Transcript Hints on Column Chromatography

Lecture 7
• Mass Spectrometry
• UV/Vis Spectroscopy
• Sample Problems - handout
Due:
Lecture Problem 5
Mass Spectrometry
Molecular ions and fragments can rearrange under MS
conditions.
For example, the benzylic cation can rearrange to a
tropollium ion:
CH2
Mass Spectrometry
Alpha-cleavage fragments
Seen in amines, alcohols, and carbonyl-containing compounds
H
N
H
N
H
CH3
OH
O
H
H
Mass Spectrometry
Loss of carbon monoxide:
O
C
O
CH2
CH2
M+ = 146
m/z 118
- CO
CH2
m/z 90
CH2
UV/Vis Spectroscopy
Basic Idea:
1. Expose compound to UV/Vis radiation
UV: 200-400 nm
Vis: 400-800 nm
2. Absorption of radiation causes electronic excitations at
specific wavelengths (lmax)
Main Use:
Detects the presence of pi systems in a compound.
Unlike NMR, IR, and MS, UV/Vis is NOT used to determine
pieces (specific bonds) of a molecule.
Electronic Transitions
Types of Electronic Transitions:


n
occupied
orbitals
Energy


The length of the arrow is roughly proportional to the energy
difference between two levels. The longer arrow means a higher
Energy transition. Higher energy means lower wavelength of radiation.
UV/Vis Spectroscopy


n
occupied
orbitals
Energy


Transitions observed by UV/Vis:
 to * (strong)
n to * (weak, often not observed)
n to * transition is lower energy (higher wavelength)
 to * transition is higher energy (lower wavelength)
UV/Vis of Acetone
 --> *
Shorter
wavelength,
stronger
absorption
non-bonding
electrons
O
C
 electrons
CH3
CH3
* are excited electrons
n --> *
Longer
wavelength,
weaker
absorption
The more conjugation, the lower the energy, the higher the
Wavelength (lmax value).
O
O
H
150 nm
H
165 nm
222 nm
215 nm
O
240 nm
244 nm
207 nm
and longer
O
295 nm
Spectra are absorbance versus wavelength in
nanometers (nm) - lmax
Examples:
Quantitative Data from UV/Vis
Extinction Coefficients or Molar Absorptivity ():
A numerical value of the intensity of the absorptions.
Calculate  by using the Beer-Lambert Law:
A (Absorbance) =  b c
Where b = path length of cell (usually 1.0 cm)
c = concentration of the solute
When reporting data, chemists only report lmax’s and ‘s from
UV/Vis data.