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Introduction to Uv-visible spectroscopy.
Applications of Uv-visible spectroscopy.
Brief introduction to woodward –fieser rules.
Introduction to UV-Visible
UV-Visible spectroscopy is also known as electronic
spectroscopy.In which the amount of light absorbed at
each wavelength of UV and Visible region of
electromagnetic spectrum is measured.
This absorption of electromagnetic radiation by the
molecules leads to molecular excitation.
This is called Electronic spectroscopy since it involves
the promotion of electron from the ground state to the
higher energy state.
It is very useful to measure the number of conjugated
double bond and also aromatic conjugation within the
It is also distinguishes between conjugated and non-
conjugated system;ᾳ,β-Unsaturated carbonyl
compound from β,ϒ-analogues; homoannular and
heteroannular conjugated diens etc.
APPLICATION OF UV- VISIBLE
UV –Visible spectroscopy has been mainly applied for the ,
Detection of unknown compound.
Detect the extent of conjugation
Detection of polynuclear compounds by comparison etc.
Extent of conjugation :
The extent of conjugation in polyene can be estimated,
Addition in unsaturation with the increase in the no.of
double bond ( increase in the value of n)shifts the
absorption to longer wavelength .
It is found that the absorption occurs in the visible
region ,i.e at about 420mᶙ.If n=8 in the above polyene,
such an alkene appears colouerd to human eye.
Distinguish in conjugated and
non conjugated compounds
It is also distinguish between a conjugated and an non
The forbidden n→π* band for the carbonyl group in
the compound (i)will appear at longer wavelength
compared to that for the compound (ii)
The alkyl substitution in an alkene causes a
Identification of an unknown
A unknown compound can be identified by comparing
its spectrum with the known spectra
If the two spectra coincide , the two compound must
If the two spectra do not coincide, then the expected
structure is different from the known compound.
Examination of polynuclear
Benzene and polynuclear hydrocarbon have
charactersitics spectra in the UV –Visible region .
Thus the identification of the polynuclear
hydrocarbon can be made by comparision with the
spectra of known polynucler compound .
The presence of substituent of on the ring , generally
,shifts the absorption maximum to longer wavelength.
Identification of a compound in
Structure of the compound changes in the solvent
.Chloral hydrate shows an absorption maxima at
290mᶙ in hexane while the absorption decreases in the
The compound in the hexane contains a carbonyl
group (a) where in aqueous solvent as (b)
Elucidation of the structure of
vitamin A and K:
Elucidation of the structure of vitamin K1& K2 and
The UV spectra of vitamin K1&K2 are due to the
presence of the same chromophore ,i.e,2,3 dimethyl
naphtha –quinone. The absorption maxima of this
compound are 243,249,260,269 and 330mᶙ .
The elucidation of the structure of vitamin A1&A2 are
possible by this method .
Vitamin A1 absorbs at 325 mᶙ and for Vitamin A2 at
287 and 351 mᶙ.
The absorption maxima occurs at longer wavelength
for Vitamin A2 due to presence of additional ethylenic
Preference over to tautomeric
Consider 2-hydroxy pyridine which exists in equilibrium
with its tautomeric form,pyridone -2.
The spectra of these two compound were found to favour
pyridone -2 which is an ᾳ,β-unsaturated ketone and
clearly ,the equilibrium is shifted towards the right ,i.e.,
Determination of configuration of
The results of absorption shows that cis-alkene
absorbs at different wavelength as compared to their
The distinction is possible when one of the isomers is
forced to be non coplanar by steric hindrence.Thus ,cis
forms suffer distortion and absorption occurs at lower
Determination of strength of
Solvents like water ,acetone forms a hydrogen bond
with the n-electron of the carbonyl oxygen .Due to this
the energy of n-electron in the ground state is lowered
depending on the strength of hydrogen bond .Thus
n→π* transition of carbonyl compound is shifted
towards shorter wavelength . Hence, by measuring the
λmax of the carbonyl compound in a non-polar and
polar protic solvent, the strength of hydrogen bond
can be determined.
n→π* transition of acetone in hexane (at 279 nm)and
that in water at 264.5 nm. Blue shift by 14.5nm
corresponds to an energy of kcal/mol. The strength of
hydrogen bond between water and acetone is
5kcal/mol. It is in fair agreement with the known
strength of hydrogen bond.
Hindered rotation and
Any deviation from the co-planarity of double bonds
will results in the reduction of π orbital overlap and
thus λ max shifts to lower values of λ max and ϵ max.
Eg 2,3-di-tertbutyl-1,3-butadiene. In this case, there is
little conjugation between 2 double bonds due to
bulky tert-butyl groups. It shows absorption at lower
wavelength (λ max= 180nm) compared to 2,3-
dimethyl-1,3-butadiene (λ max= 225nm)in which there
is no deviation from co-planarity.
Detection of impurities
UV absorption spectroscopy is one of the best method
for determination of impurities in organic molecules.
Additional peaks can be observed due to impurities in
the sample and it can be compared with that of
standard raw materials. By also measuring the
absorbance at specific wavelength, the impurities can
UV absorption spectroscopy can be used for the
quantitative determination of compound that absorbs
UV radiation. This determination is based on Beer’s
law which follows,
A= log I˳/ It = log 1/T = -log T = abc = ϵbc
where ϵ = extinction coefficient
c = concentration
b = length of the cell that is used
Molecular weight determination
Molecular weight of a compound can be measured
spectrophotometrically by preparing the suitable
derivatives of these compounds.
Eg: To determine the molecular weight of amine then
its is converted to amine picrate. Then known
concentration of amine picrate is dissolved in a litre of
solution and its optical density is measured at λ max
380nm. After this the concentration of this solution in
grams mol/litre can be calculated using
C = (log I o / It )/ ϵ max x 1
The various application follows
Woodward-Fieser rule for calculating λmax in Diens
Woodward-Fieser rule for calculating λmax in ᾳ,β-
Woodward-Fieser rule for calculating λmax in
Derivatives of Acyl benzene
Woodward-Fieser rule for calculating λmax in Diens
Woodward-Fieser rule for
calculating λmax in ᾳ,β-unsaturated
a) The basic value of ᾳ,β-unsaturated ketone as 215 mᶙ
CH-COX,If X=alkyl group is 215mᶙ,if X=H then
207mᶙ,if X=OH or OR then 193mᶙ.
b)If the double bond and the carbonyl group are
contained in a five membered ring,then the basic
value is 202mᶙ