absorptions
• Molecules vibrate and rotate in normal modes, which
are combinations of motions (relates to force
constants)
• Bond stretching dominates higher energy modes
• Light objects connected to heavy objects vibrate
fastest: C-H, N-H, O-H
• For two heavy atoms, stronger bond requires more
energy: C  C, C  N > C=C, C=O, C=N > C-C, C-O, C-N,
C-halogen
12.8 Infrared Spectra of Hydrocarbons
• C-H, C-C, C=C, C  C have characteristic peaks
– absence helps rule out C=C or C  C
12.9 Infrared Spectra of Some
Common Functional Groups
• Spectroscopic behavior of functional group is
discussed in later chapters
• Brief summaries presented here
IR: Alcohols and Amines
• O–H 3400 to 3650 cm1
– Usually broad and intense
• N–H 3300 to 3500 cm1
– Sharper and less intense than an O–H
IR: Aromatic Compounds
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Weak C–H stretch at 3030 cm1
Weak absorptions 1660 - 2000 cm1 range
Medium-intensity absorptions 1450 to 1600 cm1
See spectrum of phenylacetylene, Figure 12.15
IR: Carbonyl Compounds
• Strong, sharp C=O peak 1670 to 1780 cm1
• Exact absorption characteristic of type of carbonyl
compound
– 1730 cm1 in saturated aldehydes
– 1705 cm1 in aldehydes next to double bond or aromatic ring
C=O in Ketones
• 1715 cm1 in six-membered ring and acyclic ketones
• 1750 cm1 in 5-membered ring ketones
• 1690 cm1 in ketones next to a double bond or an
aromatic ring
C=O in Esters
• 1735 cm1 in saturated esters
• 1715 cm1 in esters next to aromatic ring or a
double bond