1. Chemistry 2412 L Dr. Sheppard
Review: Infrared Spectroscopy, Gas Chromatography and Mass Spectrometry
Chemistry 2412 L
Dr. Sheppard

2. Spectroscopy An analytical technique which helps determine structure
It destroys little or no sample
The amount of light absorbed by the sample is measured as wavelength is varied

3. Types of Spectroscopy Infrared (IR) spectroscopy
measures the bond vibration frequencies in a molecule and is used to determine the functional group
Mass spectrometry (MS)
fragments the molecule and measures the masses
Nuclear magnetic resonance (NMR) spectroscopy
detects signals from hydrogen atoms and can be used to distinguish isomers
Ultraviolet (UV) spectroscopy
uses electron transitions to determine bonding patterns

4. Infrared Spectroscopy

5. IR Spectroscopy Wavelengths usually 2.5-25 mm
More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters ( cm-1)
Measures molecular vibrations

6. IR Spectrum
Baseline
Absorbance/Peak
No two molecules will give exactly the same IR spectrum (except enantiomers)
Simple stretching: cm-1
Complex vibrations: cm-1, called the “fingerprint region”

7. Interpretation Looking for presence/absence of functional groups
Correlation tables
Klein: Ch. 15 and Padias: table 2-1
A polar bond is usually IR-active
A nonpolar bond in a symmetrical molecule will absorb weakly or not at all

8. Summary of IR Absorptions

9. Padias Correlation Table

10. Another Correlation Table

11. Alkane and Alkene Spectra

12. Some Alkyne Spectra

13. Alcohol and Amine Spectra

14. Ketone and Aldehyde Spectra

15. Carboxylic Acid and Amide Spectra

16. Strengths and Limitations of IR Spectroscopy
IR alone cannot determine a structure
Some signals may be ambiguous
The functional group is usually indicated
The absence of a signal is definite proof that the functional group is absent
Correspondence with a known sample’s IR spectrum confirms the identity of the compound

17. Gas Chromatography and Mass Spectrometry

18. Chromatography
Separation of components of a mixture based on affinity for stationary phase or mobile phase
Includes:
Thin-layer chromatography (TLC)
Paper chromatography
Column chromatography
Gas chromatography (GC)
High Performance Liquid Chromatography (HPLC)

19. Gas Chromatography Mobile phase = stream of inert gas
Stationary phase = high boiling liquid film in column
Affinity = vapor vs. liquid
Sample = injection from syringe (1 mL)
Sometimes add 1 mL air
Separated components pass by detector, send signal to recorder
Signal display = peaks
Area under peak = % of mixture
Analysis
Retention time (min.), Area, % of mixture

20. Analysis of GC How many components are in this mixture?
What is the percentage of each component in the mixture?
Which component has the higher boiling point?

21. A GC from 2411L

22. Mass Spectrometry Used with GC
Mixture of compounds separated by gas chromatography, then identified by mass spectrometry
Determines MW and provides information about structure
A beam of high-energy electrons breaks molecules into ions (fragments)
M → M•+ + e-
M•+ → A+ + X
A+ → B+ + Y
etc.
Ions are separated and detected; mass determined

23. The Mass Spectrum Plot relative abundance vs. mass-to-charge ratio

24. Isotopes Present in their usual abundance
Hydrocarbons contain 1.1% 13C, so there will be a small M+1 peak
If S is present, M+2 will be 4% of M+
If Cl is present, M+2 is one-third of M+
If Br is present, M+2 is equal to M+
If I is present, peak at 127; large gap
81Br

25. Mass Spectrum with Br

26. Mass Spectrum with I

27. Mass Spectra of Alkanes
More stable carbocations will be more abundant

28. Mass Spectra of Alcohols
Alcohols usually lose a water molecule
M+ may not be visible

29. Mass Spectrum of Methyl Benzoate51 77
105
136