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  • 1. :UV-Vis Spectroscopy Dr. Mahwish QayyumOrganic Spectral Analysis1

2. Welcome to My Class! Dr.Mahwish Qayyum 3. Table of contents introductionUV & electronic transitionsUsable ranges & observations Selection rules Band Structure Instrumentation & Spectra Beer-Lambert Law Application of UV-spec Conclusion 4. 513PHARMACEUTICAL CHEMISTRY-III (Instrumentation-I) Cr. Hr. 03Topics include: 5th Semester513 PHARMACEUTICAL CHEMISTRY-III (Instrumentation-I) (Theory) Cr. Hr. 03 Note:- The topics will be taught with special reference to their Pharmaceutical Applications. Theory, Instrumentation and Pharmaceutical Applications of the following Spectroscopic Methods 1. Atomic Absorption and Emission Spectroscopy. 2. Molecular fluorescence spectroscopy. 3. Flame Photometry. 4. I.R. Spectroscopy. 5. Mass Spectroscopy. 6. NMR Spectroscopy. 7. UV/Visible Spectroscopy 515 PHARMACEUTICAL CHEMISTRY-III (Instrumentation-I) (Laboratory) Cr. Hr. 01 NOTE:- Practical of the subject shall be designed from time to time on the basis of the above mentioned theoretical topics and availability of the requirements, e.g. Determination of the Purity and Composition of the unknown drugs by using at least each of the above techniques . 5. Reference books PHARMACEUTICAL CHEMISTRY-III (Instrumentation-I) 5th ( Semester) Lough W J, High Performance Liquid Chromatography, Blacki Academic Press, New York, 1996. William Kemp,Organic Spectroscopy, Ellsi Horwood, London, 1990. M Aminuddin & Javed Iqbal, Theory and Practice of Chromatography,University Grants Commission, Islamabad-Pakistan (2000). A H Beckett and J B Stennlake, Practical Pharmaceutical Chemistry,Part I and II, the Aulton Press, London. A M Knevel and F E Digangi, Jenkinss quantitative Pharmaceutical Chemistry, McGraw-Hill Book Company, New York. Braithwaite and F J Smith, Chromatographic Methods, Chapman andHall, London. E Heftmann, Chromatography, Von Nostrond Reinheld Co, New York,1975. Pryde and M J Gilbert, Applications of High Performance Liquid Chromatography, Chapman & Hall, London, 1979. E Stahl, Thin Layer Chromatography, Springer-Verlag, Berlin, 1969. 10. R Hamilton, Introduction to HPLC, P A Sewell, Chapman & Hall, London, 1982. Organic chemistry by M.YOUNIS 6. Our Classroom Rules 1 No food in the classroom (unless it's cake/chocolate for me)any confusion then refer to rule 2 again 3.Avoid death by powerpoint so no sleep otherwise its going to be sleep forever 4.Avoid commenting about anything in your paper chat coz save your self by not providng documented proofs (personal experience) 5.Only smarter questions are allowed (Watch out ahsan) 6.Theres a difference in a classroom & common room 7. Pleasee don't refresh your gloss or your hair by facing it towards your teacher face assuming it as a mirror (God you really wanna do that !!! DaAA) 2 Teacher is always correct if you find have 7. We are going to have a wonderful semester of learning! InshAllah sooooo 8. Lets start with the name of great Almighty AllahO My Lord! expand my chest for me. And ease my task for me; and loose a knot from my tongue, (That) they may understand my saying. (20:25-2) 9. SpectroscopySpectroscopy is a general term referring to the interactions of various types of electromagnetic radiation with matter. Exactly how the radiation interacts with matter is directly dependent on the energy of the radiation. 10. The molecular spectroscopy is the study of the interaction of electromagnetic waves and matter. The scattering of suns rays by raindrops to produce a rainbow and appearance of a colorful spectrum when a narrow beam of sunlight is passed through a triangular glass prism are the simple examples where white light is separated into the visible spectrum of primary colors. This visible light is merely a part of the whole spectrum of electromagnetic radiation, extending from the radio waves to cosmic rays. All these apparently different forms of electromagnetic radiations travel at the same velocity but characteristically differ from each other in terms of frequencies and wavelength 11. Early days of medicinal chemistry i)2 major problems Separation of a chemical substance either from mixture/natural sourceii)Chemical analysis bothqualitativeand quantitativeiii) Early days qualitative (sublimation, crystallization, distillation etc) iv) Quantitative (volumetric/gravimetric)v) Electromagnetic radiations can also give information i.e empirically to presence of certain functional groups.so can help in structural and analytical 12. Electromagnetic radiation: Transmission of energy radiant energy Heat and light energy Quantum mechanics suggests that e.m radiation dual nature Waves Particle like discrete packets energy 13. UV SpectroscopyI.Introduction A. UV radiation and Electronic Excitations1.2.This energy corresponds to EM radiation in the ultraviolet (UV) region, 100-350 nm, and visible (VIS) regions 350-700 nm of the spectrum For comparison, recall the EM spectrum:-raysX-raysUVIRMicrowaveRadioVisible3. 4. 5.RAMIVISUALIZES ULTRA XRAYS GA CARUsing IR we observed vibrational transitions with energies of 8-40 kJ/mol at wavelengths of 2500-15,000 nm For purposes of our discussion, we will refer to UV and VIS spectroscopy as UV13 14. Spectroscopy Utilises theAbsorption and Emission of electromagnetic radiation by atomsAbsorption: Low energy electrons absorb energy to move to higher energy levelEmission: Excited electrons return to lower energy states15 15. Absorption v. EmissionEnergy is emitted by electrons returningto lower energy levels3rd Excited States2nd1stEnergy is absorbed as electrons jump to higher energy levels Ground State16 16. Absorption Spectra Sodium18 17. The Spectroscopic Techniques are based on the fact thatLight absorbed(Absorption)is directly proportional to theConcentration19of the absorbing component. 18. A FLASH BACK !!!!!!!!!20 19. UV-Visible Spectroscopy A UV-visible spectrophotometer measures the amount of energy absorbed by a sample.21 20. The optics of the light source in UV-visible spectroscopy allow either visible [approx. 400nm (blue end) to 750nm (red end) ] or ultraviolet (below 400nm) to be directed at the sample under analysis.22 21. 23 22. Why are carrots orange? Carrots contain the pigment carotene which absorbs blue light strongly and reflects orange red and so the carrot appears orange.400nm500nmBLUE420nm600nmGREEN520 nm24700nmY E L LO WO R A N G E600nmRED 23. Carotene beta-Carotene forms orange to red crystals and occurs in the chromoplasts of plants and in the fatty tissues of plant-eating animals. Molecular formula: C40H56 Molar Mass 537 Melting point 178 - 179 C25 24. Absorbance is set to 0% or light transmitted using a solvent blank in a cuvet. This compensates for absorbance by the cell container and solvent and ensures that any absorbance registered is solely due to the component under analysis.The sample to be analysed is placed in a cuvetQualitative analysis is achieved by determining the radiation absorbed by a sample over a range of wavelengths. The results are plotted as a graph of absorbance/transmittance against wavelength, which is called a UV/visible spectrum.26 25. The UV- Visible absorption spectrum for carotene in the non-polar solvent, hexane I NT EN S I TY O F A B S O R P T I O N700 nm400nmultra- violet320nm27visible460nm540nminfrared 26. Although the light absorbed is dependent on pathlength through the cell, a usual standard 1cm pathlength is used so that pathlength can effectively be ignored. Quantitative analysis is achieved in a manner similar to colorimetry. The absorption of a sample at a particular wavelength (chosen by adjusting a monochromator) is measured and compared to a calibration graph of the absorptions of a series of standard solutions.What can be analysed?In its quantitative form, UV-visible spectroscopy can be used to detect coloured species in solution eg. bromine , iodine and organic compounds or metal ions that are coloured, or can be converted into a coloured compound. 28 27. Molecular orbital is the nonlocalized fields between atoms that are occupied by bonding electrons. (when two atom orbitals combine, either a low-energy bonding molecular orbital or a high energy antibonding molecular orbital results.) Sigma ( ) orbital The molecular orbital associated with single bonds in organic compounds Pi ( ) orbital The molecular orbital associated with parallel overlap of atomic P orbital. n electrons No bonding electrons 28. UV SpectroscopyI.IntroductionB. C.The Spectroscopic Process The difference in energy between molecular bonding, non-bonding and anti-bonding orbitals ranges from 125-650 kJ/mole1. 2. 3. 4.In UV spectroscopy, the sample is irradiated with the broad spectrum of the UV radiation If a particular electronic transition matches the energy of a certain band of UV, it will be absorbed The remaining UV 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 spectrum30 29. UV SpectroscopyI.Introduction C. Observed electronic transitions 1. The lowest energy transition (and most often obs. by UV) is typically that of an electron in the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO)2.3.For any bond (pair of electrons) in a molecule, the molecular orbitals are a mixture of the two contributing atomic orbitals; for every bonding orbital created from this mixing ( , ), there is a corresponding anti-bonding orbital of symmetrically * * higher energy ( , ) The lowest energy occupied orbitals are typically the anti-bonding orbital is of the highest energylikewise, the corresponding-orbitals are of somewhat higher energy, and their complementary anti-bonding orbital somewhat lower in energy than *.5.Unshared pairs lie at the energy of the original atomic orbital, most often this energy is higher than or (since no bond is formed, there is no benefit in energy)31 30. UV SpectroscopyI.Introduction C. Observed electronic transitions 6. Here is a graphical representationUnoccupied levelsEnergyAtomic orbitalnAtomic orbitalOccupied levelsMolecular orbitals32 31. UV SpectroscopyI.Introduction C. Observed electronic transitions 7. From the molecular orbital diagram, there are several possible electronic transitions that can occur, each of a different relative energy:alkanes carbonyls Energynunsat cpds. nO,N,S, halogensncarbonyls33 32. UV SpectroscopyI.Introduction C. Observed electronic transitions 7. Although the UV spectrum extends below 100 nm (high energy), oxygen in the atmosphere is not transparent below 200 nm8.Special equipment to study vacuum or far UV is required9.Routine organic UV spectra are typically collected from 200-700 nm10. This limits the transitions that can be observed: alkanes150 nmcarbonyls170 nmunsaturated cmpds.180 nmnO, N, S, halogens190 nmncarbonyls300 nm - if conjugated!34 33. UV SpectroscopyI.Introduction D. Selection Rules 1. Not all transitions that are possible are observed2.3.4.For an electron to transition, certain quantum mechanical constraints apply these are called selection rulesFor example, an electron cannot change its spin quantum number during a transition these are forbidden Other examples include: the number of electrons that can be excited at one time symmetry properties of the molecule symmetry of the electronic states To further complicate matters, forbidden transitions are sometimes observed (albeit at low intensity) due to other factors35 34. UV SpectroscopyI.Introduction E. Band Structure 1. Unlike IR (or later NMR), where there may be upwards of 5 or more resolvable peaks from which to elucidate structural information, UV tends to give wide, overlapping bands2. 3.4.It would seem that since the electronic energy levels of a pure sample of molecules would be quantized, fine, discrete bands would be observed for atomic spectra, this is the case In molecules, when a bulk sample of molecules is observed, not all bonds (read pairs of electrons) are in the same vibrational or rotational energy statesThis effect will impact the wavelength at which a transition is observed very similar to the effect of H-bonding on the O-H vibrational energy levels in neat samples36 35. UV SpectroscopyI.Introduction E. Band Structure 5. When these energy levels are superimposed, the effect can be readily explained any transition has the possibility of being observedDisassociation R1 - RnV4 R1 - RnV3R1 - RnV2 V1 R1 - RnE1VoR1 - RnDisassociationEnergyR1 - RnV4 R1 - RnV3R1 - RnV2 V1 R1 - RnE0VoR1 - Rn37 36. UV Spectroscopy Instrumentation and Spectra A. Instrumentation 1. The construction of a traditional UV-VIS spectrometer is very similar to an IR, as similar functions sample handling, irradiation, detection and output are required2.Here is a simple schematic that covers most modern UV spectrometers:I0I 200detectorUV-VIS sourcessamplelog(I0/I) = Amonochromator/ beam splitter opticsI0referenceII.700 , nmI038 37. Components of instrumentation: Sources Sample Containers Monochromators Detectors 38. Components of instrumentation: Sources: Agron, Xenon, Deuteriun, or Tungsten lamps Sample Containers: Quartz, Borosilicate, Plastic Monochromators: Quarts prisms and all gratings Detectors: Pohotomultipliers 39. Sources Deuterium and hydrogen lamps (160 375 nm) D2 + Ee D2* D + D + hExcited deuterium molecule with fixed quantized energyDissociated into two deuterium atoms with different kinetic energiesEe = ED2* = ED + ED + hvEe is the electrical energy absorbed by the molecule. ED2* is the fixed quantized energy of D2*, ED and ED are kinetic energy of the two deuterium atoms. 40. Sources Tungsten lamps (350-2500 nm)Low limit: 350 nm1)Low density 2)Glass envelope 41. General Instrument Designs Single beamRequires a stabilized voltage supply 42. General Instrument Designs Double Beam: Space resolvedNeed two detectors 43. General Instrument Designs Double Beam: Time resolved 44. Double Beam Instruments1.Compensate for all but the most short term fluctuation in radiant output of the source2.Compensate drift in transducer and amplifier 3.Compensate for wide variations in source intensity with wavelength 45. Multi-channel Design 46. UV SpectroscopyII.Instrumentation and Spectra A. Instrumentation 3. Two sources are required to scan the entire UV-VIS band: Deuterium lamp covers the UV 200-330 Tungsten lamp covers 330-7004. 5.6.As with the dispersive IR, 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 solutionThe detector measures the difference between the transmitted light through the sample (I) vs. the incident light (I0) and sends this information to the recorder48 47. UV Spectroscopy Instrumentation and Spectra A. Instrumentation 7. As with dispersive IR, time is required to cover the entire UV-VIS band due to the mechanism of changing wavelengths8. 9.A recent improvement is the diode-array spectrophotometer - here a prism (dispersion device) breaks apart the full spectrum transmitted through the sample Each individual band of UV is detected by a individual diodes on a silicon wafer simultaneously the obvious limitation is the size of the diode, so some loss of resolution over traditional instruments is observedDiode array UV-VIS sourcessampleII.Polychromator entrance slit and dispersion device 49 48. UV SpectroscopyII.Instrumentation and Spectra B. Instrumentation Sample Handling 1. Virtually all UV spectra are recorded solution-phase2.3. 4.Cells can be made of plastic, glass or quartz Only quartz is transparent in the full 200-700 nm range; plastic and glass are only suitable for visible spectra Concentration (we will cover shortly) is empirically determinedA typical sample cell (commonly called a cuvet):50 49. UV SpectroscopyII.Instrumentation and Spectra B. Instrumentation Sample Handling 5. Solvents must be transparent in the region to be observed; the wavelength where a solvent is no longer transparent is referred to as the cutoff6.Since spectra are only obtained up to 200 nm, solvents typically only need to lack conjugated systems or carbonyls Common solvents and cutoffs: acetonitrile chloroform240 cyclohexane 1,4-dioxane 95% ethanol n-hexane methanol isooctane water190 195 215 205 201 205 195 19051 50. UV SpectroscopyII.Instrumentation and Spectra B. Instrumentation Sample Handling 7. Additionally solvents must preserve the fine structure (where it is actually observed in UV!) where possible8. 9.H-bonding further complicates the effect of vibrational and rotational energy levels on electronic transitions, dipole-dipole interacts less so The more non-polar the solvent, the better (this is not always possible)52 51. UV SpectroscopyII.Instrumentation and Spectra C. The Spectrum 1. The x-axis of the spectrum is in wavelength; 200-350 nm for UV, 200-700 for UVVIS determinations2.Due to the lack of any fine structure, spectra are rarely shown in their raw form, rather, the peak maxima are simply reported as a numerical list of lamba max values or maxNH2max = OO206 nm 252 317 37653 52. UV SpectroscopyII.Instrumentation and Spectra C. The Spectrum 1. The y-axis of the spectrum is in absorbance, A2.3.From the spectrometers point of view, absorbance is the inverse of transmittance: A = log10 (I0/I)From an experimental point of view, three other considerations must be made: i. a longer path length, l through the sample will cause more UV light to be absorbed linear effectii. iii.the greater the concentration, c of the sample, the more UV light will be absorbed linear effect some electronic transitions are more effective at the absorption of photon than others molar absorptivity, this may vary by orders of magnitude54 53. UV SpectroscopyII.Instrumentation and Spectra C. The Spectrum 4. These effects are combined into the Beer-Lambert Law:i.ii. iii.clfor most UV spectrometers, l would remain constant (standard cells are typically 1 cm in path length)concentration is typically varied depending on the strength of absorption observed or expected typically dilute sub .001 M molar absorptivities vary by4orders of magnitude: values of 104-106 10 -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 transitionsA is unitless, so the units for5.A=are cm-1-1 M and are rarely expressedSince path length and concentration effects can be easily factored out, absorbance simply becomes proportional to , and the y-axis is expressed as directly or as the logarithm of55 54. UV SpectroscopyII.Instrumentation and Spectra D. Practical application of UV spectroscopy 1. UV was the first organic spectral method, however, it is rarely used as a primary method for structure determination2. 3. 4.It is most useful in combination with NMR and IR data to elucidate unique electronic features that may be ambiguous in those methods It can be used to assay (via max and molar absorptivity) the proper irradiation wavelengths for photochemical experiments, or the design of UV resistant paints and coatings The most ubiquitous use of UV is as a detection device for HPLC; since UV is utilized for solution phase samples vs. a reference solvent this is easily incorporated into LC design UV is to HPLC what mass spectrometry (MS) will be to GC56 55. UV SpectroscopyIII. Chromophores A. Definition 1. Remember the electrons present in organic molecules are involved in covalent bonds or lone pairs of electrons on atoms such as O or N2. 3. 4.Since similar functional groups will have electrons capable of discrete classes of transitions, the characteristic energy of these energies is more representative of the functional group than the electrons themselves A functional group capable of having characteristic electronic transitions is called a chromophore (color loving) Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic transitions57 56. UV SpectroscopyIII. Chromophores B. Organic Chromophores 1. Alkanes only posses energy-bonds and no lone pairs of electrons, so only the high * transition is observed in the far UVThis transition is destructive to the molecule, causing cleavage of the -bondCCCC58 57. UV SpectroscopyIII. Chromophores B. Organic Chromophores 2. Alcohols, ethers, amines and sulfur compounds in the cases of simple, aliphatic examples of these compounds the n * is the most often observed transition; like the alkane * it is most often at shorter than 200 nm Note how this transition occurs from the HOMO to the LUMOCCNNCnN spC3CNCN anitbonding orbitalNN59 58. UV SpectroscopyIII. Chromophores B. Organic Chromophores 3. Alkenes and Alkynes in the case of isolated examples of these compounds the * is observed at 175 and 170 nm, respectivelyEven though this transition is of lower energy than *, it is still in the far UV however, the transition energy is sensitive to substitution60 59. UV SpectroscopyIII. Chromophores B. Organic Chromophores 4. Carbonyls unsaturated systems incorporating N or O can undergo transitions (~285 nm) in addition to *n *Despite the fact this transition is forbidden by the selection rules ( = 15), it is the most often observed and studied transition for carbonyls This transition is also sensitive to substituents on the carbonyl Similar to alkenes and alkynes, non-substituted carbonyls undergo the * transition in the vacuum UV (188 nm, = 900); sensitive to substitution effects61 60. UV SpectroscopyIII. Chromophores B. Organic Chromophores 4. Carbonyls n * transitions (~285 nm); * (188 nm)OnCOIt has been determined from spectral studies, that carbonyl oxygen more approximates sp rather 2 than sp !OCO transitions omitted for clarity62 61. UV SpectroscopyIII. Chromophores C. Substituent EffectsGeneral from our brief study of these general chromophores, only the weak n * transition occurs in the routinely observed UV The attachment of substituent groups (other than H) can shift the energy of the transition Substituents that increase the intensity and often wavelength of an absorption are called auxochromes Common auxochromes include alkyl, hydroxyl, alkoxy and amino groups and the halogens63 62. UV SpectroscopyIII. Chromophores C. Substituent EffectsGeneral Substituents may have any of four effects on a chromophore i. Bathochromic shift (red shift) a shift to longer ; lower energy Hypsochromic shift (blue shift) shift to shorter ; higher energyiii.Hyperchromic effect an increase in intensityiv.Hypochromic effect a decrease in intensityHyperchromicii.HypsochromicBathochromicHypochromic200 nm700 nm64 63. UV SpectroscopyIII. Chromophores C. Substituent Effects 1. Conjugation most efficient means of bringing about a bathochromic and hyperchromic shift of an unsaturated chromophore: H2Cmax nmCH217515,00021721,00025835,000465125,000-caroteneOn * *280 18912 900On * *280 21327 7,10065 64. UV SpectroscopyIII. Chromophores C. Substituent Effects 1. Conjugation AlkenesThe observed shifts from conjugation imply that an increase in conjugation decreases the energy required for electronic excitation From molecular orbital (MO) theory two atomic p orbitals, 1 and 2 from two sp hybrid carbons combine to form two MOs 1 and 2* in ethylene662 65. UV SpectroscopyIII. Chromophores C. Substituent Effects 2. Conjugation AlkenesWhen we consider butadiene, we are now mixing 4 p orbitals giving 4 MOs of an energetically symmetrical distribution compared to ethyleneE for the HOMO LUMO transition is reduced67 66. UV SpectroscopyIII. Chromophores C. Substituent Effects 2. Conjugation AlkenesExtending this effect out to longer conjugated systems the energy gap becomes progressively smaller:Lower energy = Longer wavelengthsEnergyethylene butadiene hexatriene octatetraene 68 67. UV SpectroscopyIII. Chromophores C. Substituent Effects 2. Conjugation AlkenesSimilarly, the lone pairs of electrons on N, O, S, X can extend conjugated systems auxochromes Here we create 3 MOs this interaction is not as strong as that of a conjugated systemAEnergynA69 68. UV SpectroscopyIII. Chromophores C. Substituent Effects 2. Conjugation AlkenesMethyl groups also cause a bathochromic shift, even though they are devoid of or n-electrons This effect is thought to be through what is termed hyperconjugation or sigma bond resonanceH C CCH H70 69. UV Spectroscopy Next time We will find that the effect of substituent groups can be reliably quantified from empirical observation of known conjugated structures and applied to new systems This quantification is referred to as the Woodward-Fieser Rules which we will apply to three specific chromophores: 1. Conjugated dienes 2. Conjugated dienones 3. Aromatic systemsmax= 239 nm71 70. Reflection and Scattering Losses 71. LAMBERT-BEER LAWPower of radiation after passing through the solventAPPsolventTPsolutionP0log TPower of radiation after passing through the sample solutionlogP P0Aabckcaabsorptivi tybpathlengthcconcentrat ion 72. Absorption Variables 73. Beers law and mixtures Each analyte present in the solution absorbs light! The magnitude of the absorption depends on its A total = A1+A2++An A total = 1bc1+ 2bc2++ nbcn If 1 = 2 = n then simultaneous determination is impossible Need n s where s are different to solve the mixture 74. AssumptionsIngle and Crouch, Spectrochemical Analysis 75. Deviations from Beers LawIr(I0(22) 122) 1Successful at low analyte concentrations (0.01M)! High concentrations of other species may also affect 76. Effects of Stray Light PS Tstray light PPSP0 APSPSP0log TAlog abcPSP0APPSkc100 77. UV SpectroscopyIV. Structure Determination A. Dienes 1. General FeaturesFor acyclic butadiene, two conformers are possible s-cis and s-transs-transs-cisThe s-cis conformer is at an overall higher potential energy than the s-trans; therefore the HOMO electrons of the conjugated system have less of a jump to the LUMO lower energy, longer wavelength79 78. UV SpectroscopyIV. Structure Determination A. Dienes 1. General Features Two possible * transitions can occur for butadiene175 nm forb. 217 nms-transThe2and2175 nm 253 nms-cis* 4 transition is not typically observed: The energy of this transition places it outside the region typically observed 175 nm The2For the more favorable s-trans conformation, this transition is forbidden * 3 transition is observed as an intense absorption804* 79. UV SpectroscopyIV. Structure Determination A. Dienes 1. General Features *The 2 3 transition is observed as an intense absorption ( = 20,000+) based at 217 nm within the observed region of the UV While this band is insensitive to solvent (as would be expected) it is subject to the bathochromic and hyperchromic effects of alkyl substituents as well as further conjugation Consider:max = 217253220227227256263nm81 80. UV SpectroscopyIV. Structure Determination A. Dienes 2. Woodward-Fieser RulesWoodward 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 * 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, rd Ultraviolet and Visible Spectroscopy, 3 Ed., Butterworths, London, 1975)82 81. UV SpectroscopyIV. Structure Determination A. Dienes 2. Woodward-Fieser Rules - DienesThe rules begin with a base value for max of the chromophore being observed: acyclic butadiene = 217 nm The incremental contribution of substituents is added to this base value from the group tables:GroupIncrementExtended conjugation+30Each exo-cyclic C=C+5Alkyl+5-OCOCH3+0-OR+6-SR+30-Cl, -Br+5-NR2+60 83 82. UV SpectroscopyIV. Structure Determination A. Dienes 2. Woodward-Fieser Rules - Dienes For example:Isoprene - acyclic butadiene = one alkyl subs. Experimental value 220 nmAllylidenecyclohexane - acyclic butadiene = one exocyclic C=C + 5 nm 2 alkyl subs. Experimental value 237 nm217 nm + 5 nm 222 nm217 nm +10 nm 232 nm84 83. UV SpectroscopyIV. Structure Determination A. Dienes 3. Woodward-Fieser Rules Cyclic DienesThere are two major types of cyclic dienes, with two different base values Heteroannular (transoid):= 5,000 15,000 base max = 214Homoannular (cisoid):= 12,000-28,000 base max = 253The increment table is the same as for acyclic butadienes with a couple additions:Group Additional homoannularIncrement +39Where both types of diene are present, the one with the longer becomes the base 85 84. UV SpectroscopyIV. Structure Determination A. Dienes 3. Woodward-Fieser Rules Cyclic DienesIn the pre-NMR era of organic spectral determination, the power of the method for discerning isomers is readily apparent Consider abietic vs. levopimaric acid:C OH Oabietic acidC OH Olevopimaric acid86 85. UV SpectroscopyIV. Structure Determination A. Dienes 3. Woodward-Fieser Rules Cyclic Dienes For example:1,2,3,7,8,8a-hexahydro-8a-methylnaphthalene heteroannular diene = 214 nm 3 alkyl subs. (3 x 5) +15 nm 1 exo C=C+ 5 nm 234 nmExperimental value 235 nm87 86. UV SpectroscopyIV. Structure Determination A. Dienes 3. Woodward-Fieser Rules Cyclic Dienes heteroannular diene =4 alkyl subs. (4 x 5) +20 nm 1 exo C=C C OH O214 nm+ 5 nm239 nmhomoannular diene = 4 alkyl subs. (4 x 5) +20 nm 1 exo C=C C OH O253 nm+ 5 nm278 nm 88 87. UV SpectroscopyIV. Structure Determination A. Dienes 3. Woodward-Fieser Rules Cyclic DienesBe careful with your assignments three common errors: RThis compound has three exocyclic double bonds; the indicated bond is exocyclic to two ringsThis is not a heteroannular diene; you would use the base value for an acyclic dieneLikewise, this is not a homooannular diene; you would use the base value for an acyclic diene89 88. UV SpectroscopyIV. Structure Determination B. Enones 1. General FeaturesCarbonyls, as we have discussed have two primary electronic transitions:Remember, the * transition is allowed and gives a high , but lies outside the routine range of UV observation nThe n * transition is forbidden and gives a very low e, but can routinely be observed90 89. UV SpectroscopyIV. Structure Determination B. Enones 1. General FeaturesFor auxochromic substitution on the carbonyl, pronounced hypsochromic shifts are observed for the n * transition ( max): O H293 nmCH3279OO Cl235NH2214OThis 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 * which extends conjugation and causes a bathochromic shift204OOO OHIn most cases, this bathochromic shift is not enough to bring the * transition into the observed range204 91 90. UV SpectroscopyIV. Structure Determination B. Enones 1. General FeaturesConversely, if the C=O system is conjugated both the n * and are bathochromically shifted Here, several effects must be noted: i. the effect is more pronounced forii. iii. * bands *if the conjugated chain is long enough, the much higher intensity * band will overlap and drown out the n * band the shift of the n * transition is not as predictableFor these reasons, empirical Woodward-Fieser rules for conjugated enones are for the higher intensity, allowed * transition92 91. UV SpectroscopyIV. Structure Determination B. Enones 1. General FeaturesThese effects are apparent from the MO diagram for a conjugated enone:nnOO93 92. UV SpectroscopyIV. Structure Determination B. Enones 2. Woodward-Fieser Rules - EnonesC C C OGroupC C C C C O Increment6-membered ring or acyclic enoneBase 215 nm5-membered ring parent enoneBase 202 nmAcyclic dienoneBase 245 nmDouble bond extending conjugation30Alkyl group or ring residueand higher10, 12, 18-OHand higher35, 30, 18-OR -O(C=O)R35, 30, 17, 31 6-Cl15, 12-Br25, 30-NR295Exocyclic double bond5Homocyclic diene component3994 93. UV SpectroscopyIV. Structure Determination B. Enones 2. Woodward-Fieser Rules - EnonesAldehydes, esters and carboxylic acids have different base values than ketonesUnsaturated systemBase ValueAldehyde With With208oralkyl groupsorWith220alkyl groups230alkyl groups242Acid or ester With Withor oralkyl groups208alkyl groupsGroup value exocyclic Group value endocyclic or 7 membered ring217 double bond+5bond in 5+595 94. UV SpectroscopyIV. Structure Determination B. Enones 2. Woodward-Fieser Rules - EnonesUnlike conjugated alkenes, solvent does have an effect on max These effects are also described by the Woodward-Fieser rules Solvent correction WaterIncrement +8Ethanol, methanol0Chloroform-1Dioxane-5Ether-7Hydrocarbon-1196 95. UV SpectroscopyIV. Structure Determination B. Enones 2. Woodward-Fieser Rules - EnonesSome examples keep in mind these are more complex than dienes cyclic enone = 215 nm 2 x - alkyl subs. (2 x 12) +24 nm O 239 nm Experimental value 238 nmROcyclic enone = extended conj. -ring residue -ring residue exocyclic double bond215 nm +30 nm +12 nm +18 nm + 5 nm 280 nmExperimental280 nm97 96. UV SpectroscopyIV. Structure Determination B. Enones 2. Woodward-Fieser Rules - EnonesTake home problem can these two isomers be discerned by UV-specO OEremophiloneallo-Eremophilonest nd th Problem Set 1: (text) 1,2,3a,b,c,d,e,f,j, 4, 5, 6 (1 , 2 and 5 pairs), 8a, b, c Problem Set 2: outside problems/key -Tuesday98 97. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 1. General FeaturesAlthough aromatic rings are among the most widely studied and observed chromophores, the absorptions that arise from the various electronic transitions are complexOn first inspection, benzene has six -MOs, 3 filled , 3 unfilled *99 98. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 1. General FeaturesOne would expect there to be four possible HOMO-LUMO observable wavelengths (conjugation) * transitions atDue to symmetry concerns and selection rules, the actual transition energy states of benzene are illustrated at the right:u 200 nm (forbidden)u u260 nm (forbidden)180 nm (allowed)g100 99. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 1. General FeaturesThe allowed transition ( = 47,000) is not in the routine range of UV obs. at 180 nm, and is referred to as the primary band The forbidden transition ( = 7400) is observed if substituent effects shift it into the obs. region; this is referred to as the second primary band At 260 nm is another forbidden transition ( = 230), referred to as the secondary band.This transition is fleetingly allowed due to the disruption of symmetry by the vibrational energy states, the overlap of which is observed in what is called fine structure101 100. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 1. General FeaturesSubstitution, auxochromic, conjugation and solvent effects can cause shifts in wavelength and intensity of aromatic systems similar to dienes and enones However, these shifts are difficult to predict the formulation of empirical rules is for the most part is not efficient (there are more exceptions than rules) There are some general qualitative observations that can be made by classifying substituent groups --102 101. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects a. Substituents with Unshared Electrons If the group attached to the ring bears n electrons, they can induce a shift in the primary and secondary absorption bands Non-bonding electrons extend the -system through resonance lowering the energy of transition * More available n-pairs of electrons give greater shiftsGGGG103 102. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects a. Substituents with Unshared Electrons The presence of n-electrons gives the possibility of n * transitionsIf this occurs, the electron now removed from G, becomes an extra electron in the anti-bonding * orbital of the ring This state is referred to as a charge-transfer excited stateG*GGG** *104 103. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects a. Substituents with Unshared Electrons pH can change the nature of the substituent group deprotonation of oxygen gives more available n-pairs, lowering transition energy protonation of nitrogen eliminates the n-pair, raising transition energy PrimarySubstituentmaxSecondary max-H203.57,400254204-OH2116,2002701,450-O-2359,4002872,600-NH22308,6002801,430-NH3+2037,500254169-C(O)OH23011,600273970-C(O)O-2248,700268560105 104. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects b. Substituents Capable of -conjugation When the substituent is a -chromophore, it can interact with the benzene -system With benzoic acids, this causes an appreciable shift in the primary and secondary bands For the benzoate ion, the effect of extra n-electrons from the anion reduces the effect slightlyPrimary SubstituentmaxSecondary max-C(O)OH23011,600273970-C(O)O-2248,700268560106 105. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects c. Electron-donating and electron-withdrawing effects No matter what electronic influence a group exerts, the presence shifts the primary absorption band to longer Electron-withdrawing groups exert no influence on the position of the secondary absorption band Electron-donating groups increase the absorption bandandof the secondary107 106. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects c. Electron-donating and electron-withdrawing effects Primary SubstituentSecondarymaxmax7,4002542042077,000261225-Cl2107,400264190-Br2107,900261192-OH2116,2002701,450-OCH32176,4002691,480-NH22308,6002801,430-CN Electron withdrawing203.5-CH3 Electron donating-H22413,0002711,000C(O)OH23011,600273970-C(O)H25011,400-C(O)CH32249,800-NO22697,800108 107. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects d. Di-substituted and multiple group effects With di-substituted aromatics, it is necessary to consider both groups If both groups are electron donating or withdrawing, the effect is similar to the effect of the stronger of the two groups as if it were a mono-substituted ring If one group is electron withdrawing and one group electron donating and they are para- to one another, the magnitude of the shift is greater than the sum of both the group effects Consider p-nitroaniline:O H2NNO H2NON O109 108. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects d. Di-substituted and multiple group effects If the two electonically dissimilar groups are ortho- or meta- to one another, the effect is usually the sum of the two individual effects (meta- no resonance; ortho-steric hind.) For the case of substituted benzoyl derivatives, an empirical correlation of structure with observed max has been developed This is slightly less accurate than the Woodward-Fieser rules, but can usually predict within an error of 5 nmORG 110 109. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects d. Di-substituted and multiple group effects Parent ChromophoremaxR = alkyl or ring residue R=H250R = OH or O-AlkylO246 230 Substituent incrementR GmpAlkyl or ring residue3310-O-Alkyl, -OH, -O-RingGo 7725-O-112078-Cl0010-Br2215-NH2131358-NHC(O)CH3202045-NHCH3 -N(CH3)273 202085 111 110. UV SpectroscopyIV. Structure Determination C. Aromatic Compounds 2. Substituent Effects d. Polynuclear aromatics When the number of fused aromatic rings increases, thefor theprimary and secondary bands also increaseFor heteroaromatic systems spectra become complex with the addition of the n * transition and ring size effects and are unique to each case112 111. UV SpectroscopyV.Visible Spectroscopy A. Color 1. General The portion of the EM spectrum from 400-800 is observable to humans- we (and some other mammals) have the adaptation of seeing color at the expense of greater detail400500600700800, nm Violet400-420Indigo420-440Blue440-490Green490-570Yellow570-585Orange585-620Red620-780 113 112. UV SpectroscopyV.Visible Spectroscopy A. Color 1. General When white (continuum of ) light passes through, or is reflected by a surface, those ls that are absorbed are removed from the transmitted or reflected light respectivelyWhat is seen is the complimentary colors (those that are not absorbed)This is the origin of the color wheel114 113. UV SpectroscopyV.Visible Spectroscopy A. Color 1. General Organic compounds that are colored are typically those with extensively conjugated systems (typically more than five)Consider -carotene-carotene,max= 455 nmmax is at 455 in the far blue region of the spectrum this is absorbed The remaining light has the complementary color of orange115 114. UV SpectroscopyV.Visible Spectroscopy A. Color 1. General Likewise:lycopene,max =474 nmO H N N H Oindigo max for lycopene is at 474 in the near blue region of the spectrum this is absorbed, the compliment is now red max for indigo is at 602 in the orange region of the spectrum this is absorbed, the compliment is now indigo!116 115. UV SpectroscopyV.Visible Spectroscopy A. Color 1. General One of the most common class of colored organic molecules are the azo dyes:N NEWGsEDGsFrom our discussion of di-subsituted aromatic chromophores, the effect of opposite groups is greater than the sum of the individual effects more so on this heavily conjugated system Coincidentally, it is necessary for these to be opposite for the original synthetic preparation!117 116. UV SpectroscopyV.Visible Spectroscopy A. Color 1. General These materials are some of the more familiar colors of our environmentNO2HO N NH2N OHN NO3SN NNH2 SO3Para RedFast BrownSunset Yellow (Food Yellow 3)118 117. The colors of M&Ms Bright BlueRoyal BlueCommon Food Uses Beverages, dairy products, powders, jellies, confections, condiments, icing.Common Food Uses Baked goods, cereals, snack foods, ice-cream, confections, cherries.Orange-redLemon-yellowCommon Food Uses Gelatins, puddings, dairy products, confections, beverages, condiments.Common Food Uses Custards, beverages, ice-cream, confections, preserves, cereals.Orange Common Food Uses Cereals, baked goods, snack foods, ice-cream, beverages, dessert powders, confections119 118. UV SpectroscopyV.Visible Spectroscopy A. Color 1. General In the biological sciences these compounds are used as dyes to selectively stain different tissues or cell structuresBiebrich Scarlet - Used with picric acid/aniline blue for staining collagen, recticulum, muscle, and plasma. Luna's method for erythrocytes & eosinophil granules. Guard's method for sex chromatin and nuclear chromatin.HO O3SN NN N SO3120 119. UV SpectroscopyV.Visible Spectroscopy A. Color 1. General In the chemical sciences these are the acid-base indicators used for the various pH ranges:Remember the effects of pH on aromatic substituentsMethyl OrangeO3SN NYellow, pH > 4.4CH3 N CH3O3SH N NCH3 N CH3Red, pH < 3.2121 120. More Free PowerPoint Templates at SmileTemplates.com