2. GENERAL
PRINCIPLES
Degree of unsaturation
Ring plus double bond
Connectivity of structural elements
• Sensitivity is generally taken to signify the limits of
detectability of a chromophore
• In terms of overall sensitivity, i.e. the amount of
sample required, it is generally observed that:
Sensitivity
MS > UV > IR> 1H NMR > 13C NMR
3. Rules
• Empirical formula: YES/NO
• RING PLUS DOUBLE BOND
• INDEX OF HYDROGEN DEFICIENCY
• THE RULE OF THIRTEEN
• THE NITROGEN RULE
4. RPDB Rule
• To convert the formula of an open-chain, saturated hydrocarbon to a formula containing Group V
elements (N, P, As, Sb, Bi), one additional hydrogen atom must be added to the molecular formula
• To convert the formula of an open-chain, saturated hydrocarbon to a formula containing Group VI
elements (O, S, Se, Te), no change in the number of hydrogens is required.
• To convert the formula of an open-chain, saturated hydrocarbon to a formula containing Group
VII elements (F, Cl, Br, I), one hydrogen must be subtracted from the molecular formula for each
such Group VII element present.
6. Interpretation form UV spectroscopy
• Classification of UV absorption
bands
• B (for benzenoid):
• E (for ethylenic)
• R (for radical-like)
• K (for conjugated - from the
German "konjugierte‘’)
• A molecule may give rise to
more than one band in its UV
spectrum
7. Important UV Chromophores
• Dienes and Polyenes
• Carbonyl compounds
• Benzene derivatives
• Aniline and phenoxide ion have strong
UV absorptions due to the overlap of the
lone pair on the nitrogen (or oxygen)
with the n-system of the benzene ring
• THE EFFECT OF SOLVENTS
8. Infrared (IR)
Spectroscopy
• Infrared absorption spectra are calibrated in
wavelengths expressed in micrometers
1µm = 10-6 m
• Or in frequency-related wave numbers (cm-1) which
are reciprocals of wavelengths
Wave number (𝑐𝑚−1
) =
1 𝑥 104
wavelength (in μm)
• The range accessible for standard instrumentation is
usually 4000 to 667 cm-1
• Infrared absorption intensities are rarely described
quantitatively, except for the
• General classifications of s (strong), m (medium) or w
(weak).
10. Before interpretating…….
• High resolution mass spectra
• Isotope ratios
• Molecular Fragmentation
• Mode of fragmentation
• The appearance of prominent peaks at certain mass numbers can be correlated
empirically with certain structural elements
• Information can also be obtained from differences between the masses of two peaks
• Meta-stable peaks
12. Factors governing fragmentation processes
• Weak bonds tend to be broken most easily
• Stable fragments (not only ions, but also the accompanying radicals and molecules) tend
to be formed most readily
• Some fragmentation processes depend on the ability of molecules to assume cyclic
transition states.
• COMMON TYPES OF FRAGMENTATION
• Cleavage at Branch Points
• β- Cleavage
• Cleavage α to carbonyl groups.
• Cleavage α to heteroatoms
• Retro Diels-Alder reaction
• The McLafferty rearrangement
13. Nuclear Magnetic
Resonance (NMR)
spectroscopy
• Chemical shift in 1H NMR spectroscopy
• Electron withdrawing substituents (-OH, -OCOR, -OR, -N02,
halogen) attached to an aliphatic carbon chain cause a
downfield shift of 2-4 ppm when present at Cα and have less
than half of this effect when present at Cβ
• When sp2 hybridized carbon atoms (carbonyl groups, olefinic
fragments, aromatic rings) are present in an aliphatic carbon
chain they cause a downfield shift of 1-2 ppm when present at
Cα They have less than half of this effect when present at Cβ
17. NMR Correlation Chart
12 11 10 9 8 7 6 5 4 3 2 1 0
-OH -NH
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
CH2Ar
CH2NR2
CH2S
C C-H
C=C-CH2
CH2-C-
O
C-CH-C
C
C-CH2-C
C-CH3
RCOOH RCHO C=C
H
TMS
H
CHCl3 ,
d (ppm)
DOWNFIELD UPFIELD
DESHIELDED SHIELDED
Ranges can be defined for different general types of protons.
This chart is general, the next slide is more definite.
19. YOU DO NOT NEED TO MEMORIZE THE
PREVIOUS CHART
IT IS USUALLY SUFFICIENT TO KNOW WHAT TYPES
OF HYDROGENS COME IN SELECTED AREAS OF
THE NMR CHART
aliphatic
C-H
CH on C
next to
pi bonds
C-H where C is
attached to an
electronegative
atom
alkene
=C-H
benzene
CH
aldehyde
CHO
acid
COOH
2
3
4
6
7
9
10
12 0
X-C-H
X=C-C-H
MOST SPECTRA CAN BE INTERPRETED WITH
A KNOWLEDGE OF WHAT IS SHOWN HERE
20. 1 2 1
PASCAL’S TRIANGLE
1
1 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
singlet
doublet
triplet
quartet
quintet
sextet
septet
octet
The interior
entries are
the sums of
the two
numbers
immediately
above.
Intensities of
multiplet peaks
21. 13C NMR
SPECTROSCOPY
• Coupling and decoupling in 13C NMR spectra
• Broad band decoupling or noise decoupling
• 13C NMR spectra usually appear as a series of singlets
(when 1H is fully decoupled) and each distinct 13C
environment in the molecule gives rise to a separate
signal.
• Off- resonance decoupling
• DEPT experiment (Distortionless Enhancement by
Polarisation Transfer)
23. Perform all
routine
operations
• Determine the molecular weight from the Mass Spectrum.
• Determine relative numbers of protons in different environments from the 1H NMR
spectrum.
• Determine the number of carbons in different environments and the number of
quaternary carbons, methine carbons, methylene carbons and methyl carbons from
the 13CNMR spectrum.
• Examine the problem for any additional data concerning composition and determine
the molecular formula if possible. From the molecular formula, determine the degree
of unsaturation.
• Determine the molar absorbance in the UV spectrum, if applicable.
• Examine each spectrum (IR, mass spectrum, UV, 13CNMR, 1H NMR) in turn for obvious
structural elements:
• Write down all structural elements you have determined.
• Try to assemble the structural elements
• Return to each spectrum (IR, UV, mass spectrum, 13CNMR, 1H NMR) in turn and
rationalise all major features (especially all major fragments in the mass spectrum and
all features of the NMR spectra) in terms of your proposed structure.
24. PROBLEM 1
E M P I R I C A L F O R M U L A : C 4 H 8 O
