Lecture III
1) Absorption Spectrum . 2) Chromophores and Auxochromes . 3) Bathochromic shift and Hypsochromic shift .
1) Absorption spectrum according to the electronic transition that occur in each organic molecule, absorption spectrum is obtained by plotting Absorbance (A) as a function of wavelength ( ). It has characteristic shape with the of maximum absorbance ( max ). It is characteristic for each molecule according to its structure and the type of transitional energy Therefore it is used for identification of a chemical substance (qualitative analysis). Also max is used for quantitative measurement, measuremen t, in order to increase sensitivity and to
2) Chromophores and Auxochromes Chromophres : e unsaturated groups responsible for
-
* and n→
* electro
ransitions. e.g. C=C , C=O , N=N and N=O ( 200nm-800nm)
-Auxochromes Are saturated groups posses unshared electrons, and does not absorb in near UV or visible radiations e.g. 3)Bathochromic & Hypsochromic shift OH,NH2.
-Bathochromic shift (or red hift) It is the shift of max to a longer wavelength due to substitution with certain functional groups (e.g. –OH and – NH2), when two or more chromophores are present in conjugation, change in pH and effect of the medium (solvent). - Hypsochromic shift (or blue shift) It is the shift of removal
max
to a shorter wavelength due to
-
Hyperchromic effect
an increase in the intensity of absorption usually due to introduction of an auxochrome
- Hypochromic effect It involves a decrease in the intensity of absorption
-Auxochromes hen attached to chomophoric molecule, increase both its wave length and intensity of absorption maximum . Because auxochrome inters into resonance interaction with the chromophore , thus increase the extent of conjugation, shift the absorption maximum to longer wave length
Changes in Absorption spectrum
Example 1): effect of conjugation on absorption spectrum Increase in conjugation, increas absorbance of light to higher , bathochromic shift with hyperchromic
Lecture IV 1)Effect of pH on absorption spectrum. 2)Polychromatic and Monochromatic light. 3)Theory of light absorption
Example 2) Effect of pH The spectra of compounds containing acidic (phenolic-OH) or basic (-NH2) groups are dependent on the pH of the medium. The U.V spectrum of phenol .. :O -
.. :OH OH + H
+
O
-
Phenol
+ H
in acid medium, benzenoid form while in alkaline medium is the phenate anion ,quinonoid The free pair of form of electrons of O2 increasing the elocalization of the electrons, leading to the formation of conjugated So , electrons become more system. energetic and need less energy to be excited, therefore absorb longer bathochromic shift ; red shift) with hyperchromic effect
Its spectrum exhibits Aniline bathochromic shift and hyperchromic :NH + NH effect in alkaline O H medium due to its + H conversion to the quinonoid species While in acid medium (anilinium ) + NH lost the free pair electrons of N decrease the conjugation . spectrum in acid medium exhibit hypsochromic shi nd hypochromic effect due to its conversion to e benzenoid species. 3
2
2
Polychromatic light beam of light containing several wavelengths , e.g. white lig
Monochromatic light beam of light containing radiation of only one discrete wavelen
Lecture VI 1-Isosbestic point 2-Colorimetry .
Isosbestic point
-At different pH, the spectrum will be shifted to different max but all spectra intersect at certain which is known as isosbestic point At isosbestic point, the same absorbance is given for the same concentration at different pH,i.e. absorbance is not pH dependent but concentration dependent -Thus solution ; its max affected by pH , must be buffered at specific pH or measurements are carried out at the -
Colorimetry When white light passes through a colored substance, a characteristic portion of the mixed wavelengths is absorbed.
The remaining light will then assume the complementary color to the wavelength(s) absorbed. Complementary colors are diametrically opposite each other. Thus, absorption of 420-430 nm light renders a substance yellow, and absorption of 500-520 nm light makes it red.
wavelength region, nm
color
complementary color
400-435
Violet
Yellow-green
435-480
Blue
Yellow
480-490
Blue-green
Orange
490-500
Green-blue
Red
500-560
Green
Purple
560-580
Yellow-green
Violet
580-595
Yellow
Blue
595-650
Orange
Blue-green
650-750
Red
Green-blue
Colored substances appear colored because they selectively absorbed some of wavelengths of visible light and transmitted other wavelengths or colors (apparent color), Red substances absorb the blue- green wavelengths from the visible region, so the transmitted light appears red Blue substances absorb the yellow wavelengths, so the transmitted light appears blue.
Requirements for substances to be measured colorimetricaly: ubstance must be coloured e.g CuSO4, organic dyes, 2-If the substance to be analysed is colourless, it must react with certain reagent (known as chromogen) to produce equivalent coloured product. 3-If there is no suitable chromogen, the substance must be converted to a certain derivative which has a suitable chromogen.
2-If the substance to be analysed is colourless, it must react with certain reagent (known as chromogen) to produce equivalent coloured product. 1.Orthophenanthrolene reacts with ferrous (Fe2+ )in buffered medium (acidic pH) to produce intense red color. 2+
N
N
2+ + Fe
3 N
Fe N 3
3-If the sample is colorless and there is no suitable chromogen, the substance must be converted to a certain derivative which can be react with suitable reagent producing color . -Esters are first converted to hydroxamic acid derivative through the reaction with hydroxylamine. Hydroxamic acid derivative gives purple color on addition of ferric (Fe3+ ) due to the formation of iron chelate . O
O
R –C– O Et
+ H2N – OH
R –C– NH – OH + Et OH
Hydroxamic acid derivative + Fe3+ then measuring the absorbance at 520nm.
Chromogen is a compound containing chromophoric group
Requirements for ideal chromogen 1-Should be colorless or easily separated from the colored product 2-It Should be selective. 3-Its reaction to produce colored product, should be of known mechanism and proceed stoichiometrically .
4-The full development of color must be rapid. -It must produce only one color of specified
max
.
Requirements for coloured product 1-Should be of intense color, to increase the sensitivity 2-Should be unaffected by pH or the pH must be specified and maintained by suitable buffer or the measurement is carried out at of isosbestic 3-Should be stable with time
.The reaction of its formation, must be rapid and quantitative.
5-The colored product, should obey Beer-lambert’s law, i.e on plotting A versus C at fixed b, we obtain straight line passing through the origin.
Quiz I)Discuss shortly -Interaction of a substance with EMR -Factors affecting absorption spectrum II) Solve the following problem 1-A 5.00x10-4 M sample solution is measured in a cell with 1 cm bath length ; its absorbance at 592nm equals 0.446 . a-What is the molar absorptivity at 582nm. If a solution of unknown concentration of the same sample has an absorbance 0.125 at the same wave length. b-What is its concentration
2-Calculate the wave legnth in um , and in Angestron 3-Calculate the frequancy and energy of this wave length
III) Complete the answer in Exercise 1
