Transcript Hints on Column Chromatography

Lecture 21
• UV/Vis
• Pick up Lecture Problem 7
This Week in Lab:
Work on 1st Synthetic
Next Week in Lab:
• Ch 9 Final Report Due
• Synthetic #2 PreLab Due
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
 to * transitions
295 nm
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.
Plant Pigments (Chapter 9)
Class
Compound
Colors Produced
Carotenoid
Carotenes
Golden
Porphyrin
Pheophytin
Olive Green
Porphyrin
Chlorophyll a
Blue Green
Porphyrin
Chlorophyll b
Yello w Green
Carotenoid
Lutein
Yello w
Carotenoid
Xanthophylls
Yello w
Flavonoid
Anthocyanin
Red, Blue, or
Purple (pH
dependent)
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Color wheel: complementary
colors
Plant Pigments (Chapter 9)
What you’re looking for:
Example: Chlorophyll:
• Observe the visible color: green
• Absorption of red light (~ 620 to 680 nm); should be low %
transmittance
CH2
CH
N
H3C
O
CH3
N
CH2
CH2CH3
H3C
H
N
C
N
H3C
Mg
CH
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
H
N
CH2CH3
Mg
CH3
N
H 3C
N
H
H
CH3
N
H
CH2
CH2
O
COOCH3
CH3
CH2COOCH2
CH3
chlorophyll-a
CH3
CH3
CH3
O
COOCH3
CH3
CH2COOCH2
CH3
chlorophyll-b
CH3
CH3
CH3