Advanced Pharmaceutical Analysis-Lecture Two.pptx

Assistant Prof. Dr. Noor H. Naser
Aldabagh

CHROMOPHORE
 Chromophore is defined as the nucleus or any
isolated covalently bonded group responsible for
the absorption of light radiation. Any group
which exhibits absorption of electromagnetic
radiations in the visible or ultraviolet region. As
C=C , C=O ,NO2 etc.
 Some of the important chromophores are
carbonyls, acids, esters, nitrile, ethylene groups.
Assistant Prof. Dr. Noor Hatef 2

Absorption Characteristics of
Some Common
Chromophores

AUXOCHROM
E
 These are co-ordinatively saturated or
un-saturated groups which themselves do not
absorb radiation, but enhance
s
when present along with a
chromophore the absorbing properties
of chromophore.
 Also known as colour enhancing group.
 All auxochromes have one or more non-bonding
pair of electrons. Examples (-NH2 ,-OH ,-OR,-
COOH etc)
 Itextend the conjugation of a chromophore
by sharing the non-bonding electrons.

Effect of Solvent on
Absorption
 The solvent in which the absorbing species
is dissolved also has an effect on the
spectrum of the species. Peaks resulting
from n → π* transitions are shifted to
shorter wavelengths (blue shift) with
increasing solvent polarity because of
increased solvation of the lone pair in the
ground state, which lowers the energy of
the n orbital.

Absorption

Absorption
 Often
the
reverse (red shift) is seen for π →
π*
transitions. This is caused by attractive
polarization forces between the solvent and the
absorbing molecule, which lower the energy
levels of both the excited and unexcited states.
The effect being greater for the excited state, the
energy difference between the excited and
unexcited states is slightly reduced, resulting in a
small red shift. This effect can also influence n →
π* transitions but cannot be observed due to the
dominant blue shift resulting from solvation of
lone pairs.

Absorption

 For example for Mesityl oxide, following
shifts are observed for the two electronic
transitions on moving from low polarity
solvent hexane to water, which has higher
polarity.

TERMS USED IN
UV- VISIBLE
SPECTROSCOPY

Bathochromic shift(red
shift)
 When the absorption maxima(λmax)of a
compound shifts to longer wave length, it is
known as bathochromic shift or red shift.
 The effect is due to the presence of
auxochrome or by change of solvent.
 Eg: The n-π* transition for carbonyl
compounds experiences bathochromic
shift when the polarity of solvent is
decreased.

Hypsochromic shift(blue
shift)
 When the absorption maxima(λmax) of a
compound shifts to a shorter wave length,
it is known as hypsochromic shift or blue
shift.
 The effect is due to the presence of a group
causes removal of conjugation or by
change of solvent.

 Aniline shows blue shift in acidic medium science
it loses conjugation. (Aniline 280 nm, while
anilinium ion 203 nm)

Hyperchromic
effect
 When the absorption intensity (ε) of a
compound is increased, it is known as
hyperchromic shift.
 The introduction of auxochrome usually increase
the absorption intensity.

Hypochromic
effect
 When the absorption intensity (ε) of a
compound is decreased, it is known as
hypochromic shift.
 The introduction of a group that distorts the
geometry of molecule usually reduce the
absorption intensity.

SHIFTE AND
EFFECTS

Conjugated System and Unconjugated
System
 Unconjugated System
 Due to π → π* electronic transition unconjugated
alkenes absorb below 200 nm, e.g., ethene shows
an absorption at 171 nm. Alkyl substituents or the
ring residues attached to the olefinic carbon shift
the absorption band towards longer wavelength
as can be seen with absorption pattern of
compounds like 1-Octene and cyclohexene which
absorb at 177 nm and 182 nm, respectively.

Conjugated System and Unconjugated
System
 Conjugated System
 Multiple bonds that are alternative with single
bonds
are said to be conjugated
 Thus 1,3-butadiene ……… conjugated system.
 1,4- pentadiene …… non conjugated system.

 More stable than non-conjugated one because
during resonance there is transfer of electron and
it will produce some partial double bond
character between C2 and C3 thus strengthen C2-
C3 bond and stabilizing the molecule.
 conjugated system had to red shift (increase wave
length ) because the π electron spread over at
least 4 atomic center
 Generally conjugated system absorb 15 –45 nm,
longer wave length than non-conjugated system.

 Many of the pigments responsible for the brilliant
color of fruit and flowers are said to be
conjugated polyene e.g: Lycopene.
 Compound containing series of conjugated
chromophore groups will appear colored to the
eye So, absorbed at equal or more than 400 nm
 If compounds appear colored contain not less than
4,usually 5 or more conjugated chromophoric and
auxochromic groups except azo,
diazocompounds.
 If two or more chromophoric group are present in
a molecule and they are conjugated the effects
are usually additives. 21
Assistant Prof. Dr. Noor Hatef

Woodward-Fieser Rules for
Dienes
Cisoid
diene
22
Transoid
diene
Parent
values
Homoannula
r
Heteroannula
r

Substituents and Influence on
λmax
23

Examples for Wave length
Calculation
24

Calculation
25

Calculation
 Home Work ; calculate the wave length of
the following compounds.
26

Advanced Pharmaceutical Analysis-Lecture Two.pptx

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Advanced Pharmaceutical Analysis-Lecture Two.pptx