Download Organic Spectroscopy UV - Ultraviolet-Visible Spectroscopy

Organic Spectroscopy
Methods for structure determination of organic compounds:
X-ray Crystallography
Crystall structures
Mass spectroscopy
Molecular formula
----------------------------------------------------------------------------UV
UltraViolet radiation
Electron exitation
IR
InfraRed
Vibration excitation
NMR Nuclear Magnetic Resonance Nuclear spin excitation
Etot = Eelec + Evib + Erot + Ekin
E2
!E
h"
E1
E = h"
v=c/ #
E=hc/#
UV - Ultraviolet-Visible Spectroscopy
Conjugated compounds absorb energy in the ultraviolet (UV) and
visible (Vis) regions on the electromagnetic spectrum
! ! 200- 800 nm
Lambert-Beer's law : A = " · C · l
A is the observed absorbance,
" Molar absorptivity Is the intensity of the absorption
C is the molar concentration of the sample and
l is length of the sample cell in centimeters
The wavelength of maximum absorption (! max) is reported in units of
nanometers (nm).
Example: UV absorption spectrum of 2,5-dimethyl-2,4-hexadiene
Overview of methods: Ethyl propenoat
Use of UV
• Absorbance above 210 gives indication of conjugated systems
• Concentration measurements through calibration curve
Absorption Maxima for Nonconjugated and Conjugated Dienes
In UV-Vis spectroscopy the electrons are excited from lower energy levels to
higher ones
The electron is generally excited from the highest occupied molecular orbital
(HOMO) to the lowest unoccupied molecular orbital (LUMO)
Alkenes and nonconjugated dienes have absorptions below 200 nm because
the energy difference between the HOMO and LUMO is large
MS - Mass Spectroscopy
In mass spectroscopy (MS) a small quantity of a sample is vaporized and
ionized to form charged particles – usually an electron is removed from
each molecule to form charge ions. The charged particles are sorted
according to their mass-to-charge ratio (m/z) and detected. All this is taken
place in an instrument called a mass spectrometer.
Scheme:
In conjugated dienes these energy levels are much closer together and the
wavelengths of absorption are longer than 200 nm
Example: Butanone
Ethene has lmax at 171 nm and 1,3-butadiene has lmax at 217 nm
The longer the conjugated system, the smaller the energy difference
between the HOMO and the LUMO
A smaller energy gap results in longer !max in the ultraviolet -visible spectrum
#-Carotene has 11 conjugated double bonds and an absorbance maximum at 497
nm which is in the blue-green region of the visible spectrum
#-Carotene is perceived as red-orange, the complementary color of blue-green
See more examples in Tab 13-2 (p. 412)
The molecule is commonly fragmentated into “stable” carbocations at
branching points.
IR - Infrared spectroscopy
Vibration
transitions:
Streching
Each Isotope of an atom gives a specific peak according to its abundance.
Common example with different isotope distrubution from unity:
79
Br / 81Br is 1:1 Mw = 80 g/mole and 35Cl / 37Cl is 3:1 Mw= 35.45 g/mole
and
Bending
IR can be used for determination of functional groups in organic
compounds.
Important IR absorptions of fundamental bonding groups
E
4000 cm-1
3000
2000 1600
O-H
C H
N-H
C H
C H
O
C O H
All vibrations streching unless noted.
C C
C N
1000
C O C C
C
650 cm-1
C O
HBend Ar-HBend
Fingerprint area
Bending vibrations
Similar summary is presented in Fig 13,7
For more specified values see Table 13,3 (p. 416)
Stretching vibrations:1000 - 4000 cm-1 Bendning vibrations: 600 - 1600 cm-1
The fingerprint area gives specified pattern of many peaks (bending vibrations) in
the spectra for each organic molecule
Strong dipoles gives generally strong absorbance, e.g. Carbonyl, C=O
Dipole moment must be changed if IR-radiation should be absorbed.
CH3 C C CH3
No absorbance
CH3 C C H
Absorbance
Interpreting IR Spectra
Example: 1-hexene
Example: 1- hexyne
Generally only certain peaks are interpreted in the IR
-Those peaks that are large and above 1400 cm-1 are most valuable
Hydrocarbons
C-H bonds where the carbon has more s character are shorter, stronger and stiffer
and thus vibrate at higher frequency
!
C-H bonds at sp centers appear at 3000-3100 cm-1
!
C-H bonds at sp2 centers appear at about 3080 cm-1
!
C-H bonds at sp3 centers appear at about 2800-3000 cm-1
!
- C-C bond stretching frequencies are only useful for multiple bonds
! C-C double bonds give peaks at 1620-1680 cm-1
! C-C triple bonds give peaks at 2100-2260 cm-1
Example: octane
Aromatic Compounds
- The C-C bond stretching gives a set of characteristic sharp peaks between 14501600 cm -1. Substitution pattern can be analyzed by peaks at 600-800 cm -1
Example: Methyl benzene
Amines
Functional Groups:
The carbonyl group gives
a strong peak which
occurs round 1700 cm-1
Very dilute solution of 1o and 2o amines give sharp peaks at 3300-3500 cm-1
for the N-H stretching
!
1o amines give two peaks and 2o amines give one peak
!
3o have no N-H bonds and do not absorb in this region
More concentrated solutions of amines have broader peaks
Amides have amine N-H stretching peaks and a carbonyl peak
Example: p-Methylaniline
Alcohols and Phenols
The O-H stretching absorption is very characteristic
In concentrated solutions, the hydroxyl groups hydrogen bond to each other and
a very broad and large peak occurs at 3200-3550 cm-1. See the spectra above of
ethanol as an example..
Carboxylic Acids
The carbonyl peak at 1710-1780 cm-1 is very characteristic
The presence of both carbonyl and O-H stretching peaks (2500-3500 cm-1,
very broad due to extensive hydrogen bonding) is a good proof of the
presence of a carboxylic acid
Example: propanic acid
Övning:
A
B
C
Identifiera IR-spektra med rätt förening:
Table IR