Organic Spectroscopy 1

Transcription

1 rganic Spectroscopy 1 Lecture 5, 2 nd Year Michaelmas 2010! Dr Rob Paton CRL ffice 11, 1st floor! robert.paton@chem.ox.ac.uk

2 utline of Lectures 5-8 In lectures 5-6 of this course, the aspects of UV-vis and IR techniques will be introduced that are required in order to assign organic structures. Coverage of the underlying theory and instrumentation associated with each method will be kept to a bare minimum since these aspects are covered elsewhere. We will look at a variety of real spectra and learn to correlate distinguishing features in these spectra with functional groups. UV-vis and IR spectroscopy provide direct experimental data to support of a number of the underlying concepts in organic chemistry introduced last year, such as conjugation and the mesomeric effect. We will also take a moment to consider these points. N N In lectures 7-8 we will show how UV-vis, IR and NMR spectra can be used in combination to assign structures in a selection of real examples, using a selection of worked examples. The examples will be distributed in lecture 6, to give you a chance to work through them independently before lectures 7/8. andouts, problems and colour slides will also be made available on the web pages in due: 2

3 Further Reading Chemical Structure and Reactivity: an Integrated Approach J. Keeler and P. D. Wothers, UP (Chapter 11) Introduction to rganic Spectroscopy - L. M. arwood and T. D.W. Claridge, xford Chemistry Primers rganic Chemistry Clayden, Greeves, Warren and Wothers, UP (Chapter 3) rganic Spectroscopic Analysis R. J. Anderson. D. J. Bendell and P. W. Groundwater, RSC For more complete coverage including many more real examples of spectra, tables of spectroscopic data that will be useful in structural elucidation, and worked examples consult the following: Experimental rganic Chemistry L. M. arwood, C. J. Moody and J. M. Percy NMR spectroscopy Günther rganic Structure Analysis P. Crews, J. Rodriguez and M. Jaspers, UP rganic Structures from Spectra L. D. Field, S. Sternhell and J. R. Kalman Spectroscopic Methods in rganic Chemistry (6 th edition) D.. Williams and I. Fleming, Mcgraw-ill Spectrometric Identification of rganic Compounds R. M. Silverstein, F. X. Webster and D. J. Kiemle Structure Elucidation by NMR in rganic Chemistry E. Breitmaier n the Web A wealth of experimental spectra may be found on the internet, in openly accessible repositories. The following may be of interest: NMRshift DB - NMR database for organic structures: The Japanese Spectral Database for rganic Compounds (SDBS) has free access to IR, Raman, 1 and 13 C NMR and MS data: Sigma-Aldrich (chemical supplier) has IR, Raman and 1 and 13 C NMR spectra for many of their commericially available compounds: Problems in structure, combining IR with 1 and 13 C NMR courtesy of Prof Craig Merlic, UCLA: Past Paper Questions Although the course continues to evolve, the following questions are good practice material (mass spec. is no longer part of the second year course, however): General Paper I: 1993 Q6, 2000 (Q1), 2001 (Q5) and 2004 (Q8) 3

4 General Paper II: 1991 (Q3, Q5), 1992 (Q8), 1993 (Q3), 1994 (Q1), 1995 (Q3), 1996 (Q7), 1997 (Q5), 1998, Q3), 1999 (Q6), 2000 (Q9), 2002 (Q1) and 2003 (Q3) Part IA: 2004 (Q7), 2005 (Q2), 2006 (Q1), 2007 (Q8), 2008 (Q9), 2009 (Q1) and 2010 (Q1). 4

5 The Electromagnetic Spectrum By irradiating molecules at different frequencies, it is possible to gain different types of information about their structure, since these frequencies bring into resonance various modes of molecular motion, or electronic or nuclear excitation. In modern laboratories, NMR spectroscopy is the first choice method for gaining structural information, with Infrared (IR) and mass spectroscopy (MS) techniques acting in a supporting capacity and UV spectra only being required in specialized circumstances (e.g. analysis of specific compound classes such as polymers or porphyrins). c =! " E = h! E = h c / " i.e. absorbance of red light 6.63x10-34 x 3x10 8 / 700X10-9 x N a = 171 kj/mol 5

6 Ultraviolet / Visible Spectroscopy Electronic States Vibrational energy levels Rotational energy levels (energies in wavenumbers, cm -1 ) 0 0 6

7 UV-vis is a form of absorption spectroscopy. Radiation in the UV-visible region of the EM spectrum is absorbed, causing an electron to be excited to a higher energy level. "E h! ground state excited state UV and visible spectra of organic compounds are associated with excitations of electrons from the ground state to an excited state higher in energy. The transition occurs from a filled bonding or non-bonding orbital to a formerly empty antibonding orbital. The energy gap is proportional to the frequency of absorption, and so this form of spectroscopy is a source of bonding information UV spectroscopy is most important in the structural analysis of compounds containing #$-bonds, in particular conjugated systems. 7

8 3 C C 3 2 C C 2 2 C C C C 2 # " 4! " 2! " 3! " 135 nm (900 kj/mol) 162 nm 217 nm (750 kj/mol) (500 kj/mol) 1! 2! # 1! 8

9 Terminology: hyperchromic hypsos = height molar extinction coefficient, " hypsochromic hypochromic bathochromic bathos = depth hyper = above hypo = below wavelength,! (nm) Energy gap (kj/mol) " max = 450 nm 9

10 Recording UV-vis spectra The ultraviolet or visible spectrum is usually taken using a dilute solution of the sample in a glass or quartz tube, or cuvette. Typically the sides of the cuvette are 1 cm, and the total volume is 2-3 cm 3. UV or visible light is passed through the sample and the intensity of the transmitted beam is recorded across the wavelength range of the instrument (I). First the intensity of the light is recorded with pure solvent in the cuvette (I 0 ) the absorbance due to the sample can then be computed as log 10 (I 0 /I). light source * I 0 I detector l The Beer-Lambert law states that the absorption of light by a given sample is proportional to the number of absorbing molecules, and independent of the source intensity. log 10 (I 0 /I) =! l c! = " / ( l c) I 0 and I are the intensities of the incident and transmitted light respectively, l is the path length of the absorbing solution in cm and c is the concentration in moles/litre.! is the molar extinction coefficient in 1000 cm 2 mol -1. log 10 (I 0 /I) is called the absorbance. Example: A 1.12 x 10-4 M solution of paranitroaniline, in a cuvette of path length 1cm, has a measured absorbance maximum of 1.55 at 227 nm. This means the intensity of the transmitted light is = 35 times the intensity of the incident light. The % value for this absorption is: 1.55 / (1 x 1.12 x 10-4 ) = This would be quoted as " max 227 (% 13890) 10

11 Choice of solvent: The solvent and vessels must be transparent in the range of interest. cyclohexane chloroform 95% ethanol water quartz glass wavelength (nm) Absorption of common functional groups: single bonds!"!* double bonds isolated #"#* lone pairs (, N, S) n"!* conjugated #"#* n"#* wavelength (nm) Vacuum UV UV 11

12 !* "* n (LP) " The functional groups such as polyenes and poly-ynes that give rise to diagnostic absorptions in the UV-visible region of the EM spectrum are referred to as chromophores Selection Rules and Intensity The irradiation of organic compounds does not always give rise to excitations of electrons from any filled to unfilled orbital, because there are rules based on symmetry governing which transitions are allowed. The intensity of absorption is therefore related to the allowedness of a particular transition A chromophore with two double bonds conjugated together possesses a fully allowed transition, and has associated % values of about 10,000 Forbidden absorptions are in practice observed with weak absorptions, as the symmetry may be broken by a molecular vibration or by unsymmetrical substitution. allowed "forbidden"! " - "* n - "* " - "*! > 10,000! = ! = The most important point to be made is that, in general: The longer the conjugated system the more intense the absorption 12

13 Conjugated dienes: Me n Me n " max (nm)! " max (nm)! 30,000 76, , , Ph n Ph 75,000 86,500 94, , ,000 Values from Nayler, P.; Whiting, M. C. J. Chem. Soc. 1955, The most important point to be made is that, in general: The longer the conjugated system the longer the wavelength of the absorption maximum 13

14 Aromatics: Absorption maxima for substituted benzene rings (Ph-R) R N3 Me I Cl Br Me S 2N2 CN C2 C 2 N2 NAc CMe C=C2 C Ph Ph N2 C=CC 2 C=CPh " max (nm)! " max (nm)! ,400 7,500 7,000 7,000 7,400 7,900 6,200 6,400 9,700 13,000 8,700 11,600 8,600 9,400 10,500 9,800 14,000 11,400 18,300 11,000 7,800 21,000 29, " max (nm) ! Acid induced bathochromic shift: N 2 N 3! max 230 nm! max 203 nm N 2 N 2 N 2 N 14

15 Base induced hypsochromic shift: -! max nm! max 235 nm Effects of complementary EWG/EDG substituents: N 2 N 2 N 2! max 230 nm! max 269 nm " 7800 " 8600 N 2 2 N N 2 N 2 N 2 2 N 2 N! max 229 nm " 14800! max 235 nm " 16000! max 375 nm " 16000! max 260 nm " 1300 N 2 N 2 2 N N 15

16 Acid base indicators, e.g phenolphthalein:! max 231 nm (25,800)! max 275 nm (4,200)! max 230 nm (25,800)! max 553 nm (26,000) pk a 9.4 C 2 C 2 16

17 Carbonyls: 4! " 2! " 2! " 3! " 2p 2p 2! 1! 1! 1! Cyclohexanone vs. 1-cyclohexenone UV-Vis: 17

18 18

19 Predicting UV absorptions of conjugated dienes: Alkyl substitution of butadiene extends the chromophore through hyperconjugative interactions, causing a small red shift to longer values for # max. The effect of alkyl substitution on open chain dienes and dienes in six-membered rings is approximately additive, so a few rules (first formulated by Nobel Laureate R. B. Woodward in 1941) can be used to predict absorption. Woodward s rules have since been refined as a result of experience by Fieser. Woodward s rules may be applied to predict the absoroption of a diene that is either homoannular with both double bonds contained in one ring or heteroannular with two double bonds distributed between two rings. Woodward's rules for diene and triene absorption Base value for parent s-trans diene (heteroannular) Base value for parent s-cis diene (homoannular) Increments for: (a) each alkyl substituent or ring residue (b) exocyclic nature of any double bond (c) additional double bond extending conjugation (d) auxochrome: -Acyl -Alkyl -SAlkyl -Cl or -Br -NAlkyl 214 nm 253 nm +5 nm +5 nm +30 nm +0 nm -Acyl +6 nm -Alkyl +30 nm - -SAlkyl +5 nm -Cl or -Br +60 nm -NAlkyl 19

20 Examples: + 2 x alkyl s-trans diene x 5 = 224 nm + 3 x ring residue 1 x alkyl + heteroannular diene x 5 exocyclic C=C + 5 = 239 nm + 5 x ring residue 1 x alkyl + + homoannular diene C=C extending conjugation exocyclic C=C x = 318 nm More rigourous treatment particle in a box: E n = n 2 h 2 /8mL 2 20

21 Rules for the principal band of substituted benzenes RC 6 4 X Parent chromophore: X alkyl or ring residue or alkyl 246 nm 250 nm 230 nm R X Increment for each substituent: -alkyl/ring residue -, Me, Alkyl o, m - o m -Cl o, m p Examples: o, m p o, m p o m p o, m p Br -, Me, Alkyl -N 2 om -NAc o, m -NMe -NMe 2 o, m p o, m p o, m p p o, m p Me ring X = ring residue + ortho alkyl para Me = 274 nm 21

22 Steric effects on UV absorptions: trans-stilbene and cis-stilbene! max 296 nm (" 29,000)! max 280 nm (" 10,500) 2,4,6-trimethylacetophenone and para-methylacetophenone! max 242 nm (" 3,200)! max 252 nm (" 15,000) Strain release in the hydrolysis of a dilactone produced from shelloic acid. 22

23 2 no strong absoprtion >210 nm! max 227 nm (" 5,500) Tomatoes are a deeper red than carrots. Given that the conjugated systems of &-carotene and lycopene are both eleven double bonds conjugated together with a similar number of alkyl substituents, why might lycopene absorb at a longer wavelength and with greater intensity?!-carotene lycopene cis-retinal 23

24 Recent applications in organic synthesis: Dehydration of graphene oxide to graphene + (Chem. Mater. 2009, 21, 2950) 24

25 Expanding the Porphyrin #-system (rg. Lett. 2008, 10, 3945) 25

26 Infrared (IR) Spectroscopy Me Me cortisone acetate Absorbance Energy 1

27 Electronic States Vibrational energy levels Rotational energy levels (energies in wavenumbers, cm-1) E = hc/! i.e. C- bonds abs orb at around 3000 cm -1 : 6.63x10-34 x 3x10 8 x 3000x10 2 x N a = 36 kj/mol Transmission igh-resolution IR spectrum of C in the gas phase: wavenumber (cm-1)