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ULTRAVIOLET (U.V.) ABSORPTION, SPECTROSCOPY, ROSCOPY, , INTRODUCTION, , One of the important fields of activities of an organic chemist is the determination of structural, formulae of various substances. In the earlier days, the structure elucidation was mainly based upon, the study of classical physical properties (melting point, boiling point, refractive index, etc.) and, chemical reactions of the compound. The procedure involved was rather laborious and time, suming. However, in the last fifty years or so some remarkable physical methods have been, developed to serve as powerful tools for structure analysis. Of these the most useful and commonly, employed methods are the spectroscopic methods which are based upon the interaction of, magnetic radiations with the substance under examination. These methods may further involve, the study of ultraviolet (UV), infrared (IR), nuclear magnetic resonance (NMR) and mass spectra., Mass spectroscopy is employed to determine the molecular mass and molecular formula of the, substance while UV and IR are used to determine the nature of functional groups. Finally the NMR, spectroscopy is used to know the complete structure including the stereochemistry of the substance., The major advantages of employing these spectroscopic methods for structure determination, and identification of substances are as follows :, , (i) They need very little time and the information obtained is often a permanent record in the form ofa, , 4.1., , =, , chart., (ii) The substance is required in very small quantities (of the order of 1 mg.). ;, , (iii) The methods are extremely sensitive and highly reliable in establishing the structure of compounds, and analysis of mixtures of closely related compounds. ., (iv) They can be used for the detection and identification of even very short lived reaction intermediates., (v) During the spectroscopic studies (except mass spectroscopy), the compound is not generally affected, chemically and can, therefore, be recovered unchanged., , (vi) Continuous analysis is often possible facilitating the study of reaction rates, etc., In this chapter, we shall deal with the basic aspects of ultraviolet spectroscopy., , 42. THE ELECTROMAGNETIC SPECTRUM AND SPECTR@SCOPIC SIUBIES, , 4.2.1. Electromagnetic radiations 5, , Many spectroscopic techniques are based upon the interaction of a compound with light or, , - some other form of electromagnetic radiations. It is well known that light and other electromagnetic, | radiations are different forms of energy which have a dual character; they have the properties of both ., a wave and a particle. They are called electromagnetic radiations because they consist of oscillating, electric and magnetic fields which are perpendicular to each other and perpendicular to the direction of, motion of the radiations. : ; 4, , Some important characteristics of these radiations are :, , (i) The energy. of these radiations is inversely proportional to their wavelength or directly, . Proportional to their frequency. t :, , (ii) These radiations do not require any physical medium and can be transmitted through empty, Space. For example, light reaches us from the sun through empty spacé. ‘, , (iii) All types of electromagnetic radiations travel through space with the same velocity which is, , Nearly 3 x 108 m s7., , , , 4, , Scanned with CamScanner

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ANIC CHEMISTRY (8.S¢. T-SEMEsre, |, d, , naracteristics suchas waVeley | t, , , , MODERN APPROACH TO ORG, , , , 2 ‘ . ., Electrom agnetic radiations can be described interms of their ¢, slec!, . Wa * e B fi re or trou, hs ina particular $, : djacent C' rests 8! wave,, 1. Wi velength It is th distance between wo aay, , symbol A (lambda). uum), nanometres (nm) :, , ‘, , micrometres (, , , , , , , , denoted by the t ;, : The wavelength is usually expressed in metres (m), angstroms (A). — ara, , 1pm = 10% m; 1 nm =1 x10 pmjlA=1* et term for nanomety, An older term for micrometre is micron, | (1pm = 1p) and an |, nT int i ond. It is denoted byy|, allt Peaens It is the number of waves which pass across a point in one sect yt, , bol v (read as Normal). -eaply's-lor Hz, a The frequency is generally expressed in cycles per second (cps) or simply (Hert, [1 Hz = 1 eps]., , The frequency and wavelength are related to each other as:, , v=£. | where cis the constant velocity of light, , X, velength and represents the number of waves per centime, , , , , , , , , , 3. Wave-number. It is the reciprocal of wa, , It is denoted by the symbol V. ., Wave number is generally expressed in cm“ (reciprocal centimetres)., , Wave number is related to wavelength and frequency as:, ey, , , , dey,, , yso=~, , aA ¢, , where c is the velocity of radiations in cm (ie.,.3 x 10° cms“)., , InSI system, the unit of wave number is m™!., , 4. Energy of electromagnetic radiations. Electromagnetic radiations are associated with discrete, energy units called quanta or photons. The energy of a photon is directly proportional to its frequency, or inversely proportional to its wavelength as shown by the following equation :, he, , E=hv=, , , , , , , , , , , , , , , , where h is Planck’s constant (6.625 x 10-*J s)., , 4.3. ELECTROMAGNETIC SPECTRUM, Visible light; which can be detected by the human eye constitu !, eoviunietat al iations. The full range of these radiations is indeed very lanes Y small part of, vary from a very small fraction of a metre to many kilometres. The irri mene wavelengths, electromagnetic radiations in increasing order of their wavelengths or decreasing ind ent of entire range, known as electromagnetic spectrum. The complete electromagnetic spectrum, alo: er of their frequencies 8, used to identify different ranges of wave lengths, is shownin fig.4.1.. ngwith common names, , , , , , , , Frequency (Hz) 10” 10! 10'8 10!7 10% 105 10'4:. 1913, ety Pe i ! Po ae 1080 |, Cosmic and Vacuum- | Ultra- Infra Far ., Gamma rays X-rays ultra- | violet, red infra red Micro- Radio———__ .. violet waves waves, : T T —, , , , , , , , , , , , , , , , , , , , 3 ~ T T T . :, Wavelength 103 10? 0.Inm 1nm 10nm = 100nm 7 \1pm 10pm 10%4m_ 193 ! T, oN Opum 104um 104m, , 7, Z Visible light\, 7 (VIBGYOR) \, 7 «400-800 nm NN, , Fig. 4.1. Approximate ranges of wavelengths and frequencies of electromagnetic Tadiati,, tons. (Not to, scale), , , , ~, Scanned with CamScanner

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T ULTRAVIOLET (U.V.) AB, , i, , Of the complete, , ‘ with radiofrequency, , (UY). ,, , 4.3.1. Principle of absorption s, Itis interesting tonote that practic Ih ;, , anipbteces aie ae ictically all the important Spectroscopic methods used for structural, , relationship to molecular Structure, When a b, substance, the radiation m;, , SORPTION SPECTROScopy, , : Als, electromagnetic Spec!, , trum, spectroscopic techni, radiati ¢ ques are concerned primaril, adiations (NMR), infra-red radiations (IR) and ultraviolet-visible indies, , Pectroscopy, , Vibrational and rotati, natindy lowenay nan care nd rotational energy levels can be raised even by, , Avery important condition fora molecule to absorb, ofa photon of the radiation must be, , or electronic energy states of the mi, , electromagnetic radiation is that the energy, equal to the energy difference between two vibrational or rotational, Olecule. In other words, the particular wavelength of radiation that, , tption of radiation is observed instrumentally, by passage of radiation through a sample of substance and automatic analysis of the intensity of ., , transmitted radiations. A record of the amount of radiation absorbed or transmitted by a given sample asa, function of wavelength of radiation is called absorption spectrum,, , One might expect an absorption spectrum to consist of a series of lines but actuall ly the spectrum, ismade of absorption bands with peaks of maximum intensity. The instrument used for record inga, spectrum is known as spectrophotometer or spectrometer., , Having discussed the principle of spectroscopy, we proceed further to discuss ultraviolet, Spectroscopy in some detail., , 4.4. ULTRAVIOLET SPECTROSCOPY sts, i ight in the visible an, i opy involves the measurement of absorption of light in ¢, int ee aie fain 400-800 nm ; UV region 200-400 nm) by the substance under, vest ‘d os the absorption of light involves the transition from one electronic energy level to, statherin amblecule UV spectroscopy is also known as electronic spectroscopy., , Itwill be quite appropriate at this stage to study the laws which govern the absorption of light., , 4.4.1. The Absorption Laws, There are two laws which expr ~, Substance and its molar concentration an, These are:, Beer’s Law. According to this a, substance dispersed in a non-absorbing SO'venly, Concentration of the absorbing substance., , i ati ion of light by a, ectively the relationship between absorption of lig, eaind e agit of the path through which the light passes., , “When a beam of monochromatic light is passed through a, the absorption of light is directly proportional to the molar, , Scanned with CamScanner

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ay, , NIC CHEMISTRY (B.Sc. 11g, as MODERN APPROACH TO ORGA' Ele, , ic light is, , . ” beam of monochromatic lig Passed, <2. Lambert's Law, This law states that, “When a be is length ofthe sap of the sup, io, Substance, the absorption of ight is directly proportional to the path leng' Stale, 3. Beer Lambert Law. The two laws stated above can be combined a F bare ab Oty,, law, termed as BeerLambert Law. It may be stated as, “The absorption of light by a Substay,, , : _ ee, . Particular wavelength is Proportional to the number of molecules of the substance in the path of light, Mathematically :, , logit xe (Beer's law), log" «l (Lambert's law), Tessed as: |, , *; Beer-Lambert law may be exp, , logit eel, , ] ‘ ‘i, log 7 = ecl (eis read as epsilon), = Intensity of incident light, = Intensity of transmitted light, = Concentration of the absorbing substance in mol L7!, , a, i, 5, g, 5, , Limitations of Beer-Lambert Law. Beer-Lami, UV spectra of single species. However, if a samp!, in equilibrium with each other suchas in mers, the, , It is also not applicable if the solute sample and solvent j, complexes. ‘, , 4. Molar absorptivity (e). It isa proportionality constant which, , relates the ser, ved, particular wavelength (4) to the molar concentration of the absorbing substance and the leg stsorbance A) ata, light in cm. It may be expressed as : "eth () of the patho,, j . : log Ip/I°, eed, A, , ie we (. log I,/1=A), , tivity is a measure of the intensity of absorption ata Particular wave, , ital but these units are seldom ie es ength and has, he es tty of acompound and isnot effected by i Concentration i, Ifs:value.ranges tethaees to 10°. Values above 104 constitute high aoe, , le vale low 10* are low intensity absorptions., , , , , , , s ae do Re atte of incident light (1.) to the .ghee 4, age tio of intensity of incident €(L) to thew, : Percent transmission: = I oaee Scanned with CamScanner

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yuTRAVIOLET (U.V.) ABSORPTION SPECTROSCOPY 415, , light (1), the transmittance (T) of a sample will be naturally given by the relation :, ' y, , T=;, , Iy, , -. % transmittance = 100 x a, 0, , Problem 4.1. The molar absorptivity of a conjugated diene (molecular mass = 110 g mol") is 13,100., Compute the concentration of this diene in methanol which is required to give the absorbance of 1.6. The path, tenth of ight is 1.00 cm. Express your answer in (i) mol L“ and (ii) g mL"., , Solution. According to Beer-Lambert Law:, A = ec., , ~_ AL 1.6, c= a7 a an vant cant O Cem si =1.22 x 104 mol L”, ef 13100(cm* mol’) X1.00 (cm), , Since the molecular mass is 110 g mo!"},, 1.22 «104 mol L7 = 1.22 x 104 x 110 gL, , _ 122x107 «110, 10°, , = 1.23 «10% g mL", , 4.5. MECHANICS OF RECORDING THE UV SPECTRUM: PRESENTATION OF THE SPECTRUM, , Excellent ultraviolet spectrophotometers are now available which can conveniently record the, spectra over the range 200-800 nm (200-400 nm is known as near ultraviolet region while 400-800 nm, is visible region). Since oxygen of the atmosphere strongly absorbs light at about 200 nm and below,, the use of wavelengths shorter than 200 nm ( for ultraviolet region) is not practicable unless special, vacuum techniques are employed., , The ultraviolet spectra are always recorded on a wavelength scale in millimicrons (mp) or, nanometres (nm). :, , Procedure. For recording the ultraviolet spectrum, the given compound is generally dissolved in, some suitable solvent which does not itself absorb light in the region under examination. The commonly, emploved solvents are 95% ethanol; hexane and water. 2, , The solution of the compound is placed ina suitable transparent container which does not, absorb light in the region being studied; generally a quartz cell of 1 cm path length is used for, this purpose. (Glass cannot be used since it absorbs strongly in ultraviolet region.) At the same, time some solvent is taken separately in another quartz cell which serves as a reference cell. The, sample solution and the solvent are then exposed to the ultraviolet and visible radiations in a, spectrophotometer in which a hydrogen discharge lamp Is usually employed asa ae ae, , in the ultraviolet region (200-400 nm) while a tungsten filament lamp is used for the visible, , ‘ diation., region (400-800 nm) of the racia light in the beam transmitted through, , Th erates by com aring the amount of a, the cee ope th the reference fan which passes through the reference cell. This compensates for, , as : i | and the solvent. In this way, the spectrometer measures the, any absorption of light by the eeyound at each wavelength of the ultraviolet and visible region., , amount of light absorbed by ae na Tart part as a plot of wavelength of the entire region (on the, , eee ee absorbance (A) of light at each wavelength (on the vertical axis). The, xis) Vi, , Scanned with CamScanner