Embed Size (px)
DESCRIPTION
organic chemistry, analytical separation of chemistry
Citation preview
Shielding vs. Deshielding If a nuclei sees a smaller total magnetic field, it is
said to be shielded.
When a nuclei is shielded, its NMR frequency is shifted upfield lower chemical shift
Shielded lower
Deshielded higher
1 Chem 3500 - NMR spectroscopy
Mesomeric effect: definition The mesomeric effect or resonance effect in
chemistry is a property of the substituents or functional groups in a chemical compound. The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of the substituents based on relevant resonance structures and is symbolized by the letter M. The mesomeric effect is negative (-M) when the substituent is an electron-withdrawing group and the effect is positive (+M) when based on resonance the substituent is an electron releasing group.
2 Chem 3500 - NMR spectroscopy
Mesomeric effect: Examples
-M: electron-withdrawing effect
+M: electron-releasing effect
5.29 ppm
6.523.74
shieldeddeshielded
3 Chem 3500 - NMR spectroscopy
Mesomeric effect: releasing group
4 Chem 3500 - NMR spectroscopy
Mesomeric effect: withdrawing groups
5 Chem 3500 - NMR spectroscopy
Magnetic anisotropy of bonds Chemical bonds are anisotropic in space: they
have different properties along different axis
When a chemical bond is placed in a external magnetic field, it will generate a local field around itself that will also be anisotropic.
6 Chem 3500 - NMR spectroscopy
Example: C-C bond
C CB0Blocal
7 Chem 3500 - NMR spectroscopy
Example: benzene ring
8 Chem 3500 - NMR spectroscopy
More examples
9 Chem 3500 - NMR spectroscopy
Predicting 1H chemical shifts:Shoolery’s rule
Several factors can influence the 1H chemicalshift: carbon substitution, electronegativity, mesomeric effect,…
Those effects are additive
If methane is used as the base compound, the contribution of each of substituent can be addedup following the empirical Shoolery’s rule:
23.0
10 Chem 3500 - NMR spectroscopy
11 Chem 3500 - NMR spectroscopy
Alkenes The presence of a double bond affects the protons
on the allylic carbon atoms by shifting their chemical shift up (deshielding)
CH2
H H
1.5 2.1
12 Chem 3500 - NMR spectroscopy
Tobey-Simon Rule The Tobey-Simon rule is an empirical system to
predict the chemical shifts of protons attached to double bonds
transcisgem5.28δ
H
HH
R
gem
trans
cis
13 Chem 3500 - NMR spectroscopy
14 Chem 3500 - NMR spectroscopy
Aromatic compounds Substituent has a similar effect on the chemical
shifts of the aromatic ring than on alkenes.
Results from the inductive and resonance effects.
Prediction possible:
But it does NOT work when 2 substituents are ortho to each other…
iS7.27δ
15 Chem 3500 - NMR spectroscopy
Substituent Parameters for Aromatic Proton
16 Chem 3500 - NMR spectroscopy
Protons on Heteroatoms (N,S,O) Chemical shifts are very sensitive to pH,
temperature, solvent and concentration
Typical ranges are:
Acids RCOOH 10.5-12 ppm
Phenols ArOH 4.0-7.0 ppm
Alcohols ROH 0.5-5.0 ppm
Amines RNH2 0.5-5.0 ppm
Amides RCONH2 5.0-8.0 ppm
Enols CH=CH-OH >15 ppm
17 Chem 3500 - NMR spectroscopy
