Course: M. Phil (Chemistry)
Unit: I
UV - VISIBLE SPECTROSCOPY
Syllabus:
• Electronic transition
• Chromophores and Auxochromes
• Factors influencing position and intensity of absorption bands
• Effect of solvent on spectra
• Absorption spectra of Dienes, Polyene , Unsaturated carbonyl compounds
• Woodward Fieser rules
MPC102 – PHYSICAL METHODS IN CHEMISTRY
Dr. K. SIVAKUMAR
Department of Chemistry
SCSVMV University
[email protected]
1
ν
Electromagnetic Waves - Terminologies
Electromagnetic wave parameters:
Wavelength (λ): Wavelength is the distance between the consecutive peaks or crests
Wavelength is expressed in nanometers (nm)
1nm = 10-9 meters = 1/1000000000 meters
1A° = 10-10 meters = 1/10000000000 meters
2
1
ν
Electromagnetic Waves - Terminologies
Electromagnetic wave parameters:
Frequency (ν
ν ): Frequency is the number of waves passing through any point per second.
Frequency is expressed in Hertz (Hz)
3
ν
Electromagnetic Waves - Terminologies
Electromagnetic wave parameters:
Wave number (ν ): Wave number is the number of waves per cm.
Wavelength, Wave number and Frequency are interrelated as,
1
Where,
λ
=ν =
ν
c
λ is wave length
ν is wave number
ν is frequency
c is velocity of light in vacuum. i.e., 3 x 108 m/s
4
2
Electromagnetic Spectral regions
nm
EM
waves
10-4 to 10-2
γ-rays
10-2 to 100
X-rays
100 to 102
102 to 103
103 to 105
UV
Visible
IR
105 to 107
107 to 109
Microwave Radio
5
Electromagnetic Spectrum
E = hν
ν
h – Planck’s constant
6
www.spectroscopyNOW.com
3
The Electromagnetic wave lengths & Some examples
7
Electromagnetic radiation sources
EM radiation
Gamma rays
X-rays
Ultraviolet
Visible
Infrared
Microwave
Radio wave
Spectral method
Gamma spec.
X-ray spec.
UV spec.
Visible spec.
IR spec.
ESR spec.
NMR spec.
Radiation source
gamma-emitting nuclides
Synchrotron Radiation Source (SRS),
Betatron (cyclotron)
Hydrogen discharge lamp
tungsten filament lamp
rare-earth oxides rod
klystron valve
magnet of stable field strength
8
4
Electromagnetic Spectrum – Type of radiation and Energy change involved
9
Electromagnetic Spectrum – Type of radiation and Energy change involved
10
5
Electromagnetic Spectrum – Type of radiation and Energy change involved
11
Effect of electromagnetic radiations on chemical substances
The absorption spectrum of an atom often contains sharp and clear lines.
Absorption spectrum of an atom; Hydrogen
Energy levels in atom; Hydrogen
12
6
Effect of electromagnetic radiations on chemical substances
But, the absorption spectrum of a molecule is highly complicated with closely
packed lines
This is due to the fact that molecules have large number of energy levels and
certain amount of energy is required for transition between these energy levels.
Energy levels in molecule
Absorption spectrum of a molecule; Eg: H2O
13
Effect of electromagnetic radiations on chemical substances
The radiation energies absorbed by molecules may produce Rotational,
Vibrational and Electronic transitions.
14
7
Effect of electromagnetic radiations on chemical substances
Rotational transition
Microwave and far IR radiations bring about changes in the rotational energies
of the molecule
Example: Rotating HCl molecule
15
Effect of electromagnetic radiations on chemical substances
Vibrational transition
Infrared radiations bring about changes in the vibration modes (stretching,
contracting and bending) of covalent bonds in a molecule
Examples:
Example:
Vibrating HCl molecule
16
8
Effect of electromagnetic radiations on chemical substances
Electronic transition
UV and Visible radiations bring about changes in the electronic transition of a molecule
Example: Cl2 in ground and excited states
17
Effect of electromagnetic radiations on chemical substances
Cl2 in Ground state
18
9
Effect of electromagnetic radiations on chemical substances
Cl2 in Excited state
19
The Ultraviolet region [10 – 800nm]
The Ultraviolet region may be divided as follows,
1. Far (or Vacuum) Ultraviolet region [10 – 200 nm]
2. Near (or Quartz) Ultraviolet region [200 – 380 nm]
3. Visible region [380 - 800 nm]
20
10
The Ultraviolet region
Far (or Vacuum) Ultraviolet region [10 – 200nm]
• Electromagnetic spectral region from 100 – 200nm can be studied in evacuated system
and this regions is termed as “vacuum UV”
• The atmosphere absorbs the hazardous high energy UV <200nm from sunlight
• Excitation (and maximum separation) of σ - electrons occurs in 120 – 200nm
Near (or Quartz) Ultraviolet region [200 - 380nm]
• Electromagnetic spectral region from 200 – 380nm normally termed as “Ultraviolet region”
• The atmosphere is transparent in this region and quartz optics may be used to scan
from 200 – 380nm
• Excitation of p and d orbital electrons, π - electrons and π - conjugation (joining together)
systems occurs in 200 – 380nm
Example for π conjugation
Benzene
21
The Visible region
Visible region [380 – 800nm]
• Electromagnetic spectral region from 380 – 800nm is termed as “visible region”
• The atmosphere absorbs the hazardous high energy UV <200nm from sunlight
• Excitation of π-conjugation occurs in visible region; 380 – 800nm
• Conjugation of double bonds lowers the energy required for the transition and
absorption will move to longer wavelength (i.e., to low energy)
22
11
VISIBLE region in Electromagnetic Spectrum
•Violet
•Indigo
•Blue
•Green
•Yellow
•Orange
•Red
: 380 - 420 nm
: 420 - 440 nm
: 440 - 490 nm
: 490 - 570 nm
: 570 - 585 nm
: 585 - 620 nm
: 620 - 800 nm
23
UV - VISIBLE SPECTROSCOPY
•
In UV - Visible Spectroscopy, the sample is irradiated with the broad
spectrum of the UV - Visible radiation
•
If a particular electronic transition matches the energy of a certain band
of UV - Visible, it will be absorbed
•
The remaining UV - Visible light passes through the sample and is
observed
•
From this residual radiation a spectrum is obtained with “gaps” at these
discrete energies – this is called an absorption spectrum
24
12
Lambert’s law
fraction of the monochromatic light absorbed by a
homogeneous medium is independent of the intensity of the
incident light and each successive unit layer absorbs an equal
fraction of the light incident on it
Lambert
Beer’s law
fraction of the incident light absorbed is proportional to the number of
the absorbing molecules in the light-path and will increase with
increasing concentration or sample thickness.
Beer
25
Beer–Lambert law / Beer–Lambert– Bouguer law / Lambert – Beer law
log (I0/I) = ε c l = A
Where,
I0 - the intensity of incident light
I - the intensity of transmitted light
ε - molar absorptivity / molar extinction coefficient in cm2 mol-1 or L mol-1 cm-1.
c - concentration in mol L-1
l - path length in cm
A - absorbance (unitless)
Molar absorptivity
26
13
Absorption intensity ε
εmax Intensity of absorption is directly proportional to the transition probability
A fully allowed transition will have εmax > 10000
A low transition probability will have εmax < 1000
λmax wavelength of light corresponding to maximum absorption is designated as λmax and
can be read directly from the horizontal axis of the spectrum
Absorbance (A) is the vertical axis of the spectrum A = log (I0/I)
I0 - intensity of the incident light; I - intensity of transmitted light
εmax = 20000
27
Generalizations Regarding λ max
If spectrum of compound shows, Absorption band of
very low intensity (εεmax = 10-100) in the 270-350nm region, and
no other absorptions above 200 nm,
Then, the compound contains a simple, nonconjugated chromophore containing n
electrons.
The weak band is due to n → π* transitions.
If the spectrum of a compound exhibits many bands, some of which appear even in the
visible region, the compound is likely to contain long-chain conjugated or polycyclic
aromatic chromophore.
If the compound is colored, there may be at least 4 to 5 conjugated chromophores and
auxochromes.
Exceptions: some nitro-, azo-, diazo-, and nitroso-compounds will absorb visible light.
28
14
Generalizations Regarding εmax
If εmax = 10,000 - 20,000; generally a simple α, β -unsaturated ketone or diene
If εmax = 1,000 - 10,000 normally an aromatic system
Substitution on the aromatic nucleus by a functional group which extends the length of
the chromophore may give bands with εmax > 10,000 along with some which still have
εmax < 10,000.
Bands with εmax < 100 represent n → π* transitions.
molar absorptivities vary by orders of magnitude:
values of 104-106 are termed high intensity absorptions
values of 103 -104 are termed low intensity absorptions
values of 0 to 103 are the absorptions of forbidden transitions
29
Beer–Lambert law / Beer–Lambert– Bouguer law / Lambert – Beer law
Bouguer
Actually investigated the
range of absorption Vs
thickness of medium
Lambert
Extended the concepts
developed by Bouguer
Beer
Applied
Lambert’s
concept to solutions of
different concentrations
?
Bernard
Beer released the results
of Lambert’s concept just
prior to those of Bernard
30
15
Electronic Energy Levels
•
Absorption of UV - Visible radiation by an organic molecule leads to
electronic excitation among various energy levels within the molecule.
•
Electron transitions generally occur in between a occupied bonding or lone
pair orbital and an unoccupied non-bonding or antibonding orbital.
•
The energy difference between various energy levels, in most organic
molecules, varies from 30 to 150 kcal/mole
31
σ Bonding and anti-bonding formation from s atomic orbitals (Eg: H2 molecule)
Bonding between two hydrogen atoms
One molecular orbital
with 2 electrons
2 atomic orbitals of
2 hydrogen atoms
According to Molecular Orbital Theory
One antibonding orbital without
electrons and two nuclei
One bonding orbital
with 2 electrons
2 atomic orbitals of
2 hydrogen atoms
32
16
σ Bonding and anti-bonding formation from s atomic orbitals (Eg: H2 molecule)
According to Molecular Orbital Theory
Higher energy
than original atomic
orbitals and bonding
orbital - Because of
repulsion
Lower energy
than original atomic orbitals
2 atomic orbitals of
2 hydrogen atoms
Bonding orbitals are lower in energy than its original (atoms) atomic orbitals.
Because, energy is released when the bonding orbital is formed,
i.e., hydrogen molecule is more energetically stable than the original atoms.
However, an anti-bonding orbital is less energetically stable than the original atoms.
A bonding orbital is stable because of the attractions between the nuclei and the electrons.
In an anti-bonding orbital there are no equivalent attractions - instead of attraction you get repulsions.
There is very little chance of finding the electrons between the two nuclei - and in fact half-way between the nuclei
there is zero chance of finding electrons. There is nothing to stop the two nuclei from repelling each other apart.
33
So in the hydrogen case, both of the electrons go into the bonding orbital, because that produces the greatest stability
- more stable than having separate atoms, and a lot more stable than having the electrons in the anti-bonding orbital.
σ Bonding and anti-bonding formation from p atomic orbitals
34
17
π Bonding and anti-bonding formation from p atomic orbitals
35
Electronic Energy Levels
σ∗ (anti-bonding)
π∗ (anti-bonding)
n (non-bonding)
Energy
π (bonding)
σ (bonding)
σ - orbitals are the lowest energy occupied molecular orbitals
σ* - orbitals are the highest energy unoccupied molecular orbitals
π - orbitals are of somewhat higher energy occupied molecular orbitals
π* - orbitals are lower in energy (unoccupied molecular orbitals) than σ*
n - orbitals; Unshared pairs (electrons) lie at the energy of the original atomic orbital.
Most often n - orbitals energy is higher than σ and π.
since no bond is formed, there is no benefit in energy
36
18
Electronic Energy Levels
Graphically,
σ∗
Unoccupied levels
π∗
Energy Atomic orbital
Atomic orbital
n
Occupied levels
π
σ
Molecular orbitals
37
Electronic Transitions
•
The valence electrons in organic molecules are involved in bonding as
σ - bonds, π - bonds or present in the non-bonding form (lone pair)
•
Due to the absorption of UV - Visible radiation by an organic molecule different
electronic transitions within the molecule occurs depending upon the nature of
bonding.
•
The wavelength of UV - Visible radiation causing an electronic transition depends
on the energy of bonding and antibonding orbitals.
•
The lowest energy transition is typically that of an electron in the
Highest Occupied Molecular Orbital (HOMO) to the
Lowest Unoccupied Molecular Orbital (LUMO)
σ∗
π∗
Energy Atomic orbital
n
π
σ
Unoccupied levels
Atomic orbital
Occupied levels
38
Molecular orbitals
19
Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
They are of three types:
σ → σ*
π → π*
σ → π*
39
Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
σ → σ* (bonding σ to anti-bonding σ)
σ∗ (anti-bonding)
•
σ → σ* transition requires large energies in
far UV region in 120-200nm range.
π∗ (anti-bonding)
•
Molar absorptivity: Low
εmax = 1000 - 10000
n (non-bonding)
π (bonding)
σ (bonding)
•
Examples: Alkanes - transition @ 150nm
Methane
Cyclohexane
Propane
40
20
Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
σ → σ* (bonding σ to anti-bonding σ)
+
C_
_ C
+C
+
C
_
σ*
C-C
_
σ C-C
41
Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
π → π* (bonding π to anti-bonding π)
σ∗ (anti-bonding)
•
π → π* occur in 200-700nm range.
π∗ (anti-bonding)
•
Molar absorptivity: High
εmax = 1000 - 10000.
n (non-bonding)
Examples:
•Unsaturated compounds
•double or triple bonds
•aromatic rings
•Carbonyl groups
•azo groups
•Conjugated π electrons
Carbonyl
Azo
π (bonding)
σ (bonding)
εmax is high because the π and π* orbitals
are in same plane and consequently the
probability of jump of an electron from π →
π* orbital is very high.
42
21
Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
π → π* (bonding π to anti-bonding π)
_
+
C
_
C
+
+
C
C
_
π*
π
43
Types of Electronic Transitions
Transition between bonding molecular orbitals and anti-bonding molecular orbitals
σ → π* (bonding σ to anti-bonding π)
•
σ → π* occur only in <150 nm range.
•
Molar absorptivity: Low
Examples: Carbonyl compounds
σ∗ (anti-bonding)
π∗ (anti-bonding)
n (non-bonding)
π (bonding)
σ (bonding)
•σ → σ* and σ → π* transitions: high-energy, accessible in
vacuum UV (λmax <150 nm). Not usually observed in
molecular UV-Vis.
44
22
Types of Electronic Transitions
Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals
They are of two types:
σ∗ (anti-bonding)
n → π*
n → σ*
π∗ (anti-bonding)
n → π* (non-bonding n to anti-bonding π)
•
n → π* occur in 200-700nm range.
•
Molar absorptivity: Low
εmax = 10 - 100
•
n (non-bonding)
π (bonding)
σ (bonding)
Examples:
•
Compounds with double bonds involving unshared pair(s) of electrons
•
Aldehydes, Ketones
•
C=O, C=S, N=O etc.,
45
Types of Electronic Transitions
Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals
n → π* (non-bonding n to anti-bonding π)
_
+
C
+
O
_
C
O
π*
+
_
+
C
C
_
n(py)
π
46
23
Types of Electronic Transitions
•Spectra of aldehydes or ketones exhibit two bands;
A High intense band at 200-250nm due to π → π*
A low intense band at 300nm due to n → π* transition
•n
→
π*
because…….
transition
is
always
less
intense
• The electrons in the n-orbitals are situated perpendicular
to the plane of π bond and hence to the plane of π* orbital.
n ⊥ to π
Consequently, the probability of jump of an electron
from n → π* orbital is very low and in fact zero
according to symmetry selection rules.
But, vibrations of atoms bring about a partial overlap
between the perpendicular planes and so n → π*
transition does occur, but only to a limited extent.
47
Types of Electronic Transitions
Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals
n → σ* (non-bonding n to anti-bonding σ)
σ∗ (anti-bonding)
•
π∗ (anti-bonding)
Excitation of an electron in an unshared
pair on Nitrogen, oxygen, sulphur or
halogens to an antibonding σ orbital is
called n → σ* transitions.
•
n → σ* occur in 150-250nm range.
•
Molar absorptivity: Low
εmax = 100 - 3000
n (non-bonding)
π (bonding)
σ (bonding)
Example:
Methanol
λ max = 183nm
(εε = 500)
λ max = 257nm
(εε = 486)
Trimethylamine λ max = 227nm
(εε = 900)
1-Iodobutane
48
24
Types of Electronic Transitions
Transition between non-bonding atomic orbitals and anti-bonding molecular orbitals
n → σ* (non-bonding n to anti-bonding σ)
+
C_
_
+N
C-N
_
N
C
σ*
n
+
_ C
+
N _
σ C-N
49
Types of Electronic Transitions
σ → σ* (bonding σ to anti-bonding σ)
π → π* (bonding π to anti-bonding π)
σ → π* (bonding σ to anti-bonding π)
σ∗ (anti-bonding)
π∗ (anti-bonding)
n → π* (non-bonding n to anti-bonding π)
n → σ* (non-bonding n to anti-bonding σ)
n (non-bonding)
π (bonding)
σ (bonding)
Energy required for various transitions obey the order: σ → σ* > n → σ* > π → π*> n → π*
50
25
Types of Electronic Transitions
•
From the molecular orbital diagram it is clear that, In all compounds other than
alkanes there are several possible electronic transitions that can occur with different
energies.
σ∗
π∗
Energy
n
π
σ
σ∗ alkanes 150 nm
σ
π∗ carbonyls 170 nm
π
π∗ unsaturated compounds 180 nm
n
σ∗
n
π∗ carbonyls 300 nm
If conjugated
O, N, S, halogens 190 nm
σ
51
Selection Rules
•
•
Not all transitions that are possible in UV region are not generally observed.
For an electron to transition, certain quantum mechanical constraints apply – these
are called “selection rules”.
The selection rules are,
•
Rule - 1:The transitions which involve an change in the spin quantum number of an
electron during the transition are not allowed to take place or these are
“forbidden”.
•
Rule - 2: singlet –triplet transitions are forbidden
Multiplicity of states (2S+1); Where, S is total spin quantum number.
•Singlet state: have electron spin paired
•Triplet state: have two spins parallel
•Here,
•For excited singlet state: S=0; therefore, 2S+1=1 - transition allowed
•For excited triplet state: S=1; therefore, 2S+1=3 - transition forbidden
52
26
Selection Rules
Rule - 3: Symmetry of electronic states; n → π* transition in formaldehyde is
forbidden by local symmetry. i.e., Energy is always a function of molecular
geometry.
In formaldehyde (H2C=O),
In n → π* excited state an electron arrives at the antibonding π orbital,
while the electron pair in the bonding π orbital is still present.
Due to the third antibonding π electron, the C=O bond becomes
weaker and longer.
In the π → π* excited configuration, the situation is somewhat worse
because there is only one π electron in the bonding orbital, while the
other π electron is anti-bonding (i.e. π*).
Consequently, the excited state bond lengths will be longer than a
genuine C=O double bond but shorter than a σ -type single C-O bond.
In other words, these excited states will have their energy minima
somewhere in between that of H2C=O and H3C-OH.
•To further complicate matters, “forbidden” transitions are sometimes observed (albeit at low intensity) due to
other factors.
53
Franck and Condon Principle
•
Electronic transitions will take place only when the inter-nuclear distances are not
significantly different in the two states and where the nuclei have little or no velocity.
•
Thus, the forbidden transitions may arise when the inter-nuclear distances are
significantly different in the two states and where the nuclei have significant velocity.
Franck–Condon principle is the approximation that an electronic transition
is most likely to occur without changes in the positions of the nuclei in the
molecular entity and its environment.
54
27
Origin and General appearance of UV bands
•
Electronic spectra is a graphical output of transitions between electronic energy
levels.
•
We know that, electronic transitions are accompanied by changes in both vibrational
and rotational states.
•
The wavelength of absorption depends on the energy difference between
bonding/antibonding and non-bonding orbitals concerned.
•
When gaseous sample is irradiated with UV - Visible light and the spectrum is
recorded, a spectrum with number of closely spaced fine structure line is obtained.
•
When the electronic spectrum of a solution is recorded, a absorption band is obtained
in which closely spaced fine lines are merging together due to the solvent-solute
interaction.
•
Usually electronic absorption spectrums are broader bands than IR or NMR bands.
55
Designation of UV bands
•
The absorption bands in the UV - Visible spectrum may be designated either by using
electronic transitions [σ
σ → σ*, σ → π*, π → π*, n → π*, n → σ*] or the
letter designation as given below.
R – bands (German, radikalartig)
•
The bands due to n→
→ π* transitions of single chromophoric groups are referred to as
the R - Bands.
•
Example: Carbonyl group, Nitro group
•
Shows low molar absorptivity (εεmax<100) and hypsochromic shift with an increase in
solvent polarity.
K – bands (German, konjugierte)
•
The bands due to π→ π* transitions in molecules containing conjugated π systems
are referred to as the K – Bands.
•
Example: Butadiene, mesityl oxide
•
They show high molar absorptivity (εεmax<10,000).
56
28
Designation of UV bands
B and E - bands
•
The B and E bands are characteristic of the spectra of aromatic or heteroaromatic
molecules.
Examples:
•
All benzenoid compounds exhibit E and B
bands representing π → π* transitions.
•
In benzene, E1 and E2 bands occur near
180nm and 200nm respectively and their
molar absorptivity varies between (εεmax =
2000 to εmax = 14000).
•
The B-band occurs in the region from 250nm
to 255nm as a broad band containing
multiple fine structure and represents a
symmetry-forbidden transition which has
finite but low probability due to forbidden
transitions in high symmetrical benzene
molecule.
The vibrational fine structure appears only in
the B-band and disappears frequently in the
more polar solvents.
•
57
Chromophores
The coloured substances owe their colour to the presence of one or more unsaturated
groups responsible for electronic absorption. These groups are called chromophores.
Examples: C = C, C=C, C = N, C=N, C = O, N = N, etc..
Chromophores absorb intensely at the short wavelength
But, some of them (e.g, carbonyl) have less intense bands at higher wavelength due to
the participation of n electrons.
Methyl orange
58
29
Chromophores: examples
Chromophore
Example
Excitation
λmax, nm
ε
Solvent
C=C
Ethene
Π __> Π*
171
15,000
hexane
C≡C
1-Hexyne
Π __> Π*
180
10,000
hexane
C=O
Ethanal
n __> Π*
Π __> Π*
290
180
15
10,000
hexane
hexane
N=O
Nitromethane
n __> Π*
Π __> Π*
275
200
17
5,000
ethanol
ethanol
C-X; X=Br
X=I
Methyl bromide
Methyl Iodide
n __> σ*
n __> σ*
205
255
200
360
hexane
hexane
59
Auxochromes
An auxochromes is an auxillary group which interact with chromophore and deepens
colour; its presence causes a shift in the UV or visible absorption maximum to a longer
wavelength
Examples: NH2, NHR and NR2, hydroxy and alkoxy groups.
Property of an auxochromic group:
• Provides additional opportunity for charge delocalization and thus provides smaller
energy increments for transition to excited states.
• The auxochromic groups have atleast one pair of non-bonding electrons (lone pair)
that can interact with the π electrons and stabilizes the π* state
60
30
Auxochromes: examples
Auxochrome
Unsubstitued
chromophore
λ max (nm)
Substituted chromophore
λ max
(nm)
-CH3
H2C=CH-CH = CH2
217
H2C=CH-CH=CHCH3
224
-OR
H3C-CH=CH-COOH
204
H3C-C(OCH3) = CHCOOH
234
-C1
H2C=CH2
175
H2C = CHCl
185
61
Bathochromic shift (Red shift) - λmax to longer wavelength
Shift of an absorption maximum to longer wavelength is called bathochromic shift.
Occurs due to change of medium (π
π → π* transitions undergo bathochromic shift with an
increase in the polarity of the solvent)
OR
when an auxochrome is attached to a carbon-carbon double bond
Example:
Ethylene
1-butene / isobutene
: λ max = 175nm
: λ max = 188 nm
The bathochromic shift is progressive as the number of alkyl groups increases.
62
31
Hypsochromic shift (Blue shift) - λmax to Shorter wavelength
Shift of absorption maximum to shorter wavelength is known as hypsochromic shift.
Occurs due to change of medium (n → π* transitions undergo hypsochromic shift with an
increase in the polarity of solvent)
OR
when an auxochrome is attached to double bonds where n electrons (eg: C=O) are
available
Example: Acetone
λ max = 279nm in hexane
λ max = 264.5nm in water
This blue shift results from hydrogen bonding which lowers the energy of the n orbital.
63
Hyperchromic effect - increased (εεmax) absorption intensity
It is the effect leading to increased absorption intensity
Example: intensities of primary and secondary bands of phenol are increased in
phenolate
Compound
Phenol
C6H5OH
Phenolate anion C6H5O-
Primary band
Secondary band
λ max (nm)
εmax
λ max (nm)
εmax
210
6200
270
1450
235
9400
287
2600
64
32
Hypochromic effect - decreased (εεmax) absorption intensity
It is the effect leading to decreased absorption intensity
Example: intensities of primary and secondary bands of benzoic acid are decreased in
benzoate
Compound
Primary band
λ max (nm)
εmax
Secondary band
λ max (nm)
εmax
Benzoic acid
C6H5COOH
230
11600
273
970
Benzoate
C6H5COO-
224
8700
268
560
65
Effect of substituents on λmax and εmax
Graphically,
Shift to increased
εmax
Shift to shorter
λ max
Shift to Longer
λ max
Shift to decreased
εmax
66
33
Isosbestic point
A point common to all curves produced in the spectra of a compound taken at various
pH values is called isosbestic point.
If one absorbing species, X, is converted to another absorbing species, Y, in a
chemical reaction, then the characteristic behaviour shown in the figure below is
observed.
If the spectra of pure X and
pure Y cross each other at any
wavelength, then every
spectrum recorded during this
chemical reaction will cross at
the same point, called an
isosbestic point. The
observation of an isosbestic
point during a chemical
reaction is good evidence that
only two principal species are
present.
Example: Absorption spectrum of 3.7×10-4 M methyl red as a function of pH between pH 4.5 and 7.1
The aniline-anilinium or phenol-phenolate conversion as a function of pH can demonstrate the
presence of the two species in equilibrium by the appearance of an isosbestic point in the UV
spectrum.
67
UV Spectroscopy (Electronic Spectra) - Terminologies
Beer-Lambert Law
Absorbance
Molar absorptivity
Extinction coefficicent
concentration
Path length
λmax
εmax
Band
HOMO
LUMO
Chromophore
Auxochrome
A = ε.c.l
A, a measure of the amount of radiation that is absorbed
ε, absorbance of a sample of molar concentration in 1 cm cell.
An alternative term for the molar absorptivity.
c, concentration in moles / litre
l, the length of the sample cell in cm.
The wavelength at maximum absorbance
The molar absorbance at λmax
Term to describe a uv-vis absorption which are typically broad.
Highest Occupied Molecular Orbital
Lowest Unoccupied Molecular Orbital
Structural unit responsible for the absorption.
A group which extends the conjugation of a chromophore by
sharing of nonbonding electrons
Bathochromic shift
The shift of absorption to a longer wavelength.
Hypsochromic shift
shift of absorption to a shorter wavelength
Hyperchromic effect
An increase in absorption intensity
Hypochromic effect
A decrease in absorption intensity
Isosbestic point
point common to all curves produced in the spectra of a compound
taken at various pH
68
34
Instrumentation
I0
reference
I1
detector
monochromator/
beam splitter optics
I0
sample
log(I0/I) = A
UV-VIS sources
200
700
λ, nm
I
I2
69
Instrumentation…
Radiation source, monochromator and detector
Two sources are required to scan the entire UV-VIS band:
Deuterium lamp – covers the UV – 200-330
Tungsten lamp – covers 330-700
The lamps illuminate the entire band of UV or visible light; the
monochromator (grating or prism) gradually changes the small bands of
radiation sent to the beam splitter
The beam splitter sends a separate band to a cell containing the sample
solution and a reference solution
The detector (Photomultiplier, photoelectric cells) measures the difference
between the transmitted light through the sample (I) vs. the incident
light (I0) and sends this information to the recorder
70
35
Sample Handling
Virtually all UV spectra are recorded solution-phase
Only quartz is transparent in the full 200-700 nm range;
plastic and glass are only suitable for visible spectra 380 – 800nm
Concentration: 0.1 to 100mg
10-5 to 10-2 molar concentration may safely be used
Percentage of light transmitted: 20% to 65%
At high concentrations, amount of light transmitted is low, increasing the
possibility of error
A typical sample cell (commonly called a cuvet):
Cells can be made of plastic, glass or quartz
(standard cells are typically 1 cm in path length)
71
Solvents
•
•
•
•
•
•
Solvents must be transparent in the region to be observed
solvents must preserve the fine structure
solvents should dissolve the compound
Non-polar solvent does not form H-bond with the solute (and the spectrum is
similar to the spectrum of compound at gaseous state)
Polar solvent forms H-bonding leading to solute-solvent complex and the fine
structure may disappear.
The wavelength from where a solvent is no longer transparent is termed as cutoff
Common solvents and cutoffs:
acetonitrile
chloroform
cyclohexane
1,4-dioxane
95% ethanol
n-hexane
methanol
isooctane
water
nm
190
240
195
215
205
201
205
195
190
72
36
Factors affecting the position of UV bands – 1. Non-conjugated alkenes
•
A π→ π* transition can occur in simple non-conjugated alkene like ethene and
other alkenes with isolated double bonds below 200 nm.
π∗
π
73
Factors affecting the position of UV bands – 1. Non-conjugated alkenes…
•
Alkyl substitution of parent alkene moves the absorption to longer wavelengths.
• From λmax di-, tri & tetra substituted double bonds in
systems can be identified
acyclic and alicyclic
74
37
Factors affecting the position of UV bands – 1. Non-conjugated alkenes…
• This bathochromic effect of alkyl substitution is due to the extension of the
chromophore, in the sense that there is a small interaction, due to hyperconjugation,
between the σ electrons of the alkyl group and the chromophoric group.
H
C
C
C
Methyl groups also cause a
bathochromic shift, even though they
are devoid of p-or n-Electrons
H
H
This effect is thought to be through what
is termed “HYPERCONJUGATION” or
sigma bond resonance
“HYPERCONJUGATION”
• This effect is progressive as the number of alkyl groups increases.
• The intensity of alkene absorption is essentially independent of solvent because of the
non-polar nature of the alkene bond.
75
Factors affecting the position of UV bands – 2. Conjugated Dienes
A conjugated system requires lower energy for the π→ π* transition than an
unconjugated system.
Example:
Ethylene
and
Butadiene
In conjugated butadiene (λ
λmax=217nm; εmax = 21000)
π and π* orbitals have energies much closer together than those in ethylene,
resulting in a lower excitation energy
Ethylene has only two orbitals;
one ground state π bonding orbital and one excited state π* antibonding orbital.
The energy difference (∆Ε) between them is about 176 kcal/mole.
76
38
Factors affecting the position of UV bands
[i.e., From MOT, two atomic p orbitals, from two sp2 hybrid carbons combine to form two
MOs π and π* in ethylene,]
π2∗
p
p
π1
π
77
Factors affecting the position of UV bands - 2. Conjugated Dienes
In butadiene, 4 p orbitals are mixing and 4 MOs of an energetically symmetrical distribution
compared to ethylene.
Therefore, the following π and π* for ethylene and butadiene will be obtained.
π 4∗
π 2∗
π 3∗
π2
π1
π
Ethylene
π1
Butadiene
78
39
Factors affecting the position of UV bands - 2. Conjugated Dienes
Butadiene, however, with four π electrons has four available π orbitals, two bonding (π1 and π2) and two
antibonding (π*3 and π*4) orbitals.
The π1 bonding orbital encompasses all the four π electrons over the four carbon atoms of the butadiene
system and is somewhat more stable than a single π bonding orbital in ethylene.
The π2 orbital is also bonding orbital, but is of higher energy than the π1 orbital.
The two π* orbitals (π*3 and π*4) are respectively, more stable ((π*3) and less stable (π*4) than the π*
orbital of ethylene.
Energy absorption, with the appearance of an absorption band, can thus occur by a π2 (bonding) → (π*3
(antibonding transition. HOMO to LUMO), the energy difference of which (136 kcal/mole) is less than that
of the simple π→π* transition of ethylene (176 kcal/mole) giving a λmax= 217 nm; (i.e., at a longer
wavelength).
It is to be expected that the greater the number of bonding π orbitals, the lower will be the energy
between the highest bonding π orbital and the lowest excited π* orbital.
The obvious extension of this in terms of λmax is that the greater the number of conjugated double bonds,
the longer the wavelength of absorption.
79
Factors affecting the position of UV bands - 2. Conjugated Dienes
π 4∗
π 2∗
π 3∗
136 kcal/mole
∆Ε = 176 kcal/mole
π1
π2
π
π1
∆E for the HOMO LUMO transition is REDUCED
80
40
Factors affecting the position of UV bands - 2. Conjugated Dienes
Extending this effect out to longer conjugated systems the energy
gap becomes progressively smaller: For example
Energy
Lower energy = Longer wavelengths
ethylene
butadiene
hexatriene
octatetraene
81
Factors affecting the position of UV bands - 2. Conjugated Dienes - Types
•
Acyclic dienes: 1,3-Butadiene with the structural formula
•
Homo-annular conjugated dienes: Both conjugated double bonds are in same ring
•
Hetero-annular dienes: Conjugated double bonds are not present in same ring
82
41
Factors affecting the position of UV bands - 2. Conjugated Dienes - Types
•
Exocyclic and Endocyclic double bond:
Endocyclic
double bond
Exocyclic
double bond
83
Factors affecting the position of UV bands - 2. Conjugated Dienes - Types
1. Acyclic diene or Heteroannular diene
•
Most acyclic dienes have transoid conformation;
•
i.e. trans disposition of double bonds about a single
bond.
•
Base λmax=217 nm (εεmax = 5000-20000).
• Heteroannular diene, is a conjugated system in which the
two double bonds are confined to two different rings.
• Base λmax= 214 nm (εmax = 5000-20000).
s-trans
Base λmax=217 nm
εmax = 5000-20000
A
B
Base λmax=214 nm
εmax = 5000-20000
84
42
Factors affecting the position of UV bands - 2. Conjugated Dienes - Types
2. Homoannular diene
In homoannular diene, the two conjugated double bonds are
confined to a single ring.
i.e., the cyclic dienes are forced into an s-cis (cisoid) conformation.
Base λmax=253 nm
εmax = 5000-8000
Base λmax= 253 nm (εmax = 5000-8000).
Homoannular dienes contained in other ring sizes possess different
s-cis
base absorption values.
Example:
Cyclopentadiene; λmax=228nm
Cycloheptadiene; λmax= 241nm
85
Factors affecting the position of UV bands - 2. Conjugated Dienes
When two or more C=C units are conjugated,
The energy difference ∆E between the highest bonding π orbital (HOMO) and the
lowest excited π* orbital (LUMO) becomes small and results in a shift of λmax
to longer wavelength i.e., Bathochromic shift.
This concept helps to distinguish between the two isomeric diens,
1,5-hexadiene and 2, 4- hexadiene, from the relative positions of λmax.
H2C=CH-CH2-CH2-CH=CH2
1,5-Hexadiene
(non-conjugated diene)
λmax = 178 nm
CH3-CH=CH-CH=CH-CH3
2,4-Hexadiene
(conjugated diene)
λmax = 227 nm
86
43
Factors affecting the position of UV bands - 2. Conjugated Dienes
87
Factors affecting the position of UV bands - 2. Conjugated Dienes - Types
As the number of double bonds in conjugation increases, ∆E for the excitation of
an electron continues to become small and consequently there will be a
continuous increase in the value of λmax
Example:
λ max = 217
253
220
227
227
256
263
nm
Longer wavelengths = Lower energy
88
44
Factors affecting the position of UV bands - 2. Conjugation… with hetero atoms
Conjugation with a heteroatom [N, O, S, X] moves the (π → π*) absorption of
ethylene to longer wavelengths
Example: CH2=CH-OCH3 (λmax=190nm) - εmax~10000
CH2=CH-NMe2 (λmax=230nm) - εmax~10000
Methyl vinyl sulphide absorbs at 228 nm (εmax=8000)
A
Here we create 3 MOs –
this interaction is not as
strong as that of a
conjugated π-system
Ψ3∗
π∗
Energy
Ψ2
π
nA
Ψ1
89
Factors affecting the position of UV bands – 3. Effect of Geometrical isomerism - Steric effect
• In compounds where geometrical isomerism is possible.
Example:
trans - stilbene absorbs at longer wavelength [λ
λ max=295 nm] (low energy)
cis - stilbene absorbs at shorter wavelength [λ
λmax=280 nm] (high energy) due to the steric
effects.
• Coplanarity is needed for the most effective overlap of the π - orbitals and increased ease of the
π → π* transition. The cis-stilbene is forced into a nonplanar conformation due to steric effects.
90
45
Factors affecting the position of UV bands –
4. Effect of steric hindrance on coplanarity (steric inhibition of resonance)
• UV spectroscopy is very sensitive to distortion of the chromophore and consequently the steric
repulsions which oppose the coplanarity of conjugated π-electron systems can easily be
detected by comparing its UV spectrum with that of a model compound.
• Distortion of the chromophore may lead to RED or BLUE shifts depending upon the nature of
the distortion.
Example-1: Distortion leading to RED shift
The strained molecule Verbenene
exhibits λmax=245.5nm whereas the
usual calculation shows at λmax=229 nm.
Verbenene
Actual;
λmax =245.5nm
Calculated;
λmax =229nm
91
Factors affecting the position of UV bands –
4. Effect of steric hindrance on coplanarity (steric inhibition of resonance)….
Example-2: Distortion leading to BLUE shift
The diene shown here might be expected to
have a maximum at 273nm.
But, distortion of the chromophore,
presumably out of planarity with consequent
loss of conjugation, causes the maximum
to be as low as 220nm with a similar loss in
intensity (εmax =5500).
Actual;
Calculated;
λmax =220nm
λmax =273nm
92
46
Factors affecting the position of UV bands –
4. Effect of steric hindrance on coplanarity (steric inhibition of resonance)…..
Example-3: trans-azobenzene and the sterically restricted cis-azobenzene
H H
Absorption of Azobenzene (in ethanol)
Example
π→ π*
transition
n→
→ π*
transition
λmax
εmax
λmax
trans-isomer
320
21300
443
εmax
510
cis-isomer
281
5260
433
1520
Such differences between cis and trans
isomers are of some diagnostic value
93
Factors affecting the position of UV bands – 5. Effect of Solvents
• The position and intensity of an absorption band is greatly affected by the
polarity of the solvent used for running the spectrum.
• Such solvent shifts are due to the differences in the relative capabilities of the
solvents to solvate the ground and excited states of a molecule.
•
Non-polar compounds like Conjugated dienes and aromatic hydrocarbons exhibit
very little solvent shift,
94
47
Factors affecting the position of UV bands – 5. Effect of Solvents…
The following pattern of shifts are generally observed for changes to solvents of
increased polarity:
•
α, β-Un saturated carbonyl compounds display two different types of shifts.
(i) n→ π* Band moves to shorter wavelength (blue shift).
(ii) π→ π* Band moves to longer wavelength (red shift)
95
Factors affecting the position of UV bands – 5. Effect of Solvents…
α, β-Un saturated carbonyl compounds - For increased solvent polarity
•
n→
→
π*
Band
moves
to
shorter
wavelength
(blue
shift).
In n→ π * transition the ground state is more polar than excited state. The
hydrogen bonding with solvent molecules takes place to a lesser extent
with the carbonyl group in the excited state.
Example:
λmax= 279nm in hexane
π*
λmax= 264nm in water
B
D
AB < CD
n
A
Non-polar solvent
C
Polar solvent
Shorter wavelength
96
48
Factors affecting the position of UV bands – 5. Effect of Solvents…
α, β-Un saturated carbonyl compounds - For increased solvent polarity
(ii) π → π* Band moves to longer wavelength (Red shift).
In π → π * the dipole interactions with the solvent molecules lower the energy
of the excited state more than that of the ground state. Thus, the value of
λmax in ethanol will be greater than that observed in hexane. i.e., π* orbitals
are more stabilized by hydrogen bonding with polar solvents like water and
alcohol. Thus small energy will be required for such a transition and absorption
shows a red shift.
Example:
π*
B
D
AB > CD
π
A
Non-polar solvent
C
Polar solvent
Longer wavelength
97
Factors affecting the position of UV bands – 5. Effect of Solvents…
α, β-Un saturated carbonyl compounds - For increased solvent polarity
(iii) In general,
a) If the group (carbonyl) is more polar in the ground state than in the excited
state, then increasing polarity of the solvent stabilizes the non-bonding
electron
in
the
ground
state
due
to
hydrogen
bonding.
Thus, absorption is shifted to shorter wave length.
b) If the group (carbonyl) is more polar in the excited state, the absorption is
shifted to longer wavelength with increase in polarity of the solvent which
helps in stabilizing the non-bonding electrons in the excited state.
98
49
Factors affecting the position of UV bands – 6. Conformation and geometry in polyene systems
• The position of absorption depends upon the length of the conjugated system.
• Longer the conjugated system, higher will be the absorption maximum and larger
will be the value of the extinction coefficient.
• If in a structure, the π electron system is prevented from achieving
coplanarity, In long-chain conjugated polyenes, steric hindrance to coplanarity
can arise when cis-bonds are present.
• This is illustrated by the naturally occurring bixin (`all trans’ methyl carotenoid)
and its isomer with a central cis-double bonds.
• In the latter the long wavelength band is weakened and a diagnostically useful
`cis-band` probably due to partial chromophore, appears at shorter wavelength.
99
Absorption spectra of Unsaturated carbonyl compounds…….
Enones
unsaturated systems incorporating N or O can undergo
n π* transitions in addition to π π*
π π* transitions; λmax~188 nm; εmax = 900
n π* transitions; λmax~285 nm; εmax = 15
Low intensity is due to the fact this transition is forbidden by the selection rules
it is the most often observed and studied transition for carbonyls
Similar to alkenes and alkynes,
non-substituted carbonyls undergo the
(λmax=188 nm; εmax=900)
π π* transition in the vacuum UV
Both this transitions are also sensitive to substituents on the carbonyl
100
50
Absorption spectra of Unsaturated carbonyl compounds…….
π∗
n
π
Enones
Remember, the π π* transition is
allowed and gives a high ε, but lies
outside the routine range of UV
observation
The n π* transition is forbidden and
gives a very low e, but can routinely be
observed
101
Absorption spectra of Unsaturated carbonyl compounds…….
Enones
Carbonyls – n π* transitions (~285 nm); π π* (188 nm)
O
π∗
n
π
C
O
It has been determined
from spectral studies,
that carbonyl oxygen
more approximates sp
rather than sp2 !
O
σCO transitions omitted for clarity
102
51
Absorption spectra of Unsaturated carbonyl compounds…….
Enones
For auxochromic substitution on the carbonyl, pronounced hypsochromic (blue)
shifts are observed for the n π* transition (λmax):
O
H
293 nm
O
CH3
279 nm
Blue shift
O
Cl
235 nm
λmax
O
NH2
O
O
O
This is explained by the inductive
withdrawal of electrons by O, N or
halogen from the carbonyl carbon – this
causes the n-electrons on the carbonyl
oxygen to be held more firmly
214 nm
204 nm
It is important to note this is different from
the auxochromic effect on π π* which
extends conjugation and causes a
bathochromic shift
In most cases, this bathochromic shift is not
enough to bring the π π* transition into
the observed range
204 nm
OH
Absorption spectra of Unsaturated carbonyl compounds…….
103
Enones
Conversely, if the C=O system is conjugated both the n π* and π π* bands are
Bathochromically (Red) shifted
Here, several effects must be noted:
• the effect is more pronounced for π π*
• if the conjugated chain is long enough, the much higher intensity
π π* band will overlap and drown out the n π* band
• the shift of the n π* transition is not as predictable
For these reasons, empirical Woodward-Fieser rules for conjugated
enones are for the higher intensity, allowed π π* transition
104
52
Absorption spectra of Unsaturated carbonyl compounds…….
Enones
Conjugation effects are apparent; from the MO diagram for a conjugated enone:
Ψ4∗
π∗
π∗
Ψ3∗
n
n
Ψ2
π
π
Ψ1
O
O
105
Absorption spectra of Alkanes
- Miscellaneous
Alkanes – only posses σ-bonds and no lone pairs of electrons, so only the high
energy σ σ* transition is observed in the far UV
This transition is destructive to the molecule, causing cleavage of the σ-bond
C
σ∗
σ
C
C
C
106
53
Absorption spectra of Aliphatic compounds
- Miscellaneous
Alcohols, ethers, amines and sulfur compounds – in the cases of simple, aliphatic
examples of these compounds the n σ* is the most often observed transition;
like the alkane σ σ* it is most often at shorter λ than 200 nm
Note how this transition occurs from the HOMO to the LUMO
σ∗CN
C
N
C
nN sp
C
3
σCN
anitbonding
orbital
N
C
N
N
107
Woodward – Fieser rules
•
•
•
•
It is used for calculating λmax
Calculated λmax differs from observed values by 5-6%.
Effect of substituent groups can be reliably quantified by
use Woodward –Fieser Rule
Separate values for conjugated dienes and trines and αβ-unsaturated ketnones are available
Robert B. Woodward
Nobel Prize in Chemistry : 1965
108
54
Woodward – Fieser rules
Woodward-Fieser Rules
Woodward and the Fiesers performed extensive studies of terpene and
steroidal alkenes and noted similar substituents and structural features
would predictably lead to an empirical prediction of the wavelength for
the lowest energy π π* electronic transition
This work was distilled by Scott in 1964 into an extensive treatise on the
Woodward-Fieser rules in combination with comprehensive tables and
examples – (A.I. Scott, Interpretation of the Ultraviolet Spectra of Natural
Products, Pergamon, NY, 1964)
A more modern interpretation was compiled by Rao in 1975 – (C.N.R.
Rao, Ultraviolet and Visible Spectroscopy, 3rd Ed., Butterworths, London,
1975)
109
Woodward – Fieser rules for Dienes
The rules begin with a base value for λmax of the chromophore being observed:
For
acyclic butadiene = 217 nm
or 214 nm
The incremental contribution of substituents is added to this base value
from
the group tables:
Group
Increment
Extended conjugation
+30
Each exo-cyclic C=C
+5
Alkyl
+5
-OCOCH3
+0
-OR
+6
-SR
+30
-Cl, -Br
+5
-NR2
+60
110
55
Woodward – Fieser rules for Dienes – Examples -1 & 2
Isoprene - acyclic butadiene =
217 nm
one alkyl subs.
Calculated value
+ 5 nm
222 nm
Observed value
220 nm
Allylidenecyclohexane - acyclic butadiene = 217 nm
one exocyclic C=C
+ 5 nm
2 alkyl subs.
Calculated value
+10 nm
232 nm
Observed value
237 nm
111
Woodward – Fieser rules for Dienes – Problem - 1
acyclic butadiene = 217 nm
Group
Increment
Solution:
Extended conjugation
+30
acyclic butadiene
= 217 nm
Each exo-cyclic C=C
+5
extended conjugation
= +30 nm
Alkyl
+5
Calculated value
= 247 nm
-OCOCH3
+0
-OR
+6
-SR
+30
-Cl, -Br
+5
-NR2
+60
112
56
Woodward – Fieser rules for Dienes – Example-3
113
Woodward – Fieser rules for Cyclic Dienes
Heteroannular (transoid)
Homoannular (cisoid)
Base λmax = 253
Base λmax = 214
The increment table is the same as for acyclic butadienes with a couple additions:
Group
Additional homoannular
Where both types of
diene are present, the
one with the longer λ
becomes the base
Increment
+39
Group
Increment
Extended conjugation
+30
Each exo-cyclic C=C
+5
Alkyl
+5
-OCOCH3
+0
-OR
+6
-SR
+30
-Cl, -Br
+5
-NR2
+60
114
57
Woodward – Fieser rules for Cyclic Dienes – Example-4
1,2,3,7,8,8a-hexahydro-8a-methylnaphthalene
Heteroannular diene
= 214 nm
3 alkyl subs. (3 x 5)
= +15 nm
1 exo C=C
= + 5 nm
Calculated value
234 nm
Observed value
235 nm
115
Woodward – Fieser rules for Dienes – Problem - 2
Heteroannular diene = 214 nm
Group
Increment
Extended conjugation
+30
Each exo-cyclic C=C
+5
Alkyl
+5
-OCOCH3
+0
-OR
+6
-SR
+30
-Cl, -Br
+5
-NR2
+60
Solution:
Heteroannular diene
= 214 nm
Ring residues /
Alkyl substitution
3x5
= + 15 nm
Exocyclic C=C bond 1 x 5
= + 5 nm
Calculated value
= 234 nm
Observed value
= 247 nm
116
58
Woodward – Fieser rules for Cyclic Dienes – Example-5
117
Woodward – Fieser rules for Cyclic Dienes – Example-6
abietic acid
C OH
O
heteroannular diene =
214 nm
4 alkyl subs. (4 x 5)
1 exo C=C
+20 nm
+ 5 nm
239 nm
118
59
Woodward – Fieser rules for Cyclic Dienes – Example-7
levopimaric acid
homoannular diene =
253 nm
4 alkyl subs. (4 x 5)
1 exo C=C
+20 nm
+ 5 nm
278 nm
C OH
O
119
Woodward – Fieser rules for Dienes – Problem - 3
Homoannular diene = 253 nm
Group
Increment
Solution:
Additional
homoannular
+39
=
253 nm
Extended conjugation
+30
Extended conjugation 1 x 30 =
+30 nm
Each exo-cyclic C=C
+5
Alkyl substitution
=
+ 10 nm
Alkyl
+5
Calculated value
=
293 nm
-OCOCH3
+0
-OR
+6
-SR
+30
-Cl, -Br
+5
-NR2
+60
Homoannular diene
2x5
120
60
Woodward – Fieser rules for Cyclic Dienes – Example-8
121
Woodward – Fieser rules for Dienes – Examples – 9,10 & 11
122
61
Woodward – Fieser rules for Cyclic Dienes – PRECAUTIONS
Be careful with your assignments – three common errors:
R
This compound has three exocyclic
double bonds; the indicated bond is
exocyclic to two rings
This is not a heteroannular diene; you would
use the base value for an acyclic diene
Likewise, this is not a homooannular diene;
you would use the base value for an acyclic
diene
123
Woodward – Fieser rules for Enones
β α
β C C C
δ γ β α
δ C C C C C
O
Group
Increment
6-membered ring or acyclic enone
Base 215 nm
5-membered ring parent enone
Base 202 nm
Acyclic dienone
Base 245 nm
Double bond extending conjugation
30
Alkyl group or ring residue
α, β, γ and higher
10, 12, 18
-OH
α, β, γ and higher
35, 30, 18
-OR
α, β, γ, δ
35, 30, 17, 31
α, β, δ
6
-Cl
α, β
15, 12
-Br
α, β
25, 30
β
95
-O(C=O)R
-NR2
O
Exocyclic double bond
5
Homocyclic diene component
39
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Woodward – Fieser rules for Enones
Aldehydes, esters and carboxylic acids have different base values than ketones
Unsaturated system
Base Value
Aldehyde
208
With α or β alkyl groups
220
With α,β or β,β alkyl groups
230
With α,β,β alkyl groups
242
Acid or ester
With α or β alkyl groups
208
With α,β or β,β alkyl groups
217
Group value – exocyclic α,β double bond
+5
Group value – endocyclic α,β bond in 5 or 7 membered ring
+5
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Woodward – Fieser rules for Enones
Unlike conjugated alkenes, solvent does have an effect on λmax
These effects are also described by the Woodward-Fieser rules
Solvent correction
Water
Increment
+8
Ethanol, methanol
0
Chloroform
-1
Dioxane
-5
Ether
-7
Hydrocarbon
-11
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Woodward – Fieser rules for Enones – Examples – 12 & 13
Some examples – keep in mind these are more complex than dienes
cyclic enone =
215 nm
2 x β- alkyl subs. (2 x 12) +24 nm
Calculated value
239 nm
O
R
Experimental value
238 nm
cyclic enone =
extended conj.
β-ring residue
δ-ring residue
exocyclic double bond
215 nm
+30 nm
+12 nm
+18 nm
+ 5 nm
280 nm
Experimental
280 nm
O
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Woodward – Fieser rules for Enones – Problem – 4
β α
β C C C
O
δ γ β α
δ C C C C C
Group
O
Position
6-membered ring or acyclic enone
Increment
Base 215 nm
5-membered ring parent enone
Base 202 nm
Acyclic dienone
Base 245 nm
Double bond extending conjugation
30
Alkyl group or ring residue
α, β, γ and higher
-OH
α, β, γ and higher
35, 30, 18
-OR
α, β, γ, δ
35, 30, 17, 31
10, 12, 18
α, β, δ
6
-Cl
α, β
15, 12
-Br
α, β
25, 30
β
95
-O(C=O)R
-NR2
Exocyclic double bond
5
Homocyclic diene component
39
128
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Woodward – Fieser rules for Enones – Solution for Problem – 4
β α
β C C C
O
δ γ β α
δ C C C C C
O
Group
Increment
6-membered ring or acyclic enone
Base 215 nm
5-membered ring parent enone
Base 202 nm
Acyclic dienone
Base 245 nm
Double bond extending conjugation
30
Alkyl group or ring residue
α, β, γ and higher
-OH
α, β, γ and higher
35, 30, 18
-OR
α, β, γ, δ
35, 30, 17, 31
10, 12, 18
α, β, δ
6
-Cl
α, β
15, 12
-Br
α, β
25, 30
β
95
-O(C=O)R
-NR2
Exocyclic double bond
5
Homocyclic diene component
39
129
Woodward – Fieser rules for Enones – Example – 14
130
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UV Spectroscopy – For Assignment
1.
Absorption spectra of Polyenes – Lycopene, Carotene etc..
2.
Woodward Fieser rules for Polyenes – Rules and calculation for atleast 2 polyenes
3.
Applications of UV spectra - with specific examples
131
UV Spectroscopy - References
1.
Spectroscopy of Organic Compounds, by P.S. Kalsi, 2nd Edition, (1996), pp.7–50.
2.
Organic Spectroscopy: Principles and Applications, by Jag Mohan, 2nd Edition,
(2009), pp.119–152.
3.
Spectrometric Identification of Organic Compounds, by Silverstein, Bassler,
Morrill, 5th Edition, (1991), pp. 289–315.
4.
Introduction to Spectroscopy, by Pavia, Lampman, Kriz, 3rd Edition, (2001),
pp.353-389.
5.
Applied Chemistry, by K. Sivakumar, Ist Edition, (2009), pp.8.1–8.14.
6.
Instrumental Methods of Chemical Analysis, by Gurdeep.R. Chatwal, Sham
Anand, Ist Edition, (1999), pp.180-198.
7.
Selected Topics in Inorganic Chemistry, by Wahid U. Malik, G.D. Tuli, R.D. Madan,
(1996).
8.
Fundamentals of Molecular Spectroscopy, by C.N. Banwell, 3rd Edition, (1983).
9.
www.spectroscopyNOW.com
132
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Good Luck!
Dr. K. SIVAKUMAR
Department of Chemistry
134
SCSVMV University
[email protected]
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