Química Orgânica I. Ciências Farmacêuticas Bioquímica Química. Análise estrutural AFB QO I 2007/08 1 AFB QO I 2007/08 2

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1 Química Orgânica I Ciências Farmacêuticas Bioquímica Química AFB QO I 2007/08 1 Análise estrutural AFB QO I 2007/08 2 1

2 Adaptado de: Organic Chemistry, 6th Edition; L. G. Wade, Jr. Organic Chemistry, William H. Brown AFB QO I 2007/08 3 methods determine the molecular structure Crystallography Mass Spectrometry Elemental Analysis Infrared Spectroscopy UV/VIS Spectroscopy Nuclear Magnetic Resonance Coupled methods: GC-MS; HPLC-NMR... (comparative methods for know structures) AFB QO I 2007/08 4 2

3 methods Crystallography: diffraction patterns relates to atomic positions Mass Spectrometry: molecular weight and fragmentation pattern Elemental Analysis: determination of elemental and isotopic composition. Infrared Spectroscopy: determine the presence or absence of functional groups. UV/VIS Spectroscopy: Spectrophotometry uses a spectrophotometer to measure how much light is absorbed by the sample (Gives information about conjugated π electron systems). Nuclear Magnetic Resonance (NMR): NMR spectroscopy works by studying the magnetism of a nucleus by placing it in alignment with a magnetic field, and then using an electromagnetic field to disrupt this alignment. AFB QO I 2007/08 5 E = hν and spectroscopy AFB QO I 2007/08 6 3

4 AFB QO I 2007/08 7 Electromagnetic Radiation Common units used to express λ wavelength Relation Unit to Meter Meter (m) ---- Millimeter (mm) 1 mm = 10-3 m Micrometer (µm) N anometer (nm) Angstrom (Å) 1 µm = 10-6 m 1 nm = 10-9 m 1 Å = m AFB QO I 2007/08 8 4

5 Electromagnetic Radiation Electromagnetic radiation: light and other forms of radiant energy Wavelength (λ):( the distance between consecutive peaks on a wave Frequency (ν):( the number of full cycles of a wave that pass a given point in a second Hertz (Hz): the unit in which radiation frequency is reported; s -1 (read per second ) AFB QO I 2007/08 9 electronic vibrational rotational nuclear spin UV-Vis Vis infrared microwave radiofrequency AFB QO I 2007/

6 UV-vis spectroscopy AFB QO I 2007/08 11 UV-vis E = hν ν = c / λ Electromagnetic radiation is absorbed when the energy of photon corresponds to difference in energy between two states. AFB QO I 2007/

7 Why UV-vis? Many organic molecules have chromophores that absorb UV UV absorbance is about 1000 x easier to detect per mole than NMR Still used in following reactions where the chromophore changes. Useful because timescale is so fast, and sensitivity so high. Kinetics, esp. in biochemistry, enzymology. Most quantitative Analytical chemistry in organic chemistry is conducted using HPLC with UV detectors AFB QO I 2007/08 13 One of the best ways for identifying the presence of acidic or basic groups, due to big shifts in λ for a chromophore containing a phenol, carboxylic acid, etc. hypsochromic shift bathochromic shift λ AFB QO I 2007/

8 UV-vis - range Conjugated compounds can absorb light in the ultraviolet region of the spectrum The region from 2 x 10-7 m to 4 x 10-7 m (200 to 400 nm) is most useful in organic chemistry AFB QO I 2007/08 15 UV-vis range in detail σ σ* and σ π* transitions: high-energy, accessible in vacuum UV (λ max <150 nm). n σ* and π σ* transitions: non-bonding electrons (lone pairs), wavelength (λ max ) in the nm region. n π* and π π* transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with λ max = nm. AFB QO I 2007/

9 Electronic transitions σ π σ σ σ π alkanes carbonyls Energy n π n π σ unsaturated cmpds. O, N, S, halogens π n π carbonyls σ AFB QO I 2007/08 17 calculated π π* transitions Example: π π* transitions responsible for ethylene UV absorption at ~170 nm calculated with ZINDO semi-empirical excited-states methods (Gaussian 03W): hν 170nm photon HOMO π u bonding molecular orbital LUMO π g antibonding molecular orbital AFB QO I 2007/

10 1,3-butadiene The electrons in the highest occupied molecular orbital (HOMO) undergo a transition to the lowest unoccupied molecular orbital (LUMO) AFB QO I 2007/08 19 Other electronic transitions AFB QO I 2007/

11 AFB QO I 2007/08 21 AFB QO I 2007/

12 AFB QO I 2007/08 23 How Do UV spectrometers work? Rotates, to achieve scan Two photomultiplier inputs, differential voltage drives amplifier. Matched quartz cuvettes Sample in solution at ca M. System protects PM tube from stray light D2 lamp-uv Tungsten lamp-vis Double Beam makes it a difference technique AFB QO I 2007/

13 Transmittance vs Absorbance % Transmittance = (I / I 0 ) * 100% where: I = intensity of transmitted light I 0 = intensity of incident light Absorbance = -log(i / I 0 ) AFB QO I 2007/08 25 Diode Array Detectors Model from Agilent literature. Imagine replacing cell with a microflow cell for HPLC! Diode array alternative puts grating, array of photosens. Semiconductors after the light goes through the sample. Advantage, speed, sensitivity, The Multiplex advantage Disadvantage, resolution is 1 nm, vs 0.1 nm for normal UV AFB QO I 2007/

14 UV vis Spectrum 1.0 λ max with certain extinction ε UV Visible Make solution of concentration low enough that A 1 Even though a dual beam goes through a solvent blank, choose solvents that are UV transparent. Absorbance Wavelength, λ, generally in nanometers (nm) Can extract the ε value if conc. (M) and b (cm) are known UV bands are much broader than the photonic transition event. This is because vibration levels are superimposed on UV. AFB QO I 2007/08 27 Electronic Transitions and UVvisible Spectra in Molecules AFB QO I 2007/

15 Quantitative Use of UV Spectra Beers law: A = εcl A absorbance ε is molar absorptivity (extinction coefficient c is concentration in mol/l l is path of light through sample in cm AFB QO I 2007/08 29 meaning λ max : wavelength where UV absorbance for a compound is greatest Energy difference between HOMO and LUMO decreases as the extent of conjugation increases AFB QO I 2007/

16 Effect of Conjugation λ max increases as conjugation increases (lower energy) 1,3-butadiene: 217 nm 1,3,5-hexatriene: 258 nm Substituents on π system increase λ max AFB QO I 2007/08 31 Ψ 2 Ψ 4 Ψ 3 Ψ 1 π Ψ 2 Ψ 1 E for the HOMO LUMO transition is reduced AFB QO I 2007/

17 Energy Lower energy = Longer wavelengths ethylene butadiene hexatriene octatetraene AFB QO I 2007/08 33 AFB QO I 2007/

18 AFB QO I 2007/08 35 Conjugation, Color and the Chemistry of Vision Visible region is about 400 to 800 nm Extended systems of conjugation absorb in visible region β-carotene, 11 double bonds in conjugation λ max = 455 nm AFB QO I 2007/

19 AFB QO I 2007/08 37 Conjugation, Color and the Chemistry of Vision β-carotene is converted to Vitamin A, which is converted to 11-cis-retinal: AFB QO I 2007/

20 Conjugation, Color and the Chemistry of Vision 11-cis-retinal is converted to rhodopsin in the rod cells of the retina. Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain AFB QO I 2007/08 39 Transmission and Color The human eye sees the complementary color to that which is absorbed AFB QO I 2007/

21 Absorbance and Complementary Colors AFB QO I 2007/08 41 The colors of M&M s Bright Blue Common Food Uses Beverages, dairy products, powders, jellies, confections, condiments, icing. Royal Blue Common Food Uses Baked goods, cereals, snack foods, ice-cream, confections, cherries. Orange-red Common Food Uses Gelatins, puddings, dairy products, confections, beverages, condiments. Lemon-yellow Common Food Uses Custards, beverages, ice-cream, confections, preserves, cereals. Orange Common Food Uses Cereals, baked goods, snack foods, ice-cream, beverages, dessert powders, confections AFB QO I 2007/

22 Biological applications In the biological sciences these compounds are used as dyes to selectively stain different tissues or cell structures Biebrich 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 O 3 S N N N N SO 3 AFB QO I 2007/08 43 ph paper Methyl Orange In the chemical sciences these are the acid-base indicators used for the various ph ranges: Remember the effects of ph on aromatic substituents O 3 S N N CH 3 N CH 3 O 3 S H N N CH 3 N CH 3 Yellow, ph > 4.4 Red, ph < 3.2 AFB QO I 2007/

23 designing colours AFB QO I 2007/08 45 UV spectra and structure From Skoog and West et al. Ch 14 AFB QO I 2007/

24 Empirical understandind of structural effects R.B. Woodward, L.F. Fieser and others have predicted λ max for π π* in extended conjugation systems to within ca. 2-3 nm. Homoannular, base 253 nm Acyclic, base 217 nm heteroannular, base 214 nm Attached group increment, nm Extend conjugation +30 Addn exocyclic DB +5 Alkyl +5 O-Acyl 0 S-alkyl +30 O-alkyl +6 NR2 +60 Cl, Br +5 AFB QO I 2007/08 47 Expected UV (Woodward) Base value x alkyl subst. 10 exo DB 5 total 232 Obs. 237 Base value x alkyl subst. 30 exo DB 5 total 234 Obs. 235 O Base value ß alkyl subst. 24 total 239 Obs. 237 AFB QO I 2007/

25 Distinguish Isomers! HO 2 C Base value x alkyl subst. 20 exo DB 5 total 239 Obs. 238 Base value x alkyl subst. 20 total 273 Obs. 273 AFB HO 2 C QO I 2007/08 49 Substituent effects i. Bathochromic shift (red shift) a shift to longer λ; lower energy ii. Hypsochromic shift (blue shift) shift to shorter λ; higher energy iii. Hyperchromic effect an increase in intensity iv. Hypochromic effect a decrease in intensity ε Hypsochromic Hyperchromic Bathochromic Hypochromic 200 nm 700 nm AFB QO I 2007/

26 Quantitative analysis Isosbestic points Single clear point, can exclude intermediate state, exclude light scattering and Beer s law applies Great for nonaqueous titrations Example here gives detn of endpoint for bromcresol green Binding studies Form I to form II Binding of a lanthanide complex to an oligonucleotide AFB QO I 2007/08 51 Other Electronic Processes Fluorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of the same multiplicity Phosphorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of different multiplicity Singlet state: spins are paired, no net angular momentum (and no net magnetic field) Triplet state: spins are unpaired, net angular momentum (and net magnetic field) AFB QO I 2007/

27 rtualtext/spectrpy/uv- Vis/spectrum.htm#uv3b vich/teaching/316/index.html AFB QO I 2007/