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Review: Infrared Spectroscopy, Gas Chromatography and Mass Spectrometry Chemistry 2412 L Dr. Sheppard 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 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 Infrared Spectroscopy IR Spectroscopy Wavelengths usually 2.5-25 mm More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters (4000-400 cm-1) Measures molecular vibrations IR Spectrum Baseline Absorbance/ Peak No two molecules will give exactly the same IR spectrum (except enantiomers) Simple stretching: 1600-3500 cm-1 Complex vibrations: 400-1400 cm-1, called the “fingerprint region” 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 Summary of IR Absorptions Padias Correlation Table Another Correlation Table Alkane and Alkene Spectra Some Alkyne Spectra Alcohol and Amine Spectra Ketone and Aldehyde Spectra Carboxylic Acid and Amide Spectra 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 Gas Chromatography and Mass Spectrometry 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) 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 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? A GC from 2411L 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•+ + eM•+ → A+ + X A+ → B + + Y etc. Ions are separated and detected; mass determined The Mass Spectrum Plot relative abundance vs. mass-to-charge ratio – Charge = +1 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 81 Br Mass Spectrum with Br Mass Spectrum with I Mass Spectra of Alkanes More stable carbocations will be more abundant Mass Spectra of Alcohols Alcohols usually lose a water molecule M+ may not be visible Mass Spectrum of Methyl Benzoate 105 77 136 51