MPC102

1
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
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Electromagnetic Waves - Terminologies
Electromagnetic wave parameters Wavelength (?)
Wavelength is the distance between the
consecutive peaks or crests

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Electromagnetic Waves - Terminologies
Electromagnetic wave parameters
Frequency (?) Frequency is the number of waves
passing through any point per second.
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Electromagnetic Waves - Terminologies
Electromagnetic wave parameters Wave number (
) Wave number is the number of waves per cm.
Wavelength, Wave number and Frequency are
interrelated as,
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Electromagnetic Spectral regions
nm
103 to 105
10-4 to 10-2
10-2 to 100
100 to 102
102 to 103
105 to 107
107 to 109
UV
X-rays
g-rays
IR
Radio
Microwave
EM waves
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Electromagnetic Spectrum
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The Electromagnetic wave lengths Some examples
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Electromagnetic radiation sources
EM radiation Spectral method Radiation source
Gamma rays Gamma spec. gamma-emitting nuclides
X-rays X-ray spec. Synchrotron Radiation Source (SRS), Betatron (cyclotron)
Ultraviolet UV spec. Hydrogen discharge lamp
Visible Visible spec. tungsten filament lamp
Infrared IR spec. rare-earth oxides rod
Microwave ESR spec. klystron valve
Radio wave NMR spec. magnet of stable field strength
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Electromagnetic Spectrum Type of radiation and
Energy change involved
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Electromagnetic Spectrum Type of radiation and
Energy change involved
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Electromagnetic Spectrum Type of radiation and
Energy change involved
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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
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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.
Absorption spectrum of a molecule Eg H2O
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Effect of electromagnetic radiations on chemical
substances
The radiation energies absorbed by molecules may
produce Rotational, Vibrational and Electronic
transitions.
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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
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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
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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
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Effect of electromagnetic radiations on chemical
substances
Cl2 in Ground state
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Effect of electromagnetic radiations on chemical
substances
Cl2 in Excited state
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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

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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 lt200nm from sunlight
• Excitation (and maximum separation) of ? -
  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, ? -
  electrons and ? - conjugation (joining together)
  systems occurs in 200 380nm

Example for ? conjugation
Benzene
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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 lt200nm from sunlight
• Excitation of ?-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)

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VISIBLE region in Electromagnetic Spectrum
Violet 380 - 420 nm Indigo 420 - 440 nm Blue 440 - 490 nm Green 490 - 570 nm Yellow 570 - 585 nm Orange 585 - 620 nm Red 620 - 800 nm
Violet 380 - 420 nm Indigo 420 - 440 nm Blue 440 - 490 nm Green 490 - 570 nm Yellow 570 - 585 nm Orange 585 - 620 nm Red 620 - 800 nm
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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

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Lamberts 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
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BeerLambert law / BeerLambert Bouguer law /
Lambert Beer law
log (I0/I) ? c l A
Where, I0 - the intensity of incident light I
- the intensity of transmitted light ? - 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
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Absorption intensity ?
wavelength of light corresponding to maximum
absorption is designated as ?max 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
?max
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Generalizations Regarding ?max
If spectrum of compound shows, Absorption band of
very low intensity (?max 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 ? ? 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.
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Generalizations Regarding ?max
If ?max 10,000 - 20,000 generally a simple
?, ?-unsaturated ketone or diene If ?max 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 ?max gt 10,000 along with some
which still have?max lt 10,000. Bands with ?max
lt 100 represent n ? ? 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
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BeerLambert law / BeerLambert Bouguer law /
Lambert Beer law
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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

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? 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
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? 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. 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.
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? Bonding and anti-bonding formation from p
atomic orbitals
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? Bonding and anti-bonding formation from p
atomic orbitals
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Electronic Energy Levels

• ? - orbitals are the lowest energy occupied
  molecular orbitals
• ? - orbitals are the highest energy unoccupied
  molecular orbitals
• ? - orbitals are of somewhat higher energy
  occupied molecular orbitals
• ? - orbitals are lower in energy (unoccupied
  molecular orbitals) than ?
• n - orbitals Unshared pairs (electrons) lie at
  the energy of the original atomic orbital.
• Most often n - orbitals energy is higher
  than ? and ?.
• since no bond is formed, there is no
  benefit in energy

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Electronic Energy Levels
Graphically,
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Electronic Transitions

• The valence electrons in organic molecules are
  involved in bonding as ? - bonds, ? - 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 theHighest Occupied Molecular
  Orbital (HOMO) to the
  Lowest Unoccupied Molecular Orbital
  (LUMO)

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Types of Electronic Transitions
Transition between bonding molecular orbitals and
anti-bonding molecular orbitals
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Types of Electronic Transitions
Transition between bonding molecular orbitals and
anti-bonding molecular orbitals

• ? ? ? transition requires large energies in far
  UV region in 120-200nm range.
• Molar absorptivity Low?max 1000 - 10000

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Types of Electronic Transitions
Transition between bonding molecular orbitals and
anti-bonding molecular orbitals
? ? ? (bonding ? to anti-bonding ?)
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Types of Electronic Transitions
Transition between bonding molecular orbitals and
anti-bonding molecular orbitals

• ? ? ? occur in 200-700nm range.
• Molar absorptivity High
• ?max 1000 - 10000.

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Types of Electronic Transitions
Transition between bonding molecular orbitals and
anti-bonding molecular orbitals
? ? ? (bonding ? to anti-bonding ?)
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Types of Electronic Transitions
Transition between bonding molecular orbitals and
anti-bonding molecular orbitals

• ? ? ? occur only in lt150 nm range.
• Molar absorptivity Low

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Types of Electronic Transitions
Transition between non-bonding atomic orbitals
and anti-bonding molecular orbitals
They are of two types
n ? ? n ? ?

• n ? ? occur in 200-700nm range.
• Molar absorptivity Low
• ?max 10 - 100

• Examples
• Compounds with double bonds involving unshared
  pair(s) of electrons
• Aldehydes, Ketones
• CO, CS, NO etc.,

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Types of Electronic Transitions
Transition between non-bonding atomic orbitals
and anti-bonding molecular orbitals
n ? ? (non-bonding n to anti-bonding ?)
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Types of Electronic Transitions

• Spectra of aldehydes or ketones exhibit two
  bands
• A High intense band at 200-250nm due to ? ? ?
• A low intense band at 300nm due to n ? ?
  transition

Consequently, the probability of jump of an
electron from n ? ? 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 ? ? transition does occur, but only to a
limited extent.
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Types of Electronic Transitions
Transition between non-bonding atomic orbitals
and anti-bonding molecular orbitals

• Excitation of an electron in an unshared pair on
  Nitrogen, oxygen, sulphur or halogens to an
  antibonding ? orbital is called n ? ?
  transitions.
• n ? ? occur in 150-250nm range.
• Molar absorptivity Low
• ?max 100 - 3000

Example Methanol ?max 183nm (?
500) 1-Iodobutane ?max 257nm (?
486) Trimethylamine ?max 227nm (? 900)
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Types of Electronic Transitions
Transition between non-bonding atomic orbitals
and anti-bonding molecular orbitals
n ? ? (non-bonding n to anti-bonding ?)
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Types of Electronic Transitions
s (anti-bonding)
p (anti-bonding)
n (non-bonding)
p (bonding)
s (bonding)
Energy required for various transitions obey the
order ? ? ? gt n ? ? gt ? ? ?gt n ? ?
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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.

s
alkanes carbonyls unsaturated compounds O, N,
S, halogens carbonyls
s s p n n
s p p s p
150 nm
p
170 nm
180 nm
Energy
n
If conjugated
190 nm
p
300 nm
s
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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 - 1The 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 (2S1) Where, S
  is total spin quantum number.

• Singlet state have electron spin paired
• Triplet state have two spins parallel

• Here,
• For excited singlet state S0 therefore, 2S11
  - transition allowed
• For excited triplet state S1 therefore,
  2S13 - transition forbidden

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Selection Rules
Rule - 3 Symmetry of electronic states n ? ?
transition in formaldehyde is
forbidden by local symmetry. i.e., Energy is
always a function of molecular
geometry.
In formaldehyde (H2CO), In n ? ? excited state
an electron arrives at the antibonding ? orbital,
while the electron pair in the bonding ? orbital
is still present. Due to the third antibonding ?
electron, the CO bond becomes weaker and longer.
In the ? ? ? excited configuration, the
situation is somewhat worse because there is only
one ? electron in the bonding orbital, while the
other ? electron is anti-bonding (i.e. ?).
Consequently, the excited state bond lengths
will be longer than a genuine CO double bond but
shorter than a ? -type single C-O bond. In
other words, these excited states will have their
energy minima somewhere in between that of H2CO
and H3C-OH.

• To further complicate matters, forbidden
  transitions are sometimes observed (albeit at low
  intensity) due to other factors.

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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.

FranckCondon 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.
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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.

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Designation of UV bands

• The absorption bands in the UV - Visible spectrum
  may be designated either by using electronic
  transitions ? ? ?, ? ? ?, ? ? ?, n ? ?, n ?
  ? or the letter designation as given below.

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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 ? ? ? transitions.
• In benzene, E1 and E2 bands occur near 180nm and
  200nm respectively and their molar absorptivity
  varies between (?max 2000 to ?max 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.

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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, CC, C N, CN, 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
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Chromophores examples
Chromophore Example Excitation ?max, nm e Solvent
CC Ethene ? __gt ? 171 15,000 hexane
CC 1-Hexyne ? __gt ? 180 10,000 hexane
CO Ethanal n __gt ?? __gt ? 290180 1510,000 hexanehexane
NO Nitromethane n __gt ?? __gt ? 275200 175,000 ethanolethanol
C-X XBrXI Methyl bromideMethyl Iodide n __gt sn __gt s 205255 200360 hexanehexane
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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 ? electrons and stabilizes the
  ? state

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Auxochromes examples
Auxochrome Unsubstitued chromophore ?max (nm) Substituted chromophore ?max (nm)
-CH3 H2CCH-CH CH2 217 H2CCH-CHCHCH3 224
-OR H3C-CHCH-COOH 204 H3C-C(OCH3) CHCOOH 234
-C1 H2CCH2 175 H2C CHCl 185
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Bathochromic shift (Red shift) - ?max to longer
wavelength
Shift of an absorption maximum to longer
wavelength is called bathochromic shift. Occurs
due to change of medium (? ? ? 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 ?max
175nm 1-butene / isobutene ?max 188 nm The
bathochromic shift is progressive as the number
of alkyl groups increases.
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Hypsochromic shift (Blue shift) - ?max to Shorter
wavelength
Shift of absorption maximum to shorter wavelength
is known as hypsochromic shift. Occurs due to
change of medium (n ? ? 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
CO) are available Example Acetone ?max
279nm in hexane ?max 264.5nm in water This
blue shift results from hydrogen bonding which
lowers the energy of the n orbital.
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Hyperchromic effect - increased (?max) 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 Compound Primary band Primary band Secondary band Secondary band
Compound Compound ?max (nm) ?max ?max (nm) ?max
Phenol C6H5OH 210 6200 270 1450
Phenolate anion C6H5O- 235 9400 287 2600
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Hypochromic effect - decreased (?max) 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 Compound Primary band Primary band Secondary band Secondary band
Compound Compound ?max (nm) ?max ?max (nm) ?max
Benzoic acid C6H5COOH 230 11600 273 970
Benzoate C6H5COO- 224 8700 268 560
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Effect of substituents on ?max and ?max
Graphically,
Shift to increased ?max
Shift to shorter ?max
Shift to Longer ?max
Shift to decreased ?max
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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.
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.
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UV Spectroscopy (Electronic Spectra) -
Terminologies
Beer-Lambert Law A ?.c.l
Absorbance A, a measure of the amount of radiation that is absorbed
Molar absorptivity ?, absorbance of a sample of molar concentration in 1 cm cell.
Extinction coefficicent An alternative term for the molar absorptivity.
concentration c, concentration in moles / litre
Path length l, the length of the sample cell in cm.
?max The wavelength at maximum absorbance
?max The molar absorbance at ?max
Band Term to describe a uv-vis absorption which are typically broad.
HOMO Highest Occupied Molecular Orbital
LUMO Lowest Unoccupied Molecular Orbital
Chromophore Structural unit responsible for the absorption.
Auxochrome 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
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Instrumentation
log(I0/I) A
I0
I1
UV-VIS sources
sample
200
700
l, nm
I
detector
monochromator/ beam splitter optics
I0
I2
reference
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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
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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)

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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 nm
• acetonitrile 190
• chloroform 240
• cyclohexane 195
• 1,4-dioxane 215
• 95 ethanol 205
• n-hexane 201
• methanol 205
• isooctane 195
• water 190

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Factors affecting the position of UV bands 1.
Non-conjugated alkenes

• A ?? ? transition can occur in simple
  non-conjugated alkene like ethene and other
  alkenes with isolated double bonds below 200 nm.

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Factors affecting the position of UV bands 1.
Non-conjugated alkenes

• Alkyl substitution of parent alkene moves the
  absorption to longer wavelengths.

• From ?max di-, tri tetra substituted double
  bonds in acyclic and alicyclic systems can be
  identified

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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 ?
  electrons of the alkyl group and the chromophoric
  group.

Methyl groups also cause a bathochromic shift,
even though they are devoid of p-or
n-Electrons 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.

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Factors affecting the position of UV bands 2.
Conjugated Dienes
A conjugated system requires lower energy for
the ?? ? transition than an unconjugated system.
Example Ethylene and Butadiene
In conjugated butadiene (?max217nm ?max
21000) ? and ? orbitals have energies much
closer together than those in ethylene, resulting
in a lower excitation energy
Ethylene has only two orbitals one ground
state ? bonding orbital and one excited state ?
antibonding orbital. The energy difference (??)
between them is about 176 kcal/mole.
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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 ? and
? in ethylene,
p
p
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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 ?
and ? for ethylene and butadiene will be
obtained.
? 4
? 2
? 3
? 2
p
? 1
? 1
Ethylene
Butadiene
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Factors affecting the position of UV bands - 2.
Conjugated Dienes
Butadiene, however, with four ? electrons has
four available ? orbitals, two bonding (?1 and
?2) and two antibonding (?3 and ?4)
orbitals. The ?1 bonding orbital encompasses
all the four ? electrons over the four carbon
atoms of the butadiene system and is somewhat
more stable than a single ? bonding orbital in
ethylene. The ?2 orbital is also bonding
orbital, but is of higher energy than the ?1
orbital. The two ? orbitals (?3 and ?4) are
respectively, more stable ((?3) and less stable
(?4) than the ? orbital of ethylene. Energy
absorption, with the appearance of an absorption
band, can thus occur by a ?2 (bonding) ? (?3
(antibonding transition. HOMO to LUMO), the
energy difference of which (136 kcal/mole) is
less than that of the simple ??? transition of
ethylene (176 kcal/mole) giving a ?max 217 nm
(i.e., at a longer wavelength). It is to be
expected that the greater the number of bonding ?
orbitals, the lower will be the energy between
the highest bonding ? orbital and the lowest
excited ? orbital. The obvious extension of
this in terms of ?max is that the greater the
number of conjugated double bonds, the longer the
wavelength of absorption.
80
Factors affecting the position of UV bands - 2.
Conjugated Dienes
? 4
? 2
? 3
136 kcal/mole
?? 176 kcal/mole
? 2
p
? 1
? 1
DE for the HOMO ? LUMO transition is REDUCED
80
81
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
81
82
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

83
Factors affecting the position of UV bands - 2.
Conjugated Dienes - Types

• Exocyclic and Endocyclic double bond

Endocyclic double bond
Exocyclic double bond
84
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 ?max217 nm (?max 5000-20000).

Base ?max217 nm ?max 5000-20000

• Heteroannular diene, is a conjugated system in
  which the two double bonds are confined to
  two different rings.
• Base ?max 214 nm (?max 5000-20000).

Base ?max214 nm ?max 5000-20000
84
85
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 ?max 253 nm (?max
5000-8000). Homoannular dienes contained in
other ring sizes possess different base
absorption values. Example Cyclopentadiene
?max228nm Cycloheptadiene ?max 241nm
Base ?max253 nm ?max 5000-8000
85
86
Factors affecting the position of UV bands - 2.
Conjugated Dienes
When two or more CC units are conjugated, The
energy difference ?E between the highest bonding
? orbital (HOMO) and the lowest excited ?
orbital (LUMO) becomes small and results in a
shift of ?max to longer wavelength i.e.,
Bathochromic shift. This concept helps to
distinguish between the two isomeric diens,
1,5-hexadiene and 2, 4- hexadoeme, from the
relative positions of ?max. H2CCH-CH2-CH2-CHC
H2 CH3-CHCH-CHCH-CH3 1,5-Hexadiene
2,4-Hexadiene (non-conjugated diene)
(conjugated diene) ?max 178
nm ?max 227 nm
86
87
Factors affecting the position of UV bands - 2.
Conjugated Dienes
87
88
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
?max
Longer wavelengths Lower energy
88
89
Factors affecting the position of UV bands - 2.
Conjugation with hetero atoms
Conjugation with a heteroatom N, O, S, X moves
the (? ? ?) absorption of ethylene to longer
wavelengths Example CH2CH-OCH3 (?max190nm)
- ?max10000 CH2CH-NMe2 (?max230nm) -
?max10000 Methyl vinyl sulphide absorbs at 228
nm (?max8000)
Here we create 3 MOs this interaction is not as
strong as that of a conjugated ?-system
90
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
  ?max295 nm (low energy)
• cis - stilbene absorbs at shorter wavelength
  ?max280 nm (high energy) due to the steric
  effects.
• Coplanarity is needed for the most effective
  overlap of the ? - orbitals and increased ease of
  the ? ? ? transition. The cis-stilbene is
  forced into a nonplanar conformation due to
  steric effects.

91

• UV spectroscopy is very sensitive to distortion
  of the chromophore and consequently the steric
  repulsions which oppose the coplanarity of
  conjugated ?-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
?max245.5nm whereas the usual calculation shows
at ?max229 nm.
92
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 (?max 5500).
Actual ?max 220nm Calculated ?max
273nm
93
Example-3 trans-azobenzene and the sterically
restricted cis-azobenzene
Absorption of Azobenzene (in ethanol)
Example ?? ? transition ?? ? transition n? ? transition n? ? transition
Example ?max ?max ?max ?max
trans-isomer 320 21300 443 510
cis-isomer 281 5260 433 1520
Such differences between cis and trans isomers
are of some diagnostic value
94
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,

95
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
• ?, ?-Un saturated carbonyl compounds display two
  different types of shifts.
• n? ? Band moves to shorter wavelength (blue
  shift).
• ?? ? Band moves to longer wavelength (red shift)

96
Factors affecting the position of UV bands 5.
Effect of Solvents

• ?, ?-Un saturated carbonyl compounds - For
  increased solvent polarity
• n? ? Band moves to shorter wavelength (blue
  shift).
• In n? ? 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
?max 279nm in hexane
?max 264nm in water
97
Factors affecting the position of UV bands 5.
Effect of Solvents
?, ?-Un saturated carbonyl compounds - For
increased solvent polarity (ii) ? ? ? Band
moves to longer wavelength (Red shift). In ? ? ?
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 ?max in ethanol will be greater than
that observed in hexane. i.e., ? 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
B
?
D
AB gt CD
?
A
C
Non-polar solvent
Polar solvent
Longer wavelength
98
Factors affecting the position of UV bands 5.
Effect of Solvents

• ?, ?-Un saturated carbonyl compounds - For
  increased solvent polarity
• (iii) In general,
• 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.
99
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 ? 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.

100
Absorption spectra of Unsaturated carbonyl
compounds. Enones
unsaturated systems incorporating N or O can
undergo n ? ? transitions in addition to ? ?
? ? ? ? transitions ?max188 nm ?max
900 n ? ? transitions ?max285 nm ?max
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
? ? ? transition in the vacuum UV (?max188
nm ?max900) Both this transitions are also
sensitive to substituents on the carbonyl
100
101
Absorption spectra of Unsaturated carbonyl
compounds. Enones
p
Remember, the p ? p transition is allowed and
gives a high e, but lies outside the routine
range of UV observation The n ? p transition is
forbidden and gives a very low e, but can
routinely be observed
n
p
101
102
Absorption spectra of Unsaturated carbonyl
compounds. Enones
Carbonyls n ? ? transitions (285 nm) ? ? ?
(188 nm)
p
It has been determined from spectral studies,
that carbonyl oxygen more approximates sp rather
than sp2 !
n
p
sCO transitions omitted for clarity
102
103
Absorption spectra of Unsaturated carbonyl
compounds. Enones
For auxochromic substitution on the carbonyl,
pronounced hypsochromic (blue) shifts are
observed for the n ? p transition (lmax)
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 It is
important to note this is different from the
auxochromic effect on p ? p which extends
conjugation and causes a bathochromic shift In
most cases, this bathochromic shift is not enough
to bring the p ? p transition into the observed
range
103
104
Absorption spectra of Unsaturated carbonyl
compounds. Enones

• Conversely, if the CO system is conjugated both
  the n ? p and p ? p bands are
• Bathochromically (Red) shifted
• Here, several effects must be noted
• the effect is more pronounced for p ? p
• if the conjugated chain is long enough, the much
  higher intensity p ? p band will overlap and
  drown out the n ? p band
• the shift of the n ? p transition is not as
  predictable
• For these reasons, empirical Woodward-Fieser
  rules for conjugated enones are for the higher
  intensity, allowed p ? p transition

104
105
Absorption spectra of Unsaturated carbonyl
compounds. Enones
Conjugation effects are apparent from the MO
diagram for a conjugated enone
105
106
Absorption spectra of Alkanes - Miscellaneous

• Alkanes only posses s-bonds and no lone pairs
  of electrons, so only the high
• energy s ? s transition is observed in the far
  UV
• This transition is destructive to the molecule,
  causing cleavage of the s-bond

s
s
106
107
Absorption spectra of Aliphatic compounds -
Miscellaneous

• Alcohols, ethers, amines and sulfur compounds
  in the cases of simple, aliphatic examples of
  these compounds the n ? s is the most often
  observed transition like the alkane s ? s it is
  most often at shorter l than 200 nm
• Note how this transition occurs from the HOMO to
  the LUMO

sCN
nN sp3
sCN
107
108
Woodward Fieser rules

• It is used for calculating ?max
• Calculated ?max 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 a-ß-unsaturated ketnones are available

108
109
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 p ? p
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
110
Woodward Fieser rules for Dienes
The rules begin with a base value for lmax 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 CC 5
Alkyl 5
-OCOCH3 0
-OR 6
-SR 30
-Cl, -Br 5
-NR2 60
110
111
Woodward Fieser rules for Dienes Examples -1
2
Isoprene - acyclic butadiene 217 nm
one alkyl subs. 5 nm Calculated
value 222 nm Observed value 220
nm Allylidenecyclohexane - acyclic
butadiene 217 nm one exocyclic CC
5 nm 2 alkyl subs. 10 nm
Calculated value 232 nm Observed
value 237 nm
111
112
Woodward Fieser rules for Dienes Problem - 1
acyclic butadiene 217 nm
Solution
Group Increment
Extended conjugation 30
Each exo-cyclic CC 5
Alkyl 5
-OCOCH3 0
-OR 6
-SR 30
-Cl, -Br 5
-NR2 60
acyclic butadiene 217 nm
extended conjugation 30 nm
Calculated value 247 nm
112
113
Woodward Fieser rules for Dienes Example-3
113
114
Woodward Fieser rules for Cyclic Dienes
Heteroannular (transoid)
Homoannular (cisoid)
Base ?max 253
Base ?max 214
The increment table is the same as for acyclic
butadienes with a couple additions
Group Increment
Additional homoannular 39
Where both types of diene are present, the one with the longer l becomes the base
Group Increment
Extended conjugation 30
Each exo-cyclic CC 5
Alkyl 5
-OCOCH3 0
-OR 6
-SR 30
-Cl, -Br 5
-NR2 60
114
115
Woodward Fieser rules for Cyclic Dienes
Example-4
1,2,3,7,8,8a-hexahydro-8a-methylnaphthalene
115
116
Woodward Fieser rules for Dienes Problem - 2
Heteroannular diene 214 nm
Group Increment
Extended conjugation 30
Each exo-cyclic CC 5
Alkyl 5
-OCOCH3 0
-OR 6
-SR 30
-Cl, -Br 5
-NR2 60
Solution
Heteroannular diene 214 nm
Ring residues /
Alkyl substitution 3 x 5 15 nm
Exocyclic CC bond 1 x 5 5 nm
Calculated value 234 nm
Observed value 247 nm
116
117
Woodward Fieser rules for Cyclic Dienes
Example-5
117
118
Woodward Fieser rules for Cyclic Dienes
Example-6
heteroannular diene 214 nm 4 alkyl subs. (4 x
5) 20 nm 1 exo CC 5 nm 239 nm
abietic acid
118
119
Woodward Fieser rules for Cyclic Dienes
Example-7
homoannular diene 253 nm 4 alkyl subs. (4 x
5) 20 nm 1 exo CC 5 nm 278 nm
levopimaric acid
119
120
Woodward Fieser rules for Dienes Problem - 3
Homoannular diene 253 nm
Group Increment
Additional homoannular 39
Extended conjugation 30
Each exo-cyclic CC 5
Alkyl 5
-OCOCH3 0
-OR 6
-SR 30
-Cl, -Br 5
-NR2 60
Solution
Homoannular diene 253 nm
Extended conjugation 1 x 30 30 nm
Alkyl substitution 2 x 5 10 nm
Calculated value 293 nm
120
121
Woodward Fieser rules for Cyclic Dienes
Example-8
121
122
Woodward Fieser rules for Dienes Examples
9,10 11
122
123
Woodward Fieser rules for Cyclic Dienes
PRECAUTIONS
Be careful with your assignments three common
errors
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
124
Woodward Fieser rules for Enones
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 a, b, g and higher 10, 12, 18
-OH a, b, g and higher 35, 30, 18
-OR a, b, g, d 35, 30, 17, 31
-O(CO)R a, b, d 6
-Cl a, b 15, 12
-Br a, b 25, 30
-NR2 b 95
Exocyclic double bond 5
Homocyclic diene component 39
125

Woodward Fieser rules for Enones
Aldehydes, esters and carboxylic acids have
different base values than ketones
Unsaturated system Base Value
Aldehyde 208
With a or b alkyl groups 220
With a,b or b,b alkyl groups 230
With a,b,b alkyl groups 242

Acid or ester
With a or b alkyl groups 208
With a,b or b,b alkyl groups 217
Group value exocyclic a,b double bond 5
Group value endocyclic a,b bond in 5 or 7 membered ring 5
125
126

Woodward Fieser rules for Enones
Unlike conjugated alkenes, solvent does have an
effect on ?max These effects are also described
by the Woodward-Fieser rules
Solvent correction Increment
Water 8
Ethanol, methanol 0
Chloroform -1
Dioxane -5
Ether -7
Hydrocarbon -11
126
127
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 b- alkyl subs. (2
x 12) 24 nm Calculated value 239
nm Experimental value 238
nm cyclic enone 215 nm extended
conj. 30 nm b-ring residue 12
nm d-ring residue 18 nm exocyclic
double bond 5 nm 280 nm Experiment
al 280 nm
127
128
Woodward Fieser rules for Enones Problem 4
Group Position 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 a, b, g and higher 10, 12, 18
-OH a, b, g and higher 35, 30, 18
-OR a, b, g, d 35, 30, 17, 31
-O(CO)R a, b, d 6
-Cl a, b 15, 12
-Br a, b 25, 30
-NR2 b 95
Exocyclic double bond 5
Homocyclic diene component 39
128
129
Woodward Fieser rules for Enones Solution for
Problem 4
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 a, b, g and higher 10, 12, 18
-OH a, b, g and higher 35, 30, 18
-OR a, b, g, d 35, 30, 17, 31
-O(CO)R a, b, d 6
-Cl a, b 15, 12
-Br a, b 25, 30
-NR2 b 95
Exocyclic double bond 5
Homocyclic diene component 39
129
130
Woodward Fieser rules for Enones Example 14
130
131
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
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UV Spectroscopy - References

1. Spectroscopy of Organic Compounds, by P.S. Kalsi,
   2nd Edition, (1996), pp.750.
2. Organic Spectroscopy Principles and
   Applications, by Jag Mohan, 2nd Edition, (2009),
   pp.119152.
3. Spectrometric Identification of Organic
   Compounds, by Silverstein, Bassler, Morrill, 5th
   Edition, (1991), pp. 289315.
4. Introduction to Spectroscopy, by Pavia, Lampman,
   Kriz, 3rd Edition, (2001), pp.353-389.
5. Applied Chemistry, by K. Sivakumar, Ist Edition,
   (2009), pp.8.18.14.
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

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