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12. Structure Determination: Mass Spectrometry and Infrared Spectroscopy. 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

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Determining the structure of an organic compound
Determining the Structure of an Organic Compound Spectroscopy

  • 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

Determining the structure of an organic compound1
Determining the Structure of an Organic Compound Spectroscopy

  • Physical methods now permit structures to be determined directly. We will examine:

    • mass spectrometry (MS)—this chapter

    • infrared (IR) spectroscopy—this chapter

    • nuclear magnetic resonance spectroscopy (NMR)—Chapter 13

    • ultraviolet-visible spectroscopy (VIS)—Chapter 14

12 1 mass spectrometry ms
12.1 Mass Spectrometry (MS) Spectroscopy

  • Sample vaporized and bombarded by energetic electrons that remove an electron, creating a cation-radical

  • Bonds in cation radicals begin to break (fragment)

Mass spectrometer
Mass Spectrometer Spectroscopy

The mass spectrum
The Mass Spectrum Spectroscopy

  • Plot mass of ions (m/z) (x-axis) versus the intensity of the signal (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
MS Examples: Methane and Propane Spectroscopy

  • Methane produces a parent peak (m/z = 16) and fragments of 15 and 14 (See Figure 12-2 a)

Ms examples methane and propane1
MS Examples: Methane and Propane Spectroscopy

  • The Mass Spectrum of propane is more complex (Figure 12-2 b) since the molecule can break down in several ways

12 2 interpreting mass spectra
12.2 Interpreting Mass Spectra Spectroscopy

  • 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

Other mass spectral features
Other Mass Spectral Features Spectroscopy

  • 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
12.3 Interpreting Mass-Spectral Fragmentation Patterns Spectroscopy

  • 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

2 2 dimethylpropane mm 72 c 5 h 12
2,2-Dimethylpropane: SpectroscopyMM = 72 (C5H12)

Mass spectral fragmentation of hexane
Mass Spectral Fragmentation of Hexane Spectroscopy

  • Hexane (m/z = 86 for parent) has peaks at m/z = 71, 57, 43, 29

Hexane Spectroscopy

Mass spectral cleavage reactions of alcohols
Mass Spectral Cleavage Reactions of Alcohols ethylcyclopentane?

  • Alcohols undergo -cleavage (at the bond next to the C-OH) as well as loss of H-OH to give C=C

Mass spectral cleavage of amines
Mass Spectral Cleavage of Amines ethylcyclopentane?

  • Amines undergo -cleavage, generating radicals

Fragmentation of ketones and aldehydes
Fragmentation of Ketones and Aldehydes ethylcyclopentane?

  • A C-H that is three atoms away leads to an internal transfer of a proton to the C=O, called the McLafferty rearrangement

  • Carbonyl compounds can also undergo  cleavage

Wavelength and frequency
Wavelength and Frequency ethylcyclopentane?

Absorption spectra
Absorption Spectra ethylcyclopentane?

  • Organic compounds exposed to electromagnetic radiation can absorb photons of specific energies (wavelengths or frequencies)

  • 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
12.6 Infrared Spectroscopy of Organic Molecules ethylcyclopentane?

  • IR region is lower in photon 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:

  • Wavenumber (cm-1) = 1/l(cm)

  • Specific IR absorbed by organic molecule is related to its structure

Ir region and vicinity
IR region and vicinity ethylcyclopentane?

Infrared energy modes
Infrared Energy Modes ethylcyclopentane?

  • 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 molecular vibrations

Infrared energy modes1
Infrared Energy Modes ethylcyclopentane?

12 7 interpreting infrared spectra
12.7 Interpreting Infrared Spectra ethylcyclopentane?

  • Most functional groups absorb at about the same energy and intensity independent of the molecule they are in

  • Characteristic 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)

Regions of the infrared spectrum

4000-2500 cm ethylcyclopentane?-1 N-H, C-H, O-H (stretching)

3300-3600 N-H, O-H

3000 C-H

2500-2000 cm-1 CºC and C º 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

Regions of the Infrared Spectrum

Differences in infrared absorptions
Differences in Infrared Absorptions ethylcyclopentane?

  • Molecules vibrate and rotate in normal modes, which are combinations of motions (relates to force constants)

  • Bond stretching dominates higher energy (frequency) modes

Differences in infrared absorptions1
Differences in Infrared Absorptions ethylcyclopentane?

  • Light objects connected to heavy objects vibrate fastest (at higher frequencies): C-H, N-H, O-H

  • For two heavy atoms, stronger bond requires more energy (higher frequency): C º C, C º N > C=C, C=O, C=N > C-C, C-O, C-N, C-halogen

12 8 infrared spectra of hydrocarbons
12.8 ethylcyclopentane?Infrared Spectra of Hydrocarbons

  • C-H, C-C, C=C, C º C have characteristic peaks

Hexane ethylcyclopentane?

Alkenes ethylcyclopentane?

1 hexene
1-Hexene ethylcyclopentane?

Alkynes ethylcyclopentane?

12 9 infrared spectra of some common functional groups
12.9 ethylcyclopentane?Infrared Spectra of Some Common Functional Groups

  • Spectroscopic behavior of functional groups is discussed in later chapters

  • Brief summaries presented here

Ir alcohols
IR: Alcohols ethylcyclopentane?

Amines ethylcyclopentane?

Ir aromatic compounds
IR: Aromatic Compounds ethylcyclopentane?

  • Weak C–H stretch at 3030 cm1

  • Weak absorptions 1660 - 2000 cm1 range

  • Medium-intensity absorptions 1450 to 1600 cm1

Phenylacetylene ethylcyclopentane?

Ir carbonyl compounds
IR: Carbonyl Compounds ethylcyclopentane?

  • 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

Practice problem 12 7
Practice problem 12.7: ethylcyclopentane?

Phenylacetaldehyde ethylcyclopentane?

C o in ketones
C=O in Ketones ethylcyclopentane?

  • 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
C=O in Esters ethylcyclopentane?

  • 1735 cm1 in saturated esters

  • 1715 cm1 in esters next to aromatic ring or a double bond

Chromatography purifying organic compounds
Chromatography: Purifying Organic Compounds ethylcyclopentane?

  • Chromatography : a process that separates compounds using adsorption and elution

    • Mixture is dissolved in a solvent (mobile phase) and placed into a glass column of adsorbent material (stationary phase)

    • Solvent or mixtures of solvents passed through

    • Compounds adsorb to different extents and desorb differently in response to appropriate solvent (elution)

    • Purified sample in solvent is collected from end of column

    • Can be done in liquid or gas mobile phase

Principles of liquid chromatography
Principles of Liquid Chromatography ethylcyclopentane?

  • Stationary phase is alumina (Al2O3) or silica gel (hydrated SiO2)

  • Solvents of increasing polarity are used to elute more and more strongly adsorbed species

  • Polar species adsorb most strongly to stationary phase

    • For examples, alcohols adsorb more strongly than alkenes

High pressure or high performance liquid chromatography hplc
High-Pressure (or High-Performance) Liquid Chromatography (HPLC)

  • More efficient and complete separation than ordinary LC

  • Coated silica microspheres (10-25 µm diameter) in stationary phase

  • High-pressure pumps force solvent through tightly packed HPLC column

  • Detector monitors eluting material

  • Figure 12.17: HPLC analysis of a mixture of 14 pesticides, using acetonitrile/water as the mobile phase

Some useful websites
Some Useful Websites: (HPLC)

  • Interpretation of IR spectra (CSU Stanislaus): http://wwwchem.csustan.edu/Tutorials/INFRARED.HTM

  • IR Spectroscopy Tutorial (CU Boulder): http://orgchem.colorado.edu/hndbksupport/irtutor/tutorial.html

  • NIST Chemistry WebBook: http://webbook.nist.gov/chemistry/

  • SDBS Data Base: http://www.aist.go.jp/RIODB/SDBS/menu-e.html