Hints on Column Chromatography by w8S0hO

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									                  Lecture 18
• Introduction to Mass Spectrometry
• Quiz 6 (Ch 9 & NMR/IR Problems) next Tuesday in
  lecture

This week in lab:
• Ch 6 Final Report Due
• Synthetic Selection Sheet Due (Ch 9)
• Procedure 2 of Ch 8

Next week in lab:
• Synthetic Experiment #1 PreLab Due
                 Spectral Analysis
 1H NMR:
 • Bond connectivities
 • Types and kinds of protons

 IR:
 • Functional groups



Now, let’s look at Mass Spectrometry:
• Determine the mass of the sample
• Look at fragmentation patterns determine “pieces”
  of the molecule.
Mass Spectrum of Cyclohexamine




                            NH2



                      M+ = 99
Fragments
                    Mass Spectrometry

How to get the mass of the sample or pieces of the sample:

• Ionize the sample with a high energy particle to give
  a molecular ion, M+.

Note: Ions may fragment which results in pieces of the sample.
When fragmentation occurs, there is always a positive
fragment (ion) and a neutral fragment.

• Only positive species are detected by the detector. Thus,
  only the mass of the molecular ion and the masses of
  positive ion fragments will be read by the detector and appear
  on the mass spectrum for the compound. Neutral fragments
  are not read by the detector and do not appear on the spectrum.
                      Mass Spectrometry
Basic Idea:
To ionize: bash the molecule with a high energy particle in a vacuum:
• An electron (known as Electron Ionization)
  or
• A proton (known as Chemical Ionization)

A molecular ion results:
Electron Ionization (EI): M + e-  M+•
Chemical Ionization (CI): M + H+  MH+

Send molecular ion through a magnetic field

Molecular ion travels and reaches a detector. The detector
only detects positively charged species  m/z ratio (mass/
charge).

Obtain mass of molecular ion and fragments. The intensity of the signals
is proportional to the number of ions. The more intense the signal, the
more stable that ion is.
The Mass Spectrometer
                   Molecular Ions
Molecular ions of different compounds have different
stabilities and, therefore, different fates.
Some will reach the detector.
Others will fragment before they reach the detector.


Molecular ions of different compounds will have different
intensities:
     Strong in aromatics
     Weak in alkanes, alcohols & others
                                  Mass Spectrum of Hexane

                                                    57
1 00                                                                    +
                                                         CH3CH2CH2CH2
                                     +
                            CH3CH2CH2

                            43

                        41
       Ab u nd an ce




                                                             CH3CH2CH2 CH2 CH2CH3
                                  He xan e
                                                                                       M+
                                                                                       86

                       39                                             M-C H3
                                                  55                71
   34 37                                     53
  0 35   40                      45   50          55    60    65   70       75   80   85    90
                                                    m/z-->
Molecular Ions
                  M+ = 78


   Benzene
   Strong M+

                            M+ = 86


   Hexane
   Weak M+


                             M+ = 102
             OH

 Hexanol
 M+ not seen
     Nitrogen Containing Compounds
If odd # of N’s, then odd mass. If even mass,
then either zero N’s or even # of N’s.




                             M+ = 99
         Cyclohexamine

								
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